Editing Bivalvia

{{Short description|Class of molluscs}}
{{Redirect|Bivalve}}
{{Featured article}}
{{Use British English|date=July 2022}}
{{Use dmy dates|date=July 2022}}
{{Automatic taxobox
| fossil_range = {{fossil range|Cambrian Series 2|recent|Early [[Cambrian]] – recent|ref=<ref name="Jell 1980" /><ref name="Runnegar Bentley 1983" />}}
| image = Haeckel Acephala.jpg
| image_alt = [[Ernst Haeckel]]'s "Acephala"
| image_caption = "Acephala", from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'' (1904)
| taxon = Bivalvia
| authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]
| subdivision_ranks = Subclasses
| subdivision = 
*[[Heterodonta]]
*[[Palaeoheterodonta]]
*[[Protobranchia]]
*[[Pteriomorphia]]
[[#Bivalvian taxonomies|And see text]]
}}
[[File:Tridacna gigas.001 - Aquarium Finisterrae.JPG|thumb|alt=Shell of the giant clam (''Tridacna gigas'')|Empty shell of the [[giant clam]] <br /> (''Tridacna gigas'')]]
[[File:Ensis phaxoides-V.jpg|thumb|alt=Sword razor|Empty shells of the [[Ensis ensis|sword razor]] <br /> (''Ensis ensis'')]]
'''Bivalvia''' ({{IPAc-en|b|aɪ|ˈ|v|æ|l|v|i|ə}}) or '''bivalves''', in previous centuries referred to as the '''Lamellibranchiata''' and '''Pelecypoda''', is a [[class (biology)|class]] of [[aquatic animal|aquatic]] [[mollusc]]s (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified [[exoskeleton]] consisting of a hinged pair of half-[[bivalve shell|shell]]s known as [[valve (mollusc)|valve]]s. As a group, bivalves have no [[head]] and lack some typical molluscan organs such as the [[radula]] and the [[odontophore]]. Their [[gill]]s have evolved into [[ctenidium (mollusc)|ctenidia]], specialised organs for feeding and breathing.

Common bivalves include [[clam]]s, [[oyster]]s, [[Cockle (bivalve)|cockles]], [[mussel]]s, [[scallop]]s, and numerous other [[family (biology)|families]] that live in saltwater, as well as a number of families that live in freshwater. Majority of the class are [[benthic]] [[filter feeder]]s that bury themselves in sediment, where they are relatively safe from [[predation]]. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as scallops and [[file shell]]s, can [[nekton|swim]]. [[Shipworm]]s bore into wood, clay, or stone and live inside these substances.

The [[Exoskeleton|shell]] of a bivalve is composed of [[calcium carbonate]], and consists of two, usually similar, parts called [[Valve (mollusc)|valves]]. These valves are for feeding and for disposal of waste. These are joined together along one edge (the [[hinge line]]) by a flexible [[ligament (bivalve)|ligament]] that, usually in conjunction with interlocking "teeth" on each of the valves, forms the [[hinge]]. This arrangement allows the shell to be opened and closed without the two halves detaching. The shell is typically [[symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]], with the hinge lying in the [[sagittal plane]]. Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10&nbsp;cm (4&nbsp;in).

Bivalves have long been a part of the diet of coastal and [[Riparian_zone|riparian]] human populations. Oysters were [[oyster farming|cultured]] in ponds by the Romans, and [[mariculture]] has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the potential [[shellfish poisoning|hazards]] of eating raw or undercooked [[shellfish]] has led to improved storage and processing. [[Pearl oysters]] (the common name of two very different families in salt water and fresh water) are the most common source of natural [[pearl]]s. The shells of bivalves are used in craftwork, and the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution.

Bivalves appear in the [[fossil record]] first in the early [[Cambrian]] more than 500 million years ago. The total number of known living [[species]] is about 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including [[brackish water]] and [[estuarine]] species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest [[Holocene|recent]] marine families are the [[Veneridae]], with more than 680 species and the [[Tellinidae]] and [[Lucinidae]], each with over 500 species. The freshwater bivalves include seven families, the largest of which are the [[Unionidae]], with about 700 species.

== Etymology ==
The [[Linnaean taxonomy|taxonomic]] term Bivalvia was first used by [[Carl Linnaeus|Linnaeus]] in the [[10th edition of Systema Naturae|10th edition]] of his ''[[Systema Naturae]]'' in 1758 to refer to animals having shells composed of two [[Valve (mollusc)|valves]].<ref>{{cite book | last=Linnaeus | first=Carolus | author-link=Carl Linnaeus | language = la | title=''Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata'' | publisher=Laurentii Salvii | year=1758| volume=1 | page=645 | url = https://www.biodiversitylibrary.org/item/31242#page/629/mode/1up }}</ref> More recently, the class was known as Pelecypoda, meaning "[[axe]]-foot" (based on the shape of the foot of the animal when extended).

The name "bivalve" is derived from the [[Latin]] {{lang|la|bis}}, meaning 'two', and {{lang|la|valvae}}, meaning 'leaves of a door'.<ref name="definition" /> ("Leaf" is an older word for the main, movable part of a door. We normally consider this the door itself.) Paired shells have evolved independently several times among animals that are not bivalves; other animals with paired valves include certain [[gastropod]]s (small [[sea snail]]s in the family [[Juliidae]]),<ref>{{cite journal |author1=Le Renard, J. |author2=Sabelli, B. |author3=Taviani, M. | year = 1996 | title = On ''Candinia'' (Sacoglossa: Juliidae), a new fossil genus of bivalved gastropods |journal=Journal of Paleontology |volume=70 |issue=2 |pages=230–235 |doi=10.1017/S0022336000023313 |jstor=1306386 |bibcode=1996JPal...70..230R |s2cid=132821312 }}</ref> members of the phylum [[Brachiopoda]]<ref>{{cite web | url = http://www.earthlife.net/inverts/brachiopoda.html | title = The Phylum Brachiopoda |work=Earthlife Web |access-date=2012-05-05 }}</ref> and the minute crustaceans known as [[Ostracoda|ostracods]]<ref>{{cite web | url = http://oxforddictionaries.com/definition/Ostracoda | archive-url = https://web.archive.org/web/20110908063608/http://oxforddictionaries.com/definition/Ostracoda | url-status = dead | archive-date = 8 September 2011 | title = Ostracoda |work=Oxford Dictionaries |publisher=Oxford University Press |access-date=2012-07-01 }}</ref> and [[conchostraca]]ns.<ref>{{cite journal | author = Webb, J. | year = 1979 | title = A reappraisal of the palaeoecology of conchostracans (Crustacea: Branchiopoda) |journal=Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen | volume = 158 | issue = 2 | pages = 259–275 | doi = 10.1127/njgpa/158/1979/259 }}</ref>

== Anatomy ==
[[File:Margaritifiera-margaritifiera-Anatomy.png|thumb|alt=Freshwater pearl mussel anatomy|Drawing of [[freshwater pearl mussel]] (''Margaritifera margaritifera'') anatomy:
{{image key
|list type=ordered
|posterior adductor
|anterior adductor
|outer left gill demibranch
|inner left gill demibranch
|excurrent siphon
|incurrent siphon
|foot
|teeth
|hinge
|mantle
|umbo
}}]]
[[File:Valve-InternalView.png|thumb|alt=Interior of the left valve of a venerid|Interior of the left valve of a [[Veneridae|venerid]]]]
[[File:Valve-DorsalView collored.svg|thumb|alt=Main parts of a bivalve shell|Main parts of a bivalve shell:
{{image key
|list type=ordered
|[[sagittal plane]]
|growth lines
|[[ligament]]
|umbo
}}]]
Bivalves have [[bilaterally symmetrical]] and laterally flattened bodies, with a blade-shaped foot, vestigial head and no [[radula]].<ref>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=367–368 }}</ref><ref>{{cite book | title = Zoology | url = https://archive.org/details/zoology0000dori | url-access = registration | last = Dorit | first = Robert L. |author2=Walker, Warren F. Jr. |author3=Barnes, Robert D. | year = 1991 |publisher=Saunders College Publishing| isbn = 978-0-03-030504-7 | page = 672 }}</ref> At the dorsal or back region of the shell is the hinge point or line, which contain the [[umbo (bivalve)|umbo]] and [[beak (bivalve)|beak]] and the lower, curved margin is the ventral or underside region. The anterior or front of the shell is where the [[byssus]] (when present) and foot are located, and the posterior of the shell is where the siphons are located. With the hinge uppermost and with the anterior edge of the animal towards the viewer's left, the valve facing the viewer is the left valve and the opposing valve the right.<ref name="Tutorial" /><ref>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=370 }}</ref> Many bivalves such as clams, which appear upright, are evolutionarily lying on their side.

=== Mantle and shell ===
{{main|Bivalve shell}}
The shell is composed of two [[calcareous]] valves held together by a ligament. The valves are made of either [[calcite]], as is the case in oysters, or both calcite and [[aragonite]]. Sometimes, the aragonite forms an inner, [[nacre]]ous layer, as is the case in the order [[Pteriida]]. In other [[Taxon|taxa]], alternate layers of calcite and aragonite are laid down.<ref name="Mineralogy" /> The ligament and byssus, if calcified, are composed of aragonite.<ref name="Mineralogy" /> The outermost layer of the shell is the [[periostracum]], a thin layer composed of horny [[conchiolin]]. The periostracum is secreted by the outer mantle and is easily abraded.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=7|isbn=0-85238-234-0}}</ref> The outer surface of the valves is often sculpted, with clams often having concentric striations, scallops having radial ribs and oysters a latticework of irregular markings.<ref>{{cite book | title = Seashells: Jewels from the Ocean |last=Titlow |first=B. | year = 2007 |publisher=Voyageur Press| isbn = 978-0-7603-2593-3 | page = 29 | url = https://books.google.com/books?id=UpoJUR6txB0C&pg=PA29 }}</ref>

In all molluscs, the [[mantle (mollusc)|mantle]] forms a thin [[Biological membrane|membrane]] that covers the animal's body and extends out from it in flaps or lobes. In bivalves, the mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of [[ligament]], byssus threads (where present), and [[hinge teeth|teeth]].<ref name=mantlemuscule>{{cite web | url = https://www.britannica.com/EBchecked/topic/67293/bivalve/35746/The-mantle-and-musculature | title = Bivalve: The mantle and musculature | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-05-05 }}</ref> The posterior mantle edge may have two elongated extensions known as [[Siphon (mollusc)|siphons]], through one of which water is inhaled, and the other expelled.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|pages=4–5|isbn=0-85238-234-0}}</ref> The siphons retract into a cavity, known as the [[pallial sinus]].<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=16|isbn=0-85238-234-0}}</ref>

The shell grows larger when more material is secreted by the mantle edge, and the valves themselves thicken as more material is secreted from the general mantle surface. Calcareous matter comes from both its diet and the surrounding seawater. Concentric rings on the exterior of a valve are commonly used to age bivalves. For some groups, a more precise method for determining the age of a shell is by cutting a cross section through it and examining the incremental growth bands.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=7–10|isbn=0-85238-234-0}}</ref>

The [[shipworm]]s, in the family [[Teredinidae]] have greatly elongated bodies, but their shell valves are much reduced and restricted to the anterior end of the body, where they function as scraping organs that permit the animal to dig tunnels through wood.<ref>{{cite journal | author = Edmondson, C. H. | year = 1962 | title = Teredinidae, ocean travelers |journal=Occasional Papers of Bernice P. Bishop Museum |volume=23 |issue=3 |pages=45–59 | url = http://hbs.bishopmuseum.org/pubs-online/pdf/op23-3.pdf }}</ref>

=== Muscles and ligaments===
The main muscular system in bivalves is the [[Anatomical terms of location#Invertebrates|posterior and anterior]] adductor muscles. These muscles connect the two valves and contract to close the shell. The valves are also joined dorsally by the hinge [[ligament]], which is an extension of the periostracum. The ligament is responsible for opening the shell, and works against the adductor muscles when the animal opens and closes.<ref>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=369 }}</ref> Retractor muscles connect the mantle to the edge of the shell, along a line known as the [[pallial line]].<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=21|isbn=0-85238-234-0}}</ref><ref name=mantlemuscule/> These muscles pull the mantle though the valves.<ref name=mantlemuscule/>

In sedentary or recumbent bivalves that lie on one valve, such as the oysters and scallops, the anterior adductor muscle has been lost and the posterior muscle is positioned centrally.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|pages=5–6|isbn=0-85238-234-0}}</ref> In species that can swim by flapping their valves, a single, central adductor muscle occurs. These muscles are composed of two types of muscle fibres, striated muscle bundles for fast actions and smooth muscle bundles for maintaining a steady pull.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=18|isbn=0-85238-234-0}}</ref> Paired pedal protractor and retractor muscles operate the animal's foot.<ref name="Tutorial" /><ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|pages=28–30|isbn=0-85238-234-0}}</ref><ref>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=371–372 }}</ref>

=== Nervous system ===
The sedentary habits of the bivalves have meant that in general the [[nervous system]] is less complex than in most other molluscs. The animals have no [[brain]]; the nervous system consists of a [[nerve net]]work and a series of paired [[ganglia]]. In all but the most primitive bivalves, two cerebropleural ganglia are on either side of the [[oesophagus]]. The cerebral ganglia control the sensory organs, while the pleural ganglia supply nerves to the mantle cavity. The pedal ganglia, which control the foot, are at its base, and the visceral ganglia, which can be quite large in swimming bivalves, are under the posterior adductor muscle. These ganglia are both connected to the cerebropleural ganglia by [[Axon|nerve fibres]]. Bivalves with long siphons may also have siphonal ganglia to control them.<ref name="EL" /><ref name=nervous/>

==== Senses ====
The sensory organs of bivalves are largely located on the posterior mantle margins. The organs are usually [[mechanoreceptor]]s or [[chemoreceptor]]s, in some cases located on short [[tentacle]]s. The [[osphradium]] is a patch of sensory cells located below the posterior adductor muscle that may serve to taste the water or measure its [[turbidity]]. [[Statocyst]]s within the organism help the bivalve to sense and correct its orientation.<ref name=nervous>{{cite web | url = https://www.britannica.com/animal/bivalve/The-shell#ref35747 | title = Mollusk: The nervous system and organs of sensation | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-07-08 }}</ref> In the [[Order (biology)|order]] [[Anomalodesmata]], the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.<ref>{{cite journal |author1=Allen, J. A. |author2=Morgan, Rhona E. | year = 1981 | title = The functional morphology of Atlantic deep water species of the families Cuspidariidae and Poromyidae (Bivalvia): an analysis of the evolution of the septibranch condition |journal=Philosophical Transactions of the Royal Society B |volume=294 |issue=1073 |pages=413–546 |doi=10.1098/rstb.1981.0117 | bibcode = 1981RSPTB.294..413A |doi-access=free }}</ref> Many bivalves have no eyes, but a few members of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea have simple eyes on the margin of the mantle. These consist of a pit of photosensory cells and a [[lens (anatomy)|lens]].<ref name="Morton 2008" /> [[Scallop]]s have more complex eyes with a lens, a two-layered [[retina]], and a concave mirror.<ref>{{cite journal |author1=Colicchia, G. |author2=Waltner, C. |author3=Hopf, M. |author4=Wiesner, H. | year = 2009 | title = The scallop's eye—a concave mirror in the context of biology |journal=Physics Education |volume=44 |issue=2 |pages=175–179 |doi=10.1088/0031-9120/44/2/009 | bibcode = 2009PhyEd..44..175C |s2cid=122110847 }}</ref> All bivalves have [[Photoreceptor cell|light-sensitive cells]] that can detect a shadow falling over the animal.<ref name="EL" />

=== Circulation and respiration ===
[[File:Gills of Mytilus edulis 001.png|thumb|alt=Filaments from blue mussel gills|Four filaments of the gills of the blue mussel (''Mytilus edulis'') a) part of four filaments showing ciliated interfilamentar junctions (cj) b) diagram of a single filament showing the two [[lamella (anatomy)|lamellae]] connected at intervals by interlamellar junctions (ilj) and the position of the ciliated interfilamentar junctions (cp)]]

Bivalves have an open [[circulatory system]] that bathes the organs in blood ([[hemolymph]]). The [[heart]] has three chambers: two [[atrium (heart)|auricles]] receiving blood from the gills, and a single [[ventricle (heart)|ventricle]]. The ventricle is muscular and pumps hemolymph into the [[aorta]], and then to the rest of the body. Some bivalves have a single aorta, but most also have a second, usually smaller, aorta serving the hind parts of the animal.<ref>{{cite book | title = Zoology | url = https://archive.org/details/zoology0000dori | url-access = registration | last = Dorit | first = Robert L. |author2=Walker, Warren F. Jr. |author3=Barnes, Robert D. | year = 1991 |publisher=Saunders College Publishing| isbn = 978-0-03-030504-7 | page = 659 }}</ref> The hemolymph usually lacks any respiratory pigment.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=36|isbn=0-85238-234-0}}</ref> In the carnivorous genus ''[[Poromya granulata|Poromya]]'', the hemolymph has red [[amoebocyte]]s containing a haemoglobin pigment.<ref name="Archerd" />

The paired gills are located posteriorly and consist of hollow tube-like filaments with thin walls for [[gas exchange]]. The respiratory demands of bivalves are low, due to their relative inactivity. Some freshwater species, when exposed to the air, can gape the shell slightly and gas exchange can take place.<ref>{{cite web | url = https://www.britannica.com/EBchecked/topic/67293/bivalve/35750/The-respiratory-system | title = Bivalve: The respiratory system | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-07-08 }}</ref><ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=25|isbn=0-85238-234-0}}</ref> Oysters, including the [[Pacific oyster]] (''Magallana gigas''), are recognized as having varying metabolic responses to environmental stress, with changes in respiration rate being frequently observed.<ref name=":0">{{Cite journal|last1=Ducker|first1=James|last2=Falkenberg|first2=Laura J.|date=2020|title=How the Pacific Oyster Responds to Ocean Acidification: Development and Application of a Meta-Analysis Based Adverse Outcome Pathway|journal=Frontiers in Marine Science|language=en|volume=7|doi=10.3389/fmars.2020.597441|issn=2296-7745|doi-access=free}}</ref>

=== Digestive system ===

==== Modes of feeding ====
Most bivalves are [[filter feeder]]s, using their gills to capture particulate food such as [[phytoplankton]] from the water. [[Protobranchia|Protobranchs]] feed in a different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding. These primitive bivalves hold on to the bottom with a pair of tentacles at the edge of the mouth, each of which has a single [[palp]], or flap. The tentacles are covered in [[mucus]], which traps the food, and cilia, which transport the particles back to the palps. These then sort the particles, rejecting those that are unsuitable or too large to digest, and conveying others to the mouth.<ref name=Zoology75/>

In more advanced bivalves, water is drawn into the shell from the posterior [[ventral]] surface of the animal, passes upwards through the gills, and doubles back to be expelled just above the intake. There may be two elongated, retractable siphons reaching up to the seabed, one each for the inhalant and exhalant streams of water. The gills of filter-feeding bivalves are known as ctenidia and have become highly modified to increase their ability to capture food. For example, the [[cilia]] on the gills, which originally served to remove unwanted sediment, have become adapted to capture food particles, and transport them in a steady stream of mucus to the mouth. The filaments of the gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups.<ref name=Zoology75>{{cite book | title = Zoology | url = https://archive.org/details/zoology0000dori | url-access = registration | last = Dorit | first = Robert L. |author2=Walker, Warren F. Jr. |author3=Barnes, Robert D. | year = 1991 |publisher=Saunders College Publishing| isbn = 978-0-03-030504-7 | page = [https://archive.org/details/zoology0000dori/page/678 675–678] }}</ref><ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=87|isbn=0-85238-234-0}}</ref>

A few bivalves, such as the [[granular poromya]] (''Poromya granulata''), are [[Carnivore|carnivorous]], eating much larger [[Predation|prey]] than the tiny microalgae consumed by other bivalves. Muscles draw water in through the inhalant siphon which is modified into a cowl-shaped organ, sucking in prey. The siphon can be retracted quickly and inverted, bringing the prey within reach of the mouth. The gut is modified so that large food particles can be digested.<ref name="Archerd" />

The unusual genus, ''[[Entovalva nhatrangensis|Entovalva]]'', is [[Endosymbiont|endosymbiotic]], being found only in the oesophagus of [[Holothuroidea|sea cucumbers]]. It has mantle folds that completely surround its small valves. When the sea cucumber sucks in sediment, the bivalve allows the water to pass over its gills and extracts fine organic particles. To prevent itself from being swept away, it attaches itself with byssal threads to the [[host (biology)|host]]'s throat. The sea cucumber is unharmed.<ref>{{cite journal |author1=Lützen, J. |author2=Berland, B. |author3=Bristow, G. A. | year = 2011 | title = Morphology of an endosymbiotic bivalve, ''Entovalva nhatrangensis'' (Bristow, Berland, Schander & Vo, 2010) (Galeommatoidea) |journal=Molluscan Research |volume=31 |issue=2 |pages=114–124 |doi=10.11646/mr.31.2.6 | url = http://www.mapress.com/mr/content/v31/2011f/n2p124.pdf }}</ref>

==== Digestive tract ====
The digestive tract of typical bivalves consists of an [[esophagus|oesophagus]], [[stomach]], and [[intestine]]. Protobranch stomachs have a mere sac attached to them while filter-feeding bivalves have elongated rod of solidified mucus referred to as the "[[style (zoology)|crystalline style]]" projected into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into the intestine.<ref>{{cite book | title = Zoology | url = https://archive.org/details/zoology0000dori | url-access = registration | last = Dorit | first = Robert L. |author2=Walker, Warren F. Jr. |author3=Barnes, Robert D. | year = 1991 |publisher=Saunders College Publishing| isbn = 978-0-03-030504-7 | page = [https://archive.org/details/zoology0000dori/page/679 678–679] }}</ref> Waste material is consolidated in the rectum and voided as pellets into the exhalent water stream through an anal pore. Feeding and digestion are synchronized with diurnal and tidal cycles.<ref>{{cite web | url = https://www.britannica.com/EBchecked/topic/67293/bivalve/35745/The-shell#toc35748 | title = Bivalve: The digestive system and nutrition | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-05-07 }}</ref>

Carnivorous bivalves generally have reduced crystalline styles and the stomach has thick, muscular walls, extensive [[cuticle|cuticular]] linings and diminished sorting areas and gastric chamber sections.<ref>{{cite journal|author1=Tëmkin, Ilya|author2=Strong, Ellen E.|year=2013|title=New insights on stomach anatomy of carnivorous bivalves|journal=Journal of Molluscan Studies|volume=79|issue=4|pages=332–339|doi=10.1093/mollus/eyt031|doi-access=free}}</ref>

=== Excretory system ===
The excretory organs of bivalves are a pair of [[nephridium|nephridia]]. Each of these consists of a long, looped, glandular tube, which opens into the [[pericardium]], and a [[bladder]] to store urine. They also have pericardial glands either line the auricles of the heart or attach to the pericardium, and serve as extra filtration organs. Metabolic waste is voided from the bladders through a [[nephridiopore]] near the front of the upper part of the mantle cavity and excreted.<ref>{{cite web | url = https://www.britannica.com/EBchecked/topic/67293/bivalve/35749/The-excretory-system | title = Bivalve: The excretory system | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-05-07 }}</ref><ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=37|isbn=0-85238-234-0}}</ref>

=== Reproduction and development ===
The sexes are usually separate in bivalves but some [[hermaphroditism]] is known. The [[gonad]]s either open into the nephridia or through a separate pore into a chamber over the gills.<ref name="Dorit 1991 p682" /><ref>{{cite web | url = https://www.britannica.com/animal/bivalve/The-respiratory-system#ref35752 | title = Bivalve: The reproductive system | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-05-07 }}</ref> The ripe gonads of males and females release sperm and eggs into the [[water column]]. [[Spawn (biology)|Spawning]] may take place continually or be triggered by environmental factors such as day length, water temperature, or the presence of sperm in the water. Some species are "dribble spawners", releasing gametes during protracted period that can extend for weeks. Others are mass spawners and release their gametes in batches or all at once.<ref>{{cite web | url = http://www.fao.org/docrep/007/y5720e/y5720e07.htm | title = Gonadal development and spawning |author1=Helm, M. M. |author2=Bourne, N. |author3=Lovatelli, A. |year=2004 |work=Hatchery culture of bivalves: a practical manual |publisher=FAO |access-date=2012-05-08 }}</ref>

Fertilization is usually external. Typically, a short stage lasts a few hours or days before the eggs hatch into [[trochophore]] larvae. These later develop into [[veliger]] larvae which settle on the seabed and undergo [[metamorphosis]] into adults.<ref name="Dorit 1991 p682" /><ref>{{cite web | url = https://www.britannica.com/animal/bivalve/Importance#ref35736 | title = Bivalve: Reproduction and life cycles | author = Morton, Brian |publisher=Encyclopædia Britannica |access-date=2012-05-07 }}</ref> In some species, such as those in the genus ''[[Lasaea (bivalve)|Lasaea]]'', females draw water containing sperm in through their inhalant siphons and fertilization takes place inside the female. These species then brood the young inside their mantle cavity, eventually releasing them into the water column as veliger larvae or as crawl-away juveniles.<ref>{{cite journal | author = Ó Foighil, D. | year = 1989 | title = Planktotrophic larval development is associated with a restricted geographic range in ''Lasaea'', a genus of brooding, hermaphroditic bivalves |journal=Marine Biology |volume=103 |issue=3 |pages=349–358 |doi=10.1007/BF00397269 | bibcode = 1989MarBi.103..349. | s2cid = 189852952 }}</ref>

Most of the bivalve larvae that hatch from eggs in the water column feed on [[diatom]]s or other phytoplankton. In [[temperate]] regions, about 25% of species are [[Marine larval ecology|lecithotrophic]], depending on nutrients stored in the yolk of the egg where the main energy source is [[lipid]]s. The longer the period is before the larva first feeds, the larger the egg and yolk need to be. The reproductive cost of producing these energy-rich eggs is high and they are usually smaller in number. For example, the Baltic tellin (''[[Macoma balthica]]'') produces few, high-energy eggs. The larvae hatching out of these rely on the energy reserves and do not feed. After about four days, they become D-stage larvae, when they first develop hinged, D-shaped valves. These larvae have a relatively small dispersal potential before settling out. The common mussel (''[[Mytilus edulis]]'') produces 10 times as many eggs that hatch into larvae and soon need to feed to survive and grow. They can disperse more widely as they remain planktonic for a much longer time.<ref>{{cite journal |author1=Honkoop, P. J. C. |author2=Van der Meer, J. |author3=Beukema, J. J. |author4=Kwast, D. | year = 1999 | title = Reproductive investment in the intertidal bivalve ''Macoma balthica'' |journal=Journal of Sea Research |volume=41 |issue=3 |pages=203–212 | url = http://www.vliz.be/imisdocs/publications/41476.pdf | doi = 10.1016/S1385-1101(98)00053-7 | bibcode = 1999JSR....41..203H }}</ref>

Freshwater bivalves have different lifecycle. Sperm is drawn into a female's gills with the inhalant water and internal fertilization takes place. The eggs hatch into [[Glochidium|glochidia]] larvae that develop within the female's shell. Later they are released and attach themselves [[Parasitism|parasitically]] to the [[Gill filament|gills]] or fins of a fish host. After several weeks they drop off their host, undergo metamorphosis and develop into adults on the [[Substrate (biology)|substrate]].<ref name="Dorit 1991 p682" />

Some of the species in the freshwater mussel family, [[Unionidae]], commonly known as pocketbook mussels, have evolved an unusual reproductive strategy. The female's mantle protrudes from the shell and develops into an imitation small fish, complete with fish-like markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a [[conspecific]]. They approach for a closer look and the mussel releases huge numbers of larvae from its gills, dousing the inquisitive fish with its tiny, parasitic young. These glochidia larvae are drawn into the fish's gills, where they attach and trigger a tissue response that forms a small [[cyst]] around each larva. The larvae then feed by breaking down and digesting the tissue of the fish within the cysts. After a few weeks they release themselves from the cysts and fall to the stream bed as juvenile molluscs.<ref>{{cite book | title = Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals | url = https://archive.org/details/extraordinaryani0000pipe | url-access = registration |last=Piper |first=Ross | year = 2007 |publisher=Greenwood Press |pages=[https://archive.org/details/extraordinaryani0000pipe/page/224 224–225]| isbn = 978-0-313-33922-6 }}</ref>

== Comparison with brachiopods ==
[[File:AnadaraPliocene.jpg|thumb|alt=Ark clam fossil|''[[Anadara]]'', a bivalve with [[ark clam|taxodont dentition]] from the [[Pliocene]] of [[Cyprus]]]]
[[File:Liospiriferina rostrata Noir.jpg|thumb|alt=Brachiopod fossil| A fossil Jurassic brachiopod with the [[lophophore]] support intact]]

[[Brachiopod]]s are shelled marine organisms that superficially resemble bivalves in that they are of similar size and have a hinged shell in two parts. However, brachiopods evolved from a very different ancestral line, and the resemblance to bivalves only arose because they occupy similar [[ecological niche]]s. The differences between the two groups are due to their separate ancestral origins. Different initial structures have been adapted to solve the same problems, a case of [[convergent evolution]]. In modern times, brachiopods are not as common as bivalves.<ref>{{cite book | title = Zoology | url = https://archive.org/details/zoology0000dori | url-access = registration | last = Dorit | first = Robert L. |author2=Walker, Warren F. Jr. |author3=Barnes, Robert D. | year = 1991 |publisher=Saunders College Publishing| isbn = 978-0-03-030504-7 | page = [https://archive.org/details/zoology0000dori/page/774 774] }}</ref>

Both groups have a shell consisting of two valves, but the organization of the shell is quite different in the two groups. In brachiopods, the two valves are positioned on the dorsal and ventral surfaces of the body, while in bivalves, the valves are on the left and right sides of the body, and are, in most cases, mirror images of one other. Brachiopods have a [[lophophore]], a coiled, rigid cartilaginous internal apparatus adapted for filter feeding, a feature shared with two other major groups of marine invertebrates, the [[bryozoa]]ns and the [[phoronid]]s. Some brachiopod shells are made of [[calcium phosphate]] but most are [[calcium carbonate]] in the form of the biomineral [[calcite]], whereas bivalve shells are always composed entirely of calcium carbonate, often in the form of the biomineral [[aragonite]].<ref name="Synthesis140" />

== Evolutionary history ==
The [[Cambrian explosion]] took place around 540 to 520 million years ago (Mya). In this geologically brief period, most major animal [[Phylum|phyla]] diverged<!--"appeared" is incorrect; many phyla don't appear until later; see Budd & Jensen 2000--> including some of the first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time, and left their fossilized remains behind in the rocks.<ref name=Reece>{{cite book | title = Biology, Sixth Edition |last=Campbell |first=N. A. |author2=Reece, J. B. | year = 2001 |publisher=Benjamin Cummings| isbn = 978-0-201-75054-6 |page=643 }}</ref>

Possible early bivalves include ''[[Pojetaia]]'' and ''[[Fordilla]]''; these probably lie in the stem rather than crown group.  ''Watsonella'' and ''Anabarella'' are perceived to be (earlier) close relatives of these taxa.<ref>{{cite journal | last1 = Vendrasco | first1 = M. J. | last2 = Checa | first2 = A. G. | last3 = Kouchinsky | first3 = A. V. | year = 2011 | title = Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca | journal = Palaeontology | volume = 54 | issue = 4| pages = 825–850 | doi = 10.1111/j.1475-4983.2011.01056.x | bibcode = 2011Palgy..54..825V | doi-access = free }}</ref> Only five genera of supposed Cambrian "bivalves" exist, the others being ''[[Tuarangia]]'', ''[[Camya]]'' and ''[[Arhouriella]]'' and potentially ''[[Buluniella]]''.<ref name="Elicki2009">{{Cite journal | last1 = Elicki | first1 = O. | last2 = Gürsu | first2 = S. | doi = 10.1007/s12542-009-0021-9 | title = First record of ''Pojetaia runnegari'' Jell, 1980 and ''Fordilla'' Barrande, 1881 from the Middle East (Taurus Mountains, Turkey) and critical review of Cambrian bivalves | journal = Paläontologische Zeitschrift | volume = 83 | issue = 2 | pages = 267–291 | year = 2009 | bibcode = 2009PalZ...83..267E | s2cid = 49380913 | url = http://www.geo.tu-freiberg.de/~elicki/ELICKI_GUERSU_2009_PalZ_CambrianBivalves.pdf}}</ref>

Bivalve fossils can be formed when the sediment in which the shells are buried hardens into rock. Often, the impression made by the valves remains as the fossil rather than the valves. During the [[Early Ordovician]], a great increase in the diversity of bivalve species occurred, and the dysodont, heterodont, and taxodont dentitions evolved. By the [[Llandovery epoch|Early Silurian]], the gills were becoming adapted for filter feeding, and during the [[Devonian]] and [[Carboniferous]] periods, siphons first appeared, which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment.<ref name="FossilRecord" />
[[File:Bivalve, Pliocene, Java, Indonesia.png|thumb|250px|''[[cf.]] [[Paratapes textilis]]'' from the [[Pliocene]] of [[Java]], [[Indonesia]]]]

By the middle of the [[Paleozoic]], around 400 Mya, the brachiopods were among the most abundant filter feeders in the ocean, and over 12,000 fossil species are recognized.<ref>{{cite web |url=http://www.kgs.ku.edu/Extension/fossils/brachiopod.html |title=Fossil Brachiopods |author=Brosius, L. |year=2008 |work=GeoKansas |publisher=Kansas Geological Survey |access-date=2012-07-02 |archive-url=https://web.archive.org/web/20080705173419/http://www.kgs.ku.edu/Extension/fossils/brachiopod.html |archive-date=2008-07-05 |url-status=dead }}</ref> By the [[Permian–Triassic extinction event]] 250 Mya, bivalves were undergoing a huge [[Evolutionary radiation|radiation]] of diversity. The bivalves were hard hit by this event, but re-established themselves and thrived during the [[Triassic]] period that followed. In contrast, the brachiopods lost 95% of their [[species diversity]].<ref name="Synthesis140" /> The ability of some bivalves to burrow and thus avoid predators may have been a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative [[buoyancy]] in soft sediments by developing spines on the shell, gaining the ability to swim, and in a few cases, adopting predatory habits.<ref name="FossilRecord" />

For a long time, bivalves were thought to be better adapted to aquatic life than brachiopods were, [[Competition (biology)|outcompeting]] and relegating them to minor [[ecological niche|niches]] in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. All this has been broadly disproven, though; rather, the prominence of modern bivalves over brachiopods seems due to chance disparities<!--whatever this means--> in their response to [[extinction event]]s.<ref>{{cite journal|journal=Paleobiology|pages=383–396 | title = Clams and brachiopods–ships that pass in the night | doi-access=free|last=Gould|first=Stephen|author-link=Stephen Jay Gould|author2=Calloway, C. Bradford |volume=6 |issue=4 |year=1980|doi=10.1017/S0094837300003572 |jstor=2400538 |bibcode=1980Pbio....6..383G }}</ref>

== Diversity of extant bivalves ==

The adult maximum size of [[Extant taxon|living]] species of bivalve ranges from {{convert|0.52|mm|in|2|abbr=on}} in ''[[Condylonucula maya]]'',<ref>{{cite web | url = http://www.museumoftheearth.org/outreach.php?page=s_hsh_top/s_hsh_s3_biodiv/s_hsh_s3_extreme | title = ''Condylonucula maya'' | work = Extreme bivalves | access-date = 2012-04-19 | url-status = dead | archive-url = https://web.archive.org/web/20131015053855/http://www.museumoftheearth.org/outreach.php?page=s_hsh_top%2Fs_hsh_s3_biodiv%2Fs_hsh_s3_extreme | archive-date = 2013-10-15 }}</ref> a nut clam, to a length of {{convert|1,532|mm}} in ''[[Kuphus|Kuphus polythalamia]]'', an elongated, burrowing shipworm.<ref>{{cite web | url = http://www.conchologistsofamerica.org/articles/reviews/9709.asp | title = Book review: Conchologists of America | access-date = 2012-04-19 | url-status = dead | archive-url = https://web.archive.org/web/20120807040513/http://www.conchologistsofamerica.org/articles/reviews/9709.asp | archive-date = 2012-08-07 }}</ref> However, the species generally regarded as the largest living bivalve is the giant clam ''[[Giant clam|Tridacna gigas]]'', which can grow to a length of {{convert|1200|mm|in|abbr=on}} and a weight of more than 200&nbsp;kg (441&nbsp;lb).<ref>{{cite web | url = http://animals.nationalgeographic.com/animals/invertebrates/giant-clam.html | archive-url = https://web.archive.org/web/20070621060007/http://animals.nationalgeographic.com/animals/invertebrates/giant-clam.html | url-status = dead | archive-date = 21 June 2007 | title = Giant Clam: ''Tridacna gigas'' | author = Grall, George |publisher=National Geographic Society |access-date=2012-06-24 }}</ref> The largest known [[Extinction|extinct]] bivalve is a species of ''[[Platyceramus]]'' whose fossils measure up to {{convert|3000|mm|0|abbr=on}} in length.<ref>{{cite journal |author1=Kauffman, E. G. |author2=Harries, P. J. |author3=Meyer, C. |author4=Villamil, T. |author5=Arango, C. |author6=Jaecks, G. | year = 2007 | title = Paleoecology of giant Inoceramidae (Platyceramus) on a Santonian (Cretaceous) seafloor in Colorado |journal=Journal of Paleontology |volume=81 |issue=1 |pages=64–81 |doi=10.1666/0022-3360(2007)81[64:POGIPO]2.0.CO;2 |s2cid=130048023 }}</ref>

In his 2010 treatise, ''Compendium of Bivalves'', Markus Huber gives the total number of living bivalve species as about 9,200 combined in 106 families.<ref>{{cite book | author = Huber, Markus | year = 2010 | title = Compendium of Bivalves. A Full-color Guide to 3,300 of the World's Marine Bivalves. A Status on Bivalvia after 250 Years of Research |publisher= ConchBooks | page=23| isbn = 978-3-939767-28-2 }}</ref>  Huber states that the number of 20,000 living species, often encountered in literature, could not be verified and presents the following table to illustrate the known diversity:

{| class="wikitable plainrowheaders sortable"
|-
! scope="col" | Subclass
! scope="col" | Superfamilies
! scope="col" | Families
! scope="col" | Genera
! scope="col" | Species
|-
! scope="row" | [[Heterodonta]]
|
| '''64''' (incl. 1 freshwater)
| {{hs|&0800}}'''800''' (16 freshwater)
| {{hs|&5600}}'''5600''' (270 freshwater)
|-
! scope="row" |
| [[Arcticoidea]]
| 2
| {{hs|&0006}}6
| {{hs|&0013}}13
|-
! scope="row" |
| [[Cardioidea]]
| 2
| {{hs|&0038}}38
| {{hs|&0260}}260
|-
! scope="row" |
| [[Chamoidea]]
| 1
| {{hs|&0006}}6
| {{hs|&0070}}70
|-
! scope="row" |
| [[Clavagelloidea]]
| 1
| {{hs|&0002}}2
| {{hs|&0020}}20
|-
! scope="row" |
| [[Crassatelloidea]]
| 5
| {{hs|&0065}}65
| {{hs|&0420}}420
|-
! scope="row" |
| [[Cuspidarioidea]]
| 2
| {{hs|&0020}}20
| {{hs|&0320}}320
|-
! scope="row" |
| [[Cyamioidea]]
| 3
| {{hs|&0022}}22
| {{hs|&0140}}140
|-
! scope="row" |
| [[Cyrenoidea]]
| 1
| {{hs|&0006}}6 (3 freshwater)
| {{hs|&0060}}60 (30 freshwater)
|-
! scope="row" |
| [[Cyrenoidoidea]]
| 1
| {{hs|&0001}}1
| {{hs|&0006}}6
|-
! scope="row" |
| [[Dreissenoidea]]
| 1
| {{hs|&0003}}3 (2 freshwater)
| {{hs|&0020}}20 (12 freshwater)
|-
! scope="row" |
| [[Galeommatoidea]]
| {{sort|4|ca. 4}}
| {{hs|&0100}}about 100
| {{hs|&0500}}about 500
|-
! scope="row" |
| [[Gastrochaenoidea]]
| 1
| {{hs|&0007}}7
| {{hs|&0030}}30
|-
! scope="row" |
| [[Glossoidea]]
| 2
| {{hs|&0020}}20
| {{hs|&0110}}110
|-
! scope="row" |
| [[Hemidonacoidea]]
| 1
| {{hs|&0001}}1
| {{hs|&0006}}6
|-
! scope="row" |
| [[Hiatelloidea]]
| 1
| {{hs|&0005}}5
| {{hs|&0025}}25
|-
! scope="row" |
| [[Limoidea]]
| 1
| {{hs|&0008}}8
| {{hs|&0250}}250
|-
! scope="row" |
| [[Lucinoidea]]
| 2
| {{hs|&0085}}about 85
| {{hs|&0500}}about 500
|-
! scope="row" |
| [[Mactroidea]]
| 4
| {{hs|&0046}}46
| {{hs|&0220}}220
|-
! scope="row" |
| [[Myoidea]]
| 3
| {{hs|&0015}}15 (1 freshwater)
| {{hs|&0130}}130 (1 freshwater)
|-
! scope="row" |
| [[Pandoroidea]]
| 7
| {{hs|&0030}}30
| {{hs|&0250}}250
|-
! scope="row" |
| [[Pholadoidea]]
| 2
| {{hs|&0034}}34 (1 freshwater)
| {{hs|&0200}}200 (3 freshwater)
|-
! scope="row" |
| [[Pholadomyoidea]]
| 2
| {{hs|&0003}}3
| {{hs|&0020}}20
|-
! scope="row" |
| [[Solenoidea]]
| 2
| {{hs|&0017}}17 (2 freshwater)
| {{hs|&0130}}130 (4 freshwater)
|-
! scope="row" |
| [[Sphaerioidea]]
| {{sort|1|(1 freshwater)}}
| {{hs|&0005}}(5 freshwater)
| {{hs|&0200}}(200 freshwater)
|-
! scope="row" |
| [[Tellinoidea]]
| 5
| {{hs|&0110}}110 (2 freshwater)
| {{hs|&0900}}900 (15 freshwater)
|-
! scope="row" |
| [[Thyasiroidea]]
| 1
| {{hs|&0012}}about 12
| {{hs|&0100}}about 100
|-
! scope="row" |
| [[Ungulinoidea]]
| 1
| {{hs|&0016}}16
| {{hs|&0100}}100
|-
! scope="row" |
| [[Veneroidea]]
| 4
| {{hs|&0104}}104
| {{hs|&0750}}750
|-
! scope="row" |
| [[Verticordioidea]]
| 2
| {{hs|&0016}}16
| {{hs|&0160}}160
|-
! scope="row" | [[Palaeoheterodonta]]
|
| '''7''' (incl. 6 freshwater)
| {{hs|&0171}}'''171''' (170 freshwater)
| {{hs|&0908}}'''908''' (900 freshwater)
|-
! scope="row" |
| [[Trigonioidea]]
| 1
| {{hs|&0001}}1
| {{hs|&0008}}8
|-
! scope="row" |
| [[Unionoidea]]
| {{sort|6|(6 freshwater)}}
| {{hs|&0170}}(170 freshwater)
| {{hs|&0900}}(900 freshwater)
|-
! scope="row" | [[Protobranchia]]
|
| '''10'''
| {{hs|&0049}}'''49'''
| {{hs|&0700}}'''700'''
|-
! scope="row" |
| [[Manzanelloidea]]
| 1
| {{hs|&0002}}2
| {{hs|&0020}}20
|-
! scope="row" |
| [[Nuculanoidea]]
| 6
| {{hs|&0032}}32
| {{hs|&0460}}460
|-
! scope="row" |
| [[Nuculoidea]]
| 1
| {{hs|&0008}}8
| {{hs|&0170}}170
|-
! scope="row" |
| [[Sareptoidea]]
| 1
| {{hs|&0005}}about 5
| {{hs|&0010}}10
|-
! scope="row" |
| [[Solemyoidea]]
| 1
| {{hs|&0002}}2
| {{hs|&0030}}30
|-
! scope="row" | [[Pteriomorphia]]
|
| '''25'''
| {{hs|&0240}}'''240''' (2 freshwater)
| {{hs|&2000}}'''2000''' (11 freshwater)
|-
! scope="row" |
| [[Anomioidea]]
| 2
| {{hs|&0009}}9
| {{hs|&0030}}30
|-
! scope="row" |
| [[Arcoidea]]
| 7
| {{hs|&0060}}60 (1 freshwater)
| {{hs|&0570}}570 (6 freshwater)
|-
! scope="row" |
| [[Dimyoidea]]
| 1
| {{hs|&0003}}3
| {{hs|&0015}}15
|-
! scope="row" |
| [[Limoidea]]
| 1
| {{hs|&0008}}8
| {{hs|&0250}}250
|-
! scope="row" |
| [[Mytiloidea]]
| 1
| {{hs|&0050}}50 (1 freshwater)
| {{hs|&0400}}400 (5 freshwater)
|-
! scope="row" |
| [[Ostreoidea]]
| 2
| {{hs|&0023}}23
| {{hs|&0080}}80
|-
! scope="row" |
| [[Pectinoidea]]
| 4
| {{hs|&0068}}68
| {{hs|&0500}}500
|-
! scope="row" |
| [[Pinnoidea]]
| 1
| {{hs|&0003}}3 (+)
| {{hs|&0050}}50
|-
! scope="row" |
| [[Plicatuloidea]]
| 1
| {{hs|&0001}}1
| {{hs|&0020}}20
|-
! scope="row" |
| [[Pterioidea]]
| 5
| {{hs|&0009}}9
| {{hs|&0080}}80
|}

== Distribution ==
[[File:Zebra mussel GLERL 4.jpg|thumb|alt=Zebra mussels on manmade structure|Zebra mussels encrusting a water velocity meter in Lake Michigan]]
The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world. Most are [[infauna]]l and live buried in sediment on the seabed, or in the sediment in freshwater habitats. A large number of bivalve species are found in the [[Intertidal zone|intertidal]] and [[sublittoral]] zones of the oceans. A sandy sea beach may superficially appear to be devoid of life, but often a very large number of bivalves and other invertebrates are living beneath the surface of the sand. On a large beach in [[South Wales]], careful sampling produced an estimate of 1.44 million cockles (''[[Cerastoderma edule]]'') per acre of beach.<ref name="Yonge" />

Bivalves inhabit the tropics, as well as temperate and boreal waters. A number of species can survive and even flourish in extreme conditions. They are abundant in the Arctic, about 140 species being known from that zone.<ref>[http://www.arcodiv.org/seabottom/Bivalves.html Bivalves] Arctic Ocean Diversity. Retrieved 2012-04-21.</ref> The Antarctic scallop, ''[[Adamussium colbecki]]'', lives under the sea ice at the other end of the globe, where the subzero temperatures mean that growth rates are very slow.<ref>{{cite web | url = http://afg.biodiversity.aq/species/23-adamussium-colbecki | title = ''Adamussium colbecki'' (Smith, 1902) | work = Antarctic Field Guide | access-date = 2012-04-21 | archive-url = https://web.archive.org/web/20131014115125/http://afg.biodiversity.aq/species/23-adamussium-colbecki | archive-date = 2013-10-14 | url-status = dead }}</ref> The giant mussel, ''[[Bathymodiolus thermophilus]]'', and the giant white clam, ''[[Calyptogena magnifica]]'', both live clustered around [[hydrothermal vent]]s at [[Abyssal zone|abyssal]] depths in the Pacific Ocean. They have chemosymbiotic bacteria in their gills that oxidise [[hydrogen sulphide]], and the molluscs absorb nutrients synthesized by these bacteria.<ref>{{cite web | url =http://www.fathom.com/course/10701050/session6.html| archive-url =https://web.archive.org/web/20081210175652/http://www.fathom.com/course/10701050/session6.html| archive-date =2008-12-10| title = Hydrothermal vents | author = Rice, Tony |work=Deep Ocean |publisher=Fathom |access-date=2012-04-21 }}</ref> Some species are found in the [[hadal zone]], like Vesicomya sergeevi, which occurs at depths of 7600–9530 meters.<ref>{{Cite journal |last1=Krylova |first1=Elena M. |last2=Kamenev |first2=Gennady M. |last3=Vladychenskaya |first3=Irina P. |last4=Petrov |first4=Nikolai B. |date=January 2015 |title=Vesicomyinae (Bivalvia: Vesicomyidae) of the Kuril–Kamchatka Trench and adjacent abyssal regions |url=https://linkinghub.elsevier.com/retrieve/pii/S0967064514002653 |journal=Deep Sea Research Part II: Topical Studies in Oceanography |language=en |volume=111 |pages=198–209 |doi=10.1016/j.dsr2.2014.10.004|bibcode=2015DSRII.111..198K }}</ref> The saddle oyster, ''[[Enigmonia aenigmatica]]'', is a marine species that could be considered [[Amphibious fish|amphibious]]. It lives above the high tide mark in the tropical Indo-Pacific on the underside of [[mangrove]] leaves, on mangrove branches, and on sea walls in the [[splash zone]].<ref>{{cite web | url = http://www.museumoftheearth.org/outreach.php?page=s_hsh_top/s_hsh_s3_biodiv/s_hsh_s3_extreme | title = Extreme bivalves | publisher = Museum of the Earth | access-date = 2012-04-21 | url-status = dead | archive-url = https://web.archive.org/web/20131015053855/http://www.museumoftheearth.org/outreach.php?page=s_hsh_top%2Fs_hsh_s3_biodiv%2Fs_hsh_s3_extreme | archive-date = 2013-10-15 }}</ref>

Some freshwater bivalves have very restricted ranges. For example, the Ouachita creekshell mussel, ''[[Villosa arkansasensis]]'', is known only from the streams of the [[Ouachita Mountains]] in Arkansas and Oklahoma, and like several other freshwater mussel species from the southeastern US, it is in danger of extinction.<ref>{{cite web | url = http://www.wildlifearkansas.com/materials/ProjectReports/T9%20Status%20survey%20of%20Ouachita%20Creekshell%20final.pdf | title = Life History and Population Biology of the State Special Concern Ouachita Creekshell, ''Villosa arkansasensis'' (I. Lea 1862) | author = Christian, A. D. | year = 2007 |publisher=Arkansas State University |access-date=2012-04-21 }}</ref> In contrast, a few species of freshwater bivalves, including the golden mussel (''[[Limnoperna fortunei]]''), are dramatically increasing their ranges. The golden mussel has spread from Southeast Asia to Argentina, where it has become an [[invasive species]].<ref>{{cite journal |author1=Karatayev, A. Y. |author2=Burlakova, L. E. |author3=Karatayev, V. A. |author4=Boltovskoy, D. | year = 2010 | title = ''Limnoperna fortunei'' versus ''Dreissena polymorpha'': population densities and benthic community impacts of two invasive freshwater bivalves |journal=Journal of Shellfish Research |volume=29 |issue=4 |pages=975–984 |doi=10.2983/035.029.0432 |s2cid=20387549 }}</ref> Another well-travelled freshwater bivalve, the zebra mussel (''[[Dreissena polymorpha]]'') originated in southeastern Russia, and has been accidentally introduced to inland waterways in North America and Europe, where the species damages water installations and disrupts local [[ecosystem]]s.<ref>{{cite web |first=M. S. |last=Hoddle |url=http://cisr.ucr.edu/quagga_zebra_mussels.html |title=Quagga and Zebra Mussels |publisher=Center for Invasive Species Research, [[UC Riverside]] |date=13 Oct 2011 |access-date=2012-04-21 |archive-url=https://web.archive.org/web/20100623200816/http://cisr.ucr.edu/quagga_zebra_mussels.html |archive-date=23 June 2010 |url-status=dead }}</ref>

== Behaviour ==
[[File:Muscheln mit Sipho Nahaufnahme.jpg|thumb|alt=Venerids showing siphons|A large number of live venerid bivalves underwater with their [[Siphon (mollusc)|siphons]] visible]]
[[File:Electric oyster MolluSCAN eye project.jpg|thumb|right| [[Pacific oyster]] equipped with activity electrodes to follow its daily behaviour]]

Most bivalves adopt a sedentary or even [[Sessility (zoology)|sessile]] lifestyle, often spending their whole lives in the area in which they first settled as juveniles. The majority of bivalves are infaunal, living under the seabed, buried in soft substrates such as sand, silt, mud, gravel, or coral fragments. Many of these live in the [[intertidal zone]] where the sediment remains damp even when the tide is out. When buried in the sediment, burrowing bivalves are protected from the pounding of waves, desiccation, and overheating during low tide, and variations in salinity caused by rainwater. They are also out of the reach of many predators.<ref name="BCO" /> Their general strategy is to extend their siphons to the surface for feeding and respiration during high tide, but to descend to greater depths or keep their shell tightly shut when the tide goes out.<ref name="BCO" /> They use their muscular foot to dig into the substrate. To do this, the animal relaxes its adductor muscles and opens its shell wide to anchor itself in position while it extends its foot downwards into the substrate. Then it dilates the tip of its foot, retracts the adductor muscles to close the shell, shortens its foot and draws itself downwards. This series of actions is repeated to dig deeper.<ref>{{cite book | title = The Invertebrates: A New Synthesis |last=Barnes |first=R. S. K. |author2=Callow, P. |author3=Olive, P. J. W. | year = 1988 |publisher=Blackwell Scientific Publications| isbn = 978-0-632-03125-2 | page = 265 }}</ref>

Other bivalves, such as [[mussel]]s, attach themselves to hard surfaces using tough [[byssus]] threads made of [[collagen]] and [[elastin]] proteins.<ref>{{cite book|author=Gosling, Elizabeth|year=2004|title=Bivalve Molluscs: Biology, Ecology and Culture|publisher=Fishing News Books|page=27|isbn=0-85238-234-0}}</ref> Some species, including the true oysters, the [[Chamidae|jewel boxes]], the [[Anomiidae|jingle shells]], the [[Spondylus|thorny oysters]] and the [[Plicatulidae|kitten's paws]], cement themselves to stones, rock or larger dead shells.<ref name="Harper" /> In oysters, the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate. Oysters sometimes occur in dense beds in the [[neritic zone]] and, like most bivalves, are filter feeders.<ref name="Barrett" />

Bivalves filter large amounts of water to feed and breathe but they are not permanently open. They regularly shut their valves to enter a resting state, even when they are permanently submerged. In oysters, for example, their behaviour follows very strict circatidal and circadian rhythms according to the relative positions of the moon and sun. During neap tides, they exhibit much longer closing periods than during spring tides.<ref>{{cite journal |author1=Tran, D. |author2=Nadau, A. |author3=Durrieu, G. |author4=Ciret, P. |author5=Parisot, J-P. |author6=Massabuau, J-C. |year=2011 |title=Field chronobiology of a molluscan bivalve: how the moon and sun cycles interact to drive oyster activity rhythms |journal=Chronobiology International |volume=28 |issue=4 |pages=307–317 |doi=10.3109/07420528.2011.565897 |pmid=21539422 |s2cid=25356955 }}</ref>

Although many non-sessile bivalves use their muscular foot to move around, or to dig, members of the freshwater family [[Sphaeriidae]] are exceptional in that these small clams climb about quite nimbly on weeds using their long and flexible foot. The European fingernail clam (''[[Sphaerium corneum]]''), for example, climbs around on [[water weed]]s at the edges of lakes and ponds; this enables the clam to find the best position for filter feeding.<ref>{{cite journal | author = Bishop, M. J. |author2=Garis, H. | year = 1976 | title = A note on population densities of mollusca in the River Great Ouse at Ely, Cambridgeshire |journal=Hydrobiologia |volume=48 |issue=3 |pages=195–197 |doi=10.1007/BF00028690 |s2cid=13546885 }}</ref>

=== Predators and defence ===
The thick shell and rounded shape of bivalves make them awkward for potential predators to tackle. Nevertheless, a number of different creatures include them in their diet. Many species of [[demersal fish]] feed on them including the [[common carp]] (''Cyprinus carpio''), which is being used in the upper Mississippi River to try to control the invasive zebra mussel (''Dreissena polymorpha'').<ref>{{cite journal |author1=Thorp, J. H. |author2=Delong, M. D. |author3=Casper1, A. F. | year = 1998 | title = ''In situ'' experiments on predatory regulation of a bivalve mollusc (''Dreissena polymorpha'') in the Mississippi and Ohio Rivers |journal=Freshwater Biology |volume=39 |issue=4 |pages=649–661 |doi=10.1046/j.1365-2427.1998.00313.x |bibcode=1998FrBio..39..649T }}</ref> Birds such as the [[Eurasian oystercatcher]] (''Haematopus ostralegus'') have specially adapted beaks which can pry open their shells.<ref>{{cite journal | author = Hulscher, J. B. | year = 1982 | title = The oystercatcher ''Haematopus ostralegus'' as a predator of the bivalve ''Macoma balthica'' in the Dutch Wadden Sea |journal=Ardea |volume=70 |pages=89–152 | doi = 10.5253/arde.v70.p89| s2cid = 87037074 | url = https://pure.rug.nl/ws/files/133167806/arde.v70.p89.pdf }}</ref> The [[European herring gull|herring gull]] (''Larus argentatus'') sometimes drops heavy shells onto rocks in order to crack them open.<ref>{{cite journal |last=Ingolfsson |first=Agnar |author2=Bruce T. Estrella | title = The development of shell-cracking behavior in herring gulls |journal=The Auk | year = 1978 |volume=95 |issue=3 |pages=577–579 |doi=10.1093/auk/95.3.577 | url = http://sora.unm.edu/sites/default/files/journals/auk/v095n03/p0577-p0579.pdf }}</ref> [[Sea otter]]s feed on a variety of bivalve species and have been observed to use stones balanced on their chests as anvils on which to crack open the shells.<ref>{{cite journal |author1=Hall, K. R. L. |author2=Schaller, G. B. | year = 1964 | title = Tool-using behavior of the California sea otter |journal=Journal of Mammalogy |volume=45 |issue=2 |pages=287–298 |jstor=1376994 | doi = 10.2307/1376994 }}</ref> The [[Pacific walrus]] (''Odobenus rosmarus divergens'') is one of the main predators feeding on bivalves in Arctic waters.<ref>{{cite journal |author1=Fukuyamaa, A. K. |author2=Olivera, J. S. | year = 1985 | title = Sea star and walrus predation on bivalves in Norton Sound, Bering Sea, Alaska |journal=Ophelia |volume=24 |issue=1 |pages=17–36 |doi=10.1080/00785236.1985.10426616 }}</ref> Shellfish have formed part of the human diet since prehistoric times, a fact evidenced by the remains of mollusc shells found in ancient middens. Examinations of these deposits in Peru has provided a means of dating long past El Niño events because of the disruption these caused to bivalve shell growth.<ref name="Rollins" /> Further changes in shell development due to environmental stress has also been suggested to cause increased mortality in oysters due to reduced shell strength. <ref name=":0" />

Invertebrate predators include crustaceans, starfish and octopuses. Crustaceans crack the shells with their [[Chela (organ)|pincers]] and starfish use their [[water vascular system]] to force the valves apart and then insert part of their stomach between the valves to digest the bivalve's body. It has been found experimentally that both crabs and starfish preferred molluscs that are attached by byssus threads to ones that are cemented to the substrate. This was probably because they could manipulate the shells and open them more easily when they could tackle them from different angles.<ref name="Harper" /> Octopuses either pull bivalves apart by force, or they bore a hole into the shell and insert a digestive fluid before sucking out the liquified contents.<ref>{{cite journal | author = Wodinsky, Jerome | year = 1969 | title = Penetration of the shell and feeding on gastropods by octopus |journal=American Zoologist |volume=9 |issue=3 |pages=997–1010 |doi=10.1093/icb/9.3.997 |doi-access=free }}</ref> Certain carnivorous gastropod snails such as whelks ([[Buccinidae]]) and murex snails ([[Muricidae]]) feed on bivalves by boring into their shells. A dog whelk (''[[Nucella]]'') drills a hole with its [[radula]] assisted by a shell-dissolving secretion. The dog whelk then inserts its extendible proboscis and sucks out the body contents of the victim, which is typically a [[blue mussel]].<ref>{{cite web | url = http://www.asnailsodyssey.com/LEARNABOUT/WHELK/whelFeed.php | title = Learn about whelks and relatives: foods, feeding and growth | author = Carefoot, Tom | year = 2010 | work = A snail's odyssey | access-date = 2012-04-19 | archive-url = https://web.archive.org/web/20120705134610/http://www.asnailsodyssey.com/LEARNABOUT/WHELK/whelFeed.php | archive-date = 2012-07-05 | url-status = dead }}</ref>

[[Razor shell]]s can dig themselves into the sand with great speed to escape predation. When a [[Pacific razor clam]] (''Siliqua patula'') is laid on the surface of the beach, it can bury itself completely in seven seconds<ref>{{cite web | url = http://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=34320&inline=true | title = Pacific razor clam | year = 2001 |publisher=California Department of Fish and Game |access-date=2012-05-09 }}</ref> and the [[Atlantic jackknife clam]], ''Ensis directus'', can do the same within fifteen seconds.<ref>{{cite web | url = http://www.frammandearter.se/0/2english/pdf/Ensis_directus.pdf | title = American jack knife clam, (''Ensis directus'') | author = Naylor, Martin | year = 2005 | work = Alien species in Swedish seas and coastal areas | access-date = 2012-04-18 | archive-url = https://web.archive.org/web/20120904171620/http://www.frammandearter.se/0/2english/pdf/Ensis_directus.pdf | archive-date = 2012-09-04 | url-status = dead }}</ref> Scallops and [[Lima (bivalve)|file clams]] can swim by opening and closing their valves rapidly; water is ejected on either side of the hinge area and they move with the flapping valves in front.<ref name="Odyssey" /> Scallops have simple eyes around the margin of the mantle and can clap their valves shut to move sharply, hinge first, to escape from danger.<ref name="Odyssey" /> Cockles can use their foot to move across the seabed or leap away from threats. The foot is first extended before being contracted suddenly when it acts like a spring, projecting the animal forwards.<ref>{{cite web | url = http://www.manandmollusc.net/advanced_introduction/advanced_intro_printable.html | title = Bivalvia: The foot and locomotion | author = Bourquin, Avril | year = 2000 | work = The Phylum Mollusca | access-date = 2012-04-19 | url-status = dead | archive-url = https://web.archive.org/web/20010424054002/http://www.manandmollusc.net/advanced_introduction/advanced_intro_printable.html | archive-date = 2001-04-24 }}</ref>

In many bivalves that have [[siphon (mollusc)|siphon]]s, they can be retracted back into the safety of the shell. If the siphons inadvertently get attacked by a predator, in some cases, they snap off. The animal can [[Regeneration (biology)|regenerate]] them later, a process that starts when the cells close to the damaged site become activated and remodel the tissue back to its pre-existing form and size.<ref>{{cite journal | author = Hodgson, A. N. | year = 1981 | title = Studies on wound healing, and an estimation of the rate of regeneration, of the siphon of ''Scrobicularia plana'' (da Costa) |journal=Journal of Experimental Marine Biology and Ecology |volume=62 |issue=2 |pages=117–128 |doi=10.1016/0022-0981(82)90086-7 }}</ref> In some other cases, it does not snap off. If the siphon is exposed, it is the key for a predatory fish to obtain the entire body. This tactic has been observed against bivalves with an [[infaunal]] lifestyle.<ref name="Vaughn-Hoellein-2018">{{cite journal | last1=Vaughn | first1=Caryn C. | last2=Hoellein | first2=Timothy J. | title=Bivalve Impacts in Freshwater and Marine Ecosystems | journal=[[Annual Review of Ecology, Evolution, and Systematics]] | publisher=[[Annual Reviews (publisher)|Annual Reviews]] | volume=49 | issue=1 | date=2018-11-02 | issn=1543-592X | doi=10.1146/annurev-ecolsys-110617-062703 | pages=183–208| s2cid=91784258 | doi-access=free }}</ref><ref name="Dame-2011">{{cite book | last=Dame | first=Richard | title=Ecology of Marine Bivalves: An Ecosystem Approach (Marine Science | publisher=[[CRC Press]] | publication-place=[[Boca Raton]], FL | year=2011 | isbn=978-0-429-10653-8 | oclc=1229590450 | pages=1–283 | doi=10.1201/b11220 | s2cid=131914929}}</ref>

File shells such as ''[[Limaria fragilis]]'' can produce a noxious secretion when stressed. It has numerous tentacles which fringe its mantle and protrude some distance from the shell when it is feeding. If attacked, it sheds tentacles in a process known as [[autotomy]]. The toxin released by this is distasteful and the detached tentacles continue to writhe which may also serve to distract potential predators.<ref>{{cite journal |author1=Fleming, P. A. |author2=Muller, D. |author3=Bateman, P. W. | year = 2007 | title = Leave it all behind: a taxonomic perspective of autotomy in invertebrates |journal=Biological Reviews |volume=82 |issue=3 |pages=481–510 |doi=10.1111/j.1469-185X.2007.00020.x | pmid = 17624964 |s2cid=20401676 }}</ref>

== Mariculture ==
{{main|Oyster farming}}
{{main|Scallop aquaculture}}

[[File:Oyster culture in Belon, France 03.jpg|thumb|alt=Oyster culture in France|Oyster culture in Brittany, France]]
[[Oysters]], [[mussels]], clams, scallops and other bivalve species are grown with food materials that occur naturally in their culture environment in the sea and lagoons.<ref name="fao">[http://www.fao.org/docrep/016/i2727e/i2727e00.htm FAO State of Fisheries and Aquaculture 2012 ]</ref> One-third of the world's farmed food fish harvested in 2010 was achieved without the use of feed, through the production of bivalves and filter-feeding [[carp]]s.<ref name="fao" /> [[Ostrea edulis|European flat oysters]] (''Ostrea edulis'') were first farmed by the Romans in shallow ponds and similar techniques are still in use.<ref name="EFO" />  Seed oysters are either raised in a hatchery or harvested from the wild. Hatchery production provides some control of the broodstock but remains problematic because disease-resistant strains of this oyster have not yet been developed. Wild [[Spat (molluscs)|spats]] are harvested either by broadcasting empty mussel shells on the seabed or by the use of long, small-mesh nets filled with mussel shells supported on steel frames. The oyster larvae preferentially settle out on the mussel shells. Juvenile oysters are then grown on in nursery trays and are transferred to open waters when they reach {{convert|5|to|6|mm}} in length.<ref name="EFO" />

Many juveniles are further reared off the seabed in suspended rafts, on floating trays or cemented to ropes. Here they are largely free from bottom-dwelling predators such as starfish and crabs but more labour is required to tend them. They can be harvested by hand when they reach a suitable size. Other juveniles are laid directly on the seabed at the rate of {{convert|50|to|100|kg}} per hectare. They grow on for about two years before being harvested by [[dredging]]. Survival rates are low at about 5%.<ref name="EFO" />

The [[Pacific oyster]] (''Crassostrea gigas'') is cultivated by similar methods but in larger volumes and in many more regions of the world. This oyster originated in Japan where it has been cultivated for many centuries.<ref name="PCO" /> It is an estuarine species and prefers [[Salinity|salinities]] of 20 to 25 [[parts per thousand]]. Breeding programmes have produced improved stock that is available from hatcheries. A single female oyster can produce 50–80 million eggs in a batch so the selection of broodstock is of great importance. The larvae are grown on in tanks of static or moving water. They are fed high quality [[microalgae]] and diatoms and grow fast. At metamorphosis the juveniles may be allowed to settle on PVC sheets or pipes, or crushed shell. In some cases, they continue their development in "upwelling culture" in large tanks of moving water rather than being allowed to settle on the bottom. They then may be transferred to transitional, nursery beds before being moved to their final rearing quarters. Culture there takes place on the bottom, in plastic trays, in mesh bags, on rafts or on long lines, either in shallow water or in the intertidal zone. The oysters are ready for harvesting in 18 to 30 months depending on the size required.<ref name="PCO" />

Similar techniques are used in different parts of the world to cultivate other species including the [[Sydney rock oyster]] (''Saccostrea commercialis''), the northern quahog (''Mercenaria mercenaria''), the blue mussel (''Mytilus edulis''), the [[Mediterranean mussel]] (''Mytilus galloprovincialis''), the [[New Zealand green-lipped mussel]] (''Perna canaliculus''), the [[grooved carpet shell]] (''Ruditapes decussatus''), the [[Venerupis philippinarum|Japanese carpet shell]] (''Venerupis philippinarum''), the [[Venerupis pullastra|pullet carpet shell]] (''Venerupis pullastra'') and the [[Patinopecten yessoensis|Yesso scallop]] (''Patinopecten yessoensis'').<ref>{{cite web | url = http://www.fao.org/fishery/culturedspecies/search/en | title = Cultured Aquatic Species |publisher=FAO Fisheries and Aquaculture Department |access-date=2012-05-19 }}</ref>

Production of bivalve molluscs by mariculture in 2010 was 12,913,199 tons, up from 8,320,724 tons in 2000. Culture of clams, cockles and ark shells more than doubled over this time period from 2,354,730 to 4,885,179 tons. Culture of mussels over the same period grew from 1,307,243 to 1,812,371 tons, of oysters from 3,610,867 to 4,488,544 tons and of scallops from 1,047,884 to 1,727,105 tons.<ref>{{cite web | url = http://www.fao.org/fishery/statistics/global-aquaculture-production/en | title = Fishery Statistical Collections: Global Aquaculture Production |publisher=FAO Fisheries and Aquaculture Department |access-date=2012-05-23 }}</ref>

== Use as food ==
[[File:Huitres Cancale.jpg|thumb|Flat oysters (''[[Ostrea edulis]]'') from France]]
Bivalves have been an important source of food for humans at least since Roman times<ref>{{cite web | url = http://library.thinkquest.org/26602/diet.htm | title = Daily life: Roman cuisine | publisher = Oracle ThinkQuest Education Foundation | access-date = 2012-05-12 | url-status = dead | archive-url = https://web.archive.org/web/20120508031543/http://library.thinkquest.org/26602/diet.htm | archive-date = 2012-05-08 }}</ref> and empty shells found in middens at archaeological sites are evidence of earlier consumption.<ref name="Rollins" /> [[Oyster]]s, [[scallop]]s, [[clam]]s, [[ark clam]]s, [[mussel]]s and [[Cockle (bivalve)|cockles]] are the most commonly consumed kinds of bivalve, and are eaten cooked or raw. In 1950, the year in which the [[Food and Agriculture Organization]] (FAO) started making such information available, world trade in bivalve molluscs was 1,007,419 tons.<ref name="Trade" /> By 2010, world trade in bivalves had risen to 14,616,172 tons, up from 10,293,607 tons a decade earlier. The figures included 5,554,348 (3,152,826) tons of clams, cockles and ark shells, 1,901,314 (1,568,417) tons of mussels, 4,592,529 (3,858,911) tons of oysters and 2,567,981 (1,713,453) tons of scallops.<ref name="Trade" /> China increased its consumption 400-fold during the period 1970 to 1997.<ref name="outbreaks" />

It has been known for more than a century that consumption of raw or insufficiently cooked shellfish can be associated with infectious diseases. These are caused either by bacteria naturally present in the sea such as ''[[Vibrio]] spp.'' or by viruses and bacteria from sewage [[effluent]] that sometimes contaminates coastal waters. As filter feeders, bivalves pass large quantities of water through their gills, filtering out the organic particles, including the microbial pathogens. These are retained in the animals' tissues and become concentrated in their liver-like digestive glands.<ref name="outbreaks" /><ref name="Rippey" /> Another possible source of contamination occurs when bivalves contain marine [[biotoxin]]s as a result of ingesting numerous [[dinoflagellate]]s. These microalgae are not associated with sewage but occur unpredictably as [[algal bloom]]s. Large areas of a sea or lake may change colour as a result of the proliferation of millions of single-cell algae, and this condition is known as a [[red tide]].<ref name="outbreaks" />

=== Viral and bacterial infections ===
In 1816 in France, a physician, J. P. A. Pasquier, described an outbreak of [[typhoid]] linked to the consumption of raw oysters. The first report of this kind in the United States was in [[Connecticut]] in 1894. As sewage treatment programmes became more prevalent in the late 19th century, more outbreaks took place. This may have been because sewage was released through outlets into the sea providing more food for bivalves in estuaries and coastal habitats. A causal link between the bivalves and the illness was not easy to demonstrate because the illness might come on days or even weeks after the ingestion of the contaminated shellfish. One viral pathogen is the ''[[Norwalk virus|Norwalk]]'' virus. This is resistant to treatment with chlorine-containing chemicals and may be present in the marine environment even when [[coliform bacteria]] have been killed by the [[Sewage treatment|treatment of sewage]].<ref name="outbreaks" />

Since the 1970s, outbreaks of oyster-[[vector (epidemiology)|vectored]] diseases have occurs throughout the world. The mortality rate of one disease causing bacteria ''[[Vibrio vulnificus]]'', was high at 50%.<ref name="outbreaks" /> In 1978, an oyster-associated gastrointestinal infection affecting more than 2,000 people occurred in Australia. The causative agent was found to be the ''Norwalk'' virus and the epidemic caused major economic difficulties to the oyster farming industry in the country.<ref>{{cite journal |author1=Murphy A. M. |author2=Grohmann G. S. |author3=Christopher P. J. |author4=Lopez W. A. |author5=Davey G. R. |author6=Millsom R. H. | year = 1979 | title = An Australia-wide outbreak of gastroenteritis from oysters caused by Norwalk virus |journal=Medical Journal of Australia |volume=2 |issue=7 |pages=329–333 |doi=10.5694/j.1326-5377.1979.tb104133.x | pmid = 514174 |s2cid=36548443 }}</ref> In 1988, an outbreak of [[hepatitis A]] associated with the consumption of inadequately cooked clams (''[[Anadara subcrenata]]'') took place in the [[Shanghai]] area of China. An estimated 290,000 people were infected and there were 47 deaths.<ref>{{cite journal |author1=Tang Y. W. |author2=Wang J. X. |author3=Xu Z. Y. |author4=Guo Y. F. |author5=Qian W. H. |author6=Xu J. X. | year = 1991 | title = A serologically confirmed, case-control study, of a large outbreak of hepatitis A in China, associated with consumption of clams |journal=Epidemiology and Infection |volume=107 |issue=3 |pages=651–657 | pmid = 1661240 |doi=10.1017/S0950268800049347 |jstor=3864150 | pmc = 2272104 }}</ref> In the United States and the European Union, since the early 1990s regulations have been in place that are designed to prevent shellfish from contaminated waters entering restaurants.<ref name="outbreaks" />

=== Paralytic shellfish poisoning ===
[[Paralytic shellfish poisoning]] (PSP) is primarily caused by the consumption of bivalves that have accumulated toxins by feeding on toxic dinoflagellates, single-celled protists found naturally in the sea and inland waters. [[Saxitoxin]] is the most virulent of these. In mild cases, PSP causes tingling, numbness, sickness and diarrhoea. In more severe cases, the muscles of the chest wall may be affected leading to paralysis and even death. In 1937, researchers in [[California]] established the connection between blooms of these phytoplankton and PSP.<ref name="PSP" /> The biotoxin remains potent even when the shellfish are well-cooked.<ref name="PSP" /> In the United States, there is a regulatory limit of 80&nbsp;[[microgram|μg]]/g of saxitoxin equivalent in shellfish meat.<ref name="PSP" />

=== Amnesic shellfish poisoning ===
[[Amnesic shellfish poisoning]] (ASP) was first reported in eastern Canada in 1987. It is caused by the substance [[domoic acid]] found in certain diatoms of the genus ''[[Pseudo-nitzschia]]''. Bivalves can become toxic when they filter these microalgae out of the water. Domoic acid is a low-molecular weight [[amino acid]] that is able to destroy brain cells causing memory loss, [[gastroenteritis]], long-term neurological problems or death. In an outbreak in the western United States in 1993, finfish were also implicated as vectors, and seabirds and mammals suffered neurological symptoms.<ref name="PSP" /> In the United States and Canada, a regulatory limit of 20&nbsp;μg/g of domoic acid in shellfish meat is set.<ref>{{cite web | url = http://www.whoi.edu/redtide/page.do?pid=14275 | title = Amnesic Shellfish Poisoning | year = 2007 |work=Harmful Algae |publisher=Woods Hole Oceanographic Institution |access-date=2012-05-14 }}</ref>

==Ecosystem services==
[[File:Nutrient extraction services provided by bivalves.png|thumb|upright=1.4|left| Nutrient extraction services provided by bivalves. [[Blue mussel]]s are used as examples but other bivalves like [[oyster]]s can also provide these nutrient extraction services.<ref name=Petersen2019>Petersen, J.K., Holmer, M., Termansen, M. and Hasler, B. (2019) "Nutrient extraction through bivalves". In: Smaal A., Ferreira J., Grant J., Petersen J., Strand Ø. (eds) ''Goods and Services of Marine Bivalves'', pages 179–208. Springer. {{doi|10.1007/978-3-319-96776-9_10}}. {{ISBN|9783319967769}} [[File:CC-BY icon.svg|50px|class=noviewer]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]]

[[Ecosystem services]] provided by marine bivalves in relation to nutrient extraction from the coastal environment have gained increased attention to mitigate adverse effects of excess nutrient loading from human activities, such as agriculture and sewage discharge. These activities damage coastal ecosystems and require action from local, regional, and national environmental management. Marine bivalves filter particles like [[phytoplankton]], thereby transforming [[particulate organic matter]] into bivalve tissue or larger [[faecal pellet]]s that are transferred to the [[benthos]]. [[Nutrient cycle|Nutrient extraction]] from the coastal environment takes place through two different pathways: (i) harvest/removal of the bivalves – thereby returning nutrients back to land; or (ii) through increased [[denitrification]] in proximity to dense bivalve aggregations, leading to loss of nitrogen to the atmosphere. Active use of marine bivalves for nutrient extraction may include a number of secondary effects on the ecosystem, such as filtration of [[particulate]] material. This leads to partial transformation of particulate-bound nutrients into dissolved nutrients via bivalve excretion or enhanced mineralization of faecal material.<ref name=Petersen2019 />

When they live in polluted waters, bivalve molluscs have a tendency to accumulate substances such as [[Heavy metal (chemistry)|heavy metals]] and [[persistent organic pollutants]] in their tissues. This is because they ingest the chemicals as they feed but their enzyme systems are not capable of metabolising them and as a result, the levels build up. This may be a health hazard for the molluscs themselves, and is one for humans who eat them. It also has certain advantages in that bivalves can be used in [[Biomonitoring|monitoring]] the presence and quantity of pollutants in their environment.<ref>{{cite journal |author1=Burns, K. A. |author2=Smith, J. L. | year = 1981 | title = Biological monitoring of ambient water quality: the case for using bivalves as sentinel organisms for monitoring petroleum pollution in coastal waters |journal=[[Estuarine, Coastal and Shelf Science]] |volume=30 |issue=4 |pages=433–443 |doi=10.1016/S0302-3524(81)80039-4 | bibcode = 1981ECSS...13..433B }}</ref>

[[File:Economic value of bivalve nutrient extraction.png|thumb|upright=1.4|right| Economic value of bivalve nutrient extraction, linking processes to services to economic values.<ref name=Petersen2019 />]]

There are limitations to the use of bivalves as [[bioindicator]]s. The level of pollutants found in the tissues varies with species, age, size, time of year and other factors. The quantities of pollutants in the water may vary and the molluscs may reflect past rather than present values. In a study near [[Vladivostok]] it was found that the level of pollutants in the bivalve tissues did not always reflect the high levels in the surrounding sediment in such places as harbours. The reason for this was thought to be that the bivalves in these locations did not need to filter so much water as elsewhere because of the water's high nutritional content.<ref name="Shulkin Kavun 1995" />

A study of nine different bivalves with widespread distributions in tropical marine waters concluded that the mussel, ''[[Trichomya hirsuta]]'', most nearly reflected in its tissues the level of heavy metals (Pb, Cd, Cu, Zn, Co, Ni, and Ag) in its environment. In this species there was a linear relationship between the sedimentary levels and the tissue concentration of all the metals except zinc.<ref>{{cite journal |author1=Klumpp, D. W. |author2=Burdon-Jones, C. | year = 1982 | title = Investigations of the potential of bivalve molluscs as indicators of heavy metal levels in tropical marine waters |journal=Australian Journal of Marine and Freshwater Research |volume=32 |issue=2 |pages=285–300 |doi=10.1071/MF9820285 }}</ref> In the [[Persian Gulf]], the Atlantic pearl-oyster (''[[Pinctada radiata]]'') is considered to be a useful bioindicator of heavy metals.<ref>{{cite journal|last=Al-Madfa |first=H. |author2=Abdel-Moati, A. R. M. |author3=Al-Gimaly, F. H. |title=''Pinctada radiata'' (pearl oyster): a bioindicator for metal pollution monitoring in the Qatari waters |journal=Bulletin of Environmental Contamination and Toxicology |year=1998 |volume=60 |issue=2 |pages=245–251 |url=http://www.scsagr.com/upimg/2008530162135.pdf |doi=10.1007/s001289900617 |pmid=9470985 |s2cid=35586676 |url-status=dead |archive-url=https://web.archive.org/web/20110716022626/http://www.scsagr.com/upimg/2008530162135.pdf |archive-date=2011-07-16 }}</ref>

Crushed shells, available as a by-product of the seafood canning industry, can be used to remove pollutants from water.  It has been found that, as long as the water is maintained at an alkaline [[pH]], crushed shells will remove cadmium, lead and other heavy metals from contaminated waters by swapping the calcium in their constituent aragonite for the heavy metal, and retaining these pollutants in a solid form.<ref>{{cite news|title=Sea Shells Used to Clean Up Heavy Metals |author=Reilly, Michael |url=http://dsc.discovery.com/news/2009/04/27/shells-metals-water.html |newspaper=Discovery News |date=2009-04-27 |access-date=2012-05-18 |url-status=dead |archive-url=https://web.archive.org/web/20120331014620/http://dsc.discovery.com/news/2009/04/27/shells-metals-water.html |archive-date=March 31, 2012 }}</ref> The rock oyster (''[[Saccostrea cucullata]]'') has been shown to reduce the levels of [[copper]] and cadmium in contaminated waters in the Persian Gulf. The live animals acted as biofilters, selectively removing these metals, and the dead shells also had the ability to reduce their concentration.<ref>{{cite journal |author1=Azarbad, H. |author2=Khoi, A. J. |author3=Mirvaghefi, A. |author4=Danekar, A. |author5=Shapoori, M. |year=2010 |title=Biosorption and bioaccumulation of heavy metals by rock oyster ''Saccostrea cucullata'' in the Persian Gulf |journal=International Aquatic Research |volume=2010 |issue=2 |pages=61–69 |issn=2008-4935 |url=http://www.intelaquares.com/doc/7b.pdf |url-status=dead |archive-url=https://web.archive.org/web/20131017015815/http://www.intelaquares.com/doc/7b.pdf |archive-date=2013-10-17 }}</ref>

== Other uses ==
[[File:Calc1.jpg|thumb|alt=Carved shell miniatures|Carved shell miniatures]]
[[Conchology]] is the scientific study of mollusc shells, but the term conchologist is also sometimes used to describe a collector of shells. Many people pick up shells on the beach or purchase them and display them in their homes. There are many private and public collections of mollusc shells, but the largest one in the world is at the [[Smithsonian Institution]], which houses in excess of 20 million specimens.<ref>{{cite web | url = http://www.smithsonianmag.com/videos/category/at-the-smithsonian/the-magnificent-shells-of-the-smithsonian/?no-ist | title = The magnificent shells of the Smithsonian |publisher=Smithsonian.com |access-date=2012-05-05}}</ref>

[[File:PSM V28 D311 Wampum belt commemorating the iroquois confederacy.jpg|thumb|alt=1885 wampum belt|1885 wampum belt]]
[[File:ButtonShell.jpg|thumb|alt=Freshwater mussel shell used for making buttons|Freshwater mussel shell used for making buttons]]
[[File:Flügelretabel Perlmutt Augsburg um 1520.jpg|thumb|alt=Altarpiece with carved nacre|Carved nacre in a 16th-century [[altarpiece]]]]
Shells are used decoratively in many ways. They can be pressed into concrete or plaster to make decorative paths, steps or walls and can be used to embellish picture frames, mirrors or other craft items. They can be stacked up and glued together to make ornaments. They can be pierced and threaded onto necklaces or made into other forms of jewellery. Shells have had various uses in the past as body decorations, utensils, scrapers and cutting implements. Carefully cut and shaped shell tools dating back 32,000 years have been found in a cave in Indonesia. In this region, shell technology may have been developed in preference to the use of stone or bone implements, perhaps because of the scarcity of suitable rock materials.<ref>{{cite news | title = Shell Tools Rewrite Australasian Prehistory | author = Catling, Chris | url = http://www.world-archaeology.com/news/shell-tools-rewrite-australasian-prehistory/ | newspaper = World Archaeology | date = 2009-07-06 | access-date = 2012-05-18 | url-status = dead | archive-url = https://web.archive.org/web/20130317051507/http://www.world-archaeology.com/news/shell-tools-rewrite-australasian-prehistory/ | archive-date = 2013-03-17 }}</ref>

The [[indigenous peoples of the Americas]] living near the east coast used pieces of shell as [[wampum]]. The [[channeled whelk]] (''Busycotypus canaliculatus'') and the [[quahog]] (''Mercenaria mercenaria'') were used to make white and purple traditional patterns. The shells were cut, rolled, polished and drilled before being strung together and woven into belts. These were used for personal, social and ceremonial purposes and also, at a later date, for currency.<ref>{{cite book | title = North American Indian Jewelry and Adornment: From Prehistory to the Present |last=Dubin |first=Lois Sherr | year = 1999 |publisher=Harry N. Abrams | isbn = 978-0-8109-3689-8 | pages = 170–171 }}</ref> The [[Ho-Chunk|Winnebago Tribe]] from Wisconsin had numerous uses for freshwater mussels including using them as spoons, cups, ladles and utensils. They notched them to provide knives, graters and saws. They carved them into fish hooks and lures. They incorporated powdered shell into clay to [[Temper (pottery)|temper]] their pottery vessels. They used them as scrapers for removing flesh from hides and for separating the scalps of their victims. They used shells as scoops for gouging out fired logs when building canoes and they drilled holes in them and fitted wooden handles for tilling the ground.<ref>{{cite journal | author = Kuhm, H. W. | year = 2007 | title = Aboriginal uses of shell |journal=The Wisconsin Archeologist |volume=17 |issue=1 |pages=1–8 | url = https://archive.org/stream/wisconsinarcheol17wiscrich/wisconsinarcheol17wiscrich_djvu.txt }}</ref>

[[button (clothing)|Button]]s have traditionally been made from a variety of freshwater and marine [[seashell|shells]].<ref name="Hesse&amp;Hesse" /> At first they were used decoratively rather than as fasteners and the earliest known example dates back five thousand years and was found at [[Mohenjo-daro]] in the [[Indus Valley]].<ref>{{cite book | title = An Encyclopaedia of the History of Technology | last = McNeil | first = Ian | year = 1990 | publisher = Taylor & Francis | isbn = 978-0-415-01306-2 | page = [https://archive.org/details/encyclopaediaofh00mcne/page/852 852] | url = https://archive.org/details/encyclopaediaofh00mcne/page/852 }}</ref>

[[Sea silk]] is a fine fabric woven from the [[byssus]] threads of bivalves, particularly the pen shell (''[[Pinna nobilis]]''). It used to be produced in the Mediterranean region where these shells are [[Endemism|endemic]]. It was an expensive fabric and [[overfishing]] has much reduced populations of the pen shell.<ref>{{cite journal |author1=Šiletić, T. |author2=Peharda, M. | year = 2003 | title = Population study of the fan shell ''Pinna nobilis'' L. in Malo and Veliko Jezero of the Mljet National Park (Adriatic Sea) |journal=Scientia Marina |volume=67 |issue=1 |pages=971–998 | url = http://scientiamarina.revistas.csic.es/index.php/scientiamarina/article/view/457/469 | doi=10.3989/scimar.2003.67n191|doi-access=free }}</ref>

Crushed shells are added as a calcareous supplement to the diet of laying poultry. Oyster shell and cockle shell are often used for this purpose and are obtained as a by-product from other industries.<ref>{{cite web | url = http://ascott-dairy.co.uk/acatalog/Poultry_Grit_and_Oystershell.html | title = Poultry Grit, Oystershell and Wood Shavings | publisher = Ascott Smallholding Supplies Ltd | access-date = 2012-05-18 | archive-url = https://web.archive.org/web/20130807223058/http://ascott-dairy.co.uk/acatalog/Poultry_Grit_and_Oystershell.html | archive-date = 2013-08-07 | url-status = dead }}</ref>

=== Pearls and mother-of-pearl ===
Mother-of-pearl or nacre is the naturally occurring lustrous layer that lines some mollusc shells. It is used to make pearl buttons and in artisan craftwork to make organic jewellery. It has traditionally been [[Marquetry|inlaid]] into furniture and boxes, particularly in China. It has been used to decorate musical instruments, watches, pistols, fans and other products. The import and export of goods made with nacre are controlled in many countries under the [[CITES|International Convention of Trade in Endangered Species of Wild Fauna and Flora]].<ref>{{cite news | title = Contraband Chic: Mother-of-Pearl Items Sell With Export Restrictions | author = Hodin, Jessica | url = http://observer.com/2010/10/20/contraband-chic-motherofpearl-items-sell-with-export-restrictions/ |newspaper=New York Observer |date=2010-10-20 |access-date=2012-05-18 }}</ref>

A [[pearl]] is created in the mantle of a mollusk when an irritant particle is surrounded by layers of nacre. Although most bivalves can create pearls, [[Pearl oyster|oysters]] in the family [[Pteriidae]] and freshwater mussels in the families [[Unionidae]] and [[Margaritiferidae]] are the main source of commercially available pearls because the calcareous concretions produced by most other species have no lustre. Finding pearls inside oysters is a very chancy business as hundreds of shells may need to be pried open before a single pearl can be found. Most pearls are now obtained from cultured shells where an irritant substance has been purposefully introduced to induce the formation of a pearl. A "mabe" (irregular) pearl can be grown by the insertion of an implant, usually made of plastic, under a flap of the mantle and next to the mother-of-pearl interior of the shell. A more difficult procedure is the grafting of a piece of oyster mantle into the gonad of an adult specimen together with the insertion of a shell bead nucleus. This produces a superior, spherical pearl. The animal can be opened to extract the pearl after about two years and reseeded so that it produces another pearl. Pearl oyster farming and pearl culture is an important industry in Japan and many other countries bordering the Indian and Pacific Oceans.<ref>{{cite web | url = http://www.fao.org/docrep/field/003/AB726E/AB726E00.htm | title = Pearl Oyster Farming and Pearl Culture | year = 1991 |work=Training manual produced by the Central Marine Fisheries Research Institute at Tuticorin, India |publisher=FAO Fisheries and Aquaculture Department |access-date=2012-05-18 }}</ref>

=== Symbolism ===
The scallop is the symbol of [[James, son of Zebedee|St James]] and is called ''[[Coquille Saint-Jacques]]'' in [[French language|French]]. It is an emblem carried by pilgrims on their [[Way of St James|way]] to the shrine of [[Santiago de Compostela]] in Galicia. The shell became associated with the pilgrimage and came to be used as a symbol showing hostelries along the route and later as a sign of hospitality, food and lodging elsewhere.<ref>{{cite web | url = http://hermetism.free.fr/Jacques_Coeur_facade.htm | title = Fulcanelli et la façade du palais Jacques Coeur |publisher=Fulcanelli, La rue de l'alchimie à travers l'architecture, les livres et les alchimistes |access-date=2012-06-14 |language=fr }}</ref>

[[Roman mythology|Roman]] myth has it that [[Venus (mythology)|Venus]], the goddess of love, was born in the sea and emerged accompanied by fish and dolphins, with [[Botticelli]] depicting her as arriving in a scallop shell. The Romans revered her and erected shrines in her honour in their gardens, praying to her to provide water and verdant growth.<ref>{{cite news | title = ''Venus honored in Roman garden shrines'' | author = Gilmer, Maureen | url = http://www.highbeam.com/doc/1P2-1524524.html | archive-url = https://web.archive.org/web/20130511160600/http://www.highbeam.com/doc/1P2-1524524.html | url-status = dead | archive-date = 2013-05-11 |newspaper=Chicago Sun-Times via [[HighBeam Research]] |date=2004-03-19 |access-date=2012-05-21 }}</ref> From this, the scallop and other bivalve shells came to be used as a symbol for fertility.<ref>{{cite book | title = The Secret Language of Symbols: A Visual Key to Symbols and Their Meanings |last=Fontana |first=D. | year = 1994 |publisher=Chronicle Books| isbn = 978-0-8118-0462-2 | pages = 88, 103 }}</ref> Its depiction is used in architecture, furniture and fabric design and it is the logo of [[Royal Dutch Shell]], the global oil and gas company.<ref>{{cite web | url = http://www.shell.com/ | title = The Shell global homepage |access-date=2012-05-21 }}</ref>

== Bivalvian taxonomies ==
[[File:CornishMussels.JPG|thumb|alt=Mussels in Cornwall|Mussels in the intertidal zone in [[Cornwall]], England]]
[[File:Mytilarca cingulosa.jpg|thumb|''Mytilarca'' is a distant relative of the mussels; from the Middle [[Devonian]] of [[Wisconsin]].]]
[[File:JurassicMarineIsrael.JPG|thumb|alt=Fossil gastropod and bivalves from Israel|Fossil [[gastropod]] and attached [[Mytilidae|mytilid]] bivalves in a Jurassic [[limestone]] ([[Matmor Formation]]) in southern [[Israel]]]]
[[File:Aviculopecten subcardiformis01.JPG|thumb|alt=Fossil scallop from Ohio|''[[Aviculopecten subcardiformis]]''; a [[fossil]] of an extinct [[scallop]] from the [[Logan Formation]] of [[Wooster, Ohio]] (external mold)]]

For the past two centuries no consensus has existed on bivalve [[phylogeny]] from the many classifications developed. In earlier taxonomic systems, experts used a single characteristic feature for their classifications, choosing among shell morphology, hinge type or gill type.  Conflicting naming schemes proliferated due to these taxonomies based on single organ systems. One of the most widely accepted systems was that put forward by [[Norman D. Newell]] in Part N of the ''[[Treatise on Invertebrate Paleontology]]'',<ref>{{cite book | title = Treatise on Invertebrate Paleontology, Part N: Mollusca 6, Bivalvia (Vol. 3) |last=Moore |first=R. C. | year = 1969 |publisher=Geological Society of America| isbn = 978-0-8137-3026-4 | page = 275 }}</ref> which employed a classification system based on general shell shape, microstructures and hinge configuration.<ref name="Newell1969" /> Because features such as hinge morphology, dentition, mineralogy, shell morphology and shell composition change slowly over time, these characteristics can be used to define major taxonomic groups.

Since the year 2000, taxonomic studies using [[cladistics|cladistical]] analyses of multiple organ systems, shell morphology (including fossil species) and modern [[molecular phylogenetics]] have resulted in the drawing up of what experts believe is a more accurate phylogeny of the Bivalvia.<ref name="Giribet & Wheeler 2002" /><ref name="Bieler & Mikkelsen 2006" /><ref name="Mikkelsen Bieler_ 2006" /><ref name="Taylor Williams_ 2007" /><ref name="Taylor Williams 2009" /> Based upon these studies, a new proposed classification system for the Bivalvia was published in 2010 by Bieler, Carter & Coan.<ref name="Bouchet Rocroi_ 2010" /> In 2012, this new system was adopted by the [[World Register of Marine Species]] (WoRMS) for the classification of the Bivalvia. Some experts still maintain that Anomalodesmacea should be considered a separate subclass, whereas the new system treats it as the order Anomalodesmata, within the subclass [[Heterodonta]]. Molecular phylogenetic work continues, further clarifying which Bivalvia are most closely related and thus refining the classification.<ref name="Tëmkin 2010" /><ref name="Taylor Glover_ 2011" />

=== Practical taxonomy of R.C. Moore ===

[[R.C. Moore]], in Moore, Lalicker, and Fischer, 1952, ''Invertebrate Fossils'', gives a practical and useful classification of pelecypods (Bivalvia) even if somewhat antiquated, based on shell structure, gill type, and hinge teeth configuration.  Subclasses and orders given are:

: Subclass:[[Prionodesmacea]]
:: Order
:: [[Paleoconcha]]
:: Taxodonta: Many teeth (e.g. order [[Nuculida]])
:: [[Schizodonta]]: Big bifurcating teeth (e.g. ''[[Trigonia]]'' spp.)
:: [[Isodonta]]: Equal teeth (e.g. ''[[Spondylus]]'' spp.)
:: [[Dysodonta]]: Absent teeth and ligaments joins the valves.

: Subclass:[[Teleodesmacea]]
:: Order
:: [[Heterodonta]]: Different teeth (e.g. family [[Cardiidae]]). [ [[Lower Ordovician]]&nbsp;– [[Holocene|Recent]]]
:: [[Pachydonta]]: Large, different, deformed teeth (e.g. [[rudist]] spp.). [ [[Late Jurassic]]&nbsp;– [[Upper Cretaceous]]]
:: [[Desmodonta]]: Hinge-teeth absent or irregular with ligaments (e.g. family [[Anatinidae]]).

Prionodesmacea have a prismatic and nacreous shell structure, separated mantle lobes, poorly developed siphons, and hinge teeth that are lacking or unspecialized. Gills range from protobranch to eulamellibranch. Teleodesmacea on the other hand have a porcelanous and partly nacreous shell structure; Mantle lobes that are generally connected, well developed siphons, and specialized hinge teeth. In most, gills are eulamellibranch.

=== 1935 taxonomy ===
In his 1935 work ''Handbuch der systematischen Weichtierkunde'' (Handbook of Systematic Malacology), [[Johannes Thiele (zoologist)|Johannes Thiele]] introduced a mollusc taxonomy based upon the 1909 work by Cossmann and Peyrot. Thiele's system divided the bivalves into three orders. ''Taxodonta'' consisted of forms that had taxodont dentition, with a series of small parallel teeth perpendicular to the hinge line. ''Anisomyaria'' consisted of forms that had either a single adductor muscle or one adductor muscle much smaller than the other. ''Eulamellibranchiata'' consisted of forms with ctenidial gills. The Eulamellibranchiata was further divided into four suborders: ''Schizodonta'', ''Heterodonta'', ''Adapedonta'' and ''Anomalodesmata''.<ref name="schneider2001" /><ref name="ponder_lindberg2008" />

=== Taxonomy based upon hinge tooth morphology ===
The systematic layout presented here follows Newell's 1965 classification based on hinge tooth [[morphology (biology)|morphology]] (all taxa marked † are extinct) :<ref name="Newell1969" />

{| class="wikitable"
|-
! [[Class (biology)|Subclass]]
! [[Order (biology)|Order]]
|-
| [[Palaeotaxodonta]]
| [[Nuculoida]] (nut shells)
|-
| [[Cryptodonta]]
| † [[Praecardioida]]
[[Solemyoida]]
|-
| [[Pteriomorphia]]
| [[Arcoida]] ([[ark shell]]s)
† [[Cyrtodontoida]]

[[Limoida]] (file shells)

[[Mytiloida]] (true [[mussels]])

[[Ostreoida]] ([[oysters]], formerly included in Pterioida)

† [[Praecardioida]]

[[Pterioida]] ([[pearl oysters]], [[pen shell]]s)
|-
| [[Palaeoheterodonta]]
| [[Trigonioida]] (''[[Neotrigonia]]'' is the only extant genus)

[[Unionoida]] ([[freshwater mussel]]s)

† [[Modiomorpha]]
|-
| [[Heterodonta]]
| † [[Cycloconchidae]]

† [[Hippuritoida]]

† [[Lyrodesmatidae]]

[[Myoida]] (soft-shell clams, [[geoduck]]s, [[shipworm]]s)

† [[Redoniidae]]

[[Veneroida]] (hard-shell clams, [[cockle (bivalve)|cockles]], [[razor shell]]s)
|-
| [[Anomalodesmata]]
| [[Pholadomyoida]]
|}

The [[monophyly]] of the subclass Anomalodesmata is disputed. The standard view now is that it resides within the subclass Heterodonta.<ref name="Giribet & Wheeler 2002" /><ref name="Taylor Williams_ 2007" /><ref name="Harper Dreyer_ 2006" />

=== Taxonomy based upon gill morphology ===
An alternative systematic scheme exists using gill morphology.<ref>{{cite book |chapter=Classe de Bivalves |last=Franc |first=A. |editor-last=Grassé, P.-P. | title = Traité de Zoologie: Anatomie, Systématique, Biologie |volume=5 | year = 1960 |publisher=Masson et Cie | pages=1845–2164 | url = http://clade.ansp.org/obis/search.php/reference1589 |language=fr }}</ref> This distinguishes between Protobranchia, Filibranchia and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta and Cryptodonta, the second to his Pteriomorphia, with the last corresponding to all other groups. In addition, Franc separated the [[Septibranchia]] from his eulamellibranchs because of the morphological differences between them. The septibranchs belong to the superfamily [[Poromyoidea]] and are carnivorous, having a muscular septum instead of filamentous gills.<ref>{{cite web | url = http://www.answers.com/topic/septibranchia | title = Septibranchia |work=McGraw-Hill Dictionary of Scientific and Technical Terms |publisher=McGraw-Hill Companies |access-date=2012-05-07 }}</ref>

=== 2010 taxonomy ===

{{main|Taxonomy of the Bivalvia (Bouchet, Rocroi, Bieler, Carter & Coan, 2010)}}

In May 2010, a new taxonomy of the Bivalvia was published in the journal ''[[Malacologia]]''. In compiling this, the authors used a variety of phylogenetic information including molecular analysis, anatomical analysis, shell morphology and shell microstructure as well as bio-geographic, paleobiogeographic and [[stratigraphy|stratigraphic]] information. In this classification 324 families are recognized as valid, 214 of which are known exclusively from fossils and 110 of which occur in the recent past, with or without a fossil record.<ref name="Bouchet Rocroi_ 2010" /> This classification has since been adopted by WoRMS.<ref>{{cite WoRMS | author = Gofas, Serge | year = 2012 | title = Bivalvia |id=105 |access-date=2012-06-30 }}</ref>

Proposed classification of '''Class Bivalvia''' (under the redaction of Rüdiger Bieler, Joseph G. Carter and Eugene V. Coan) (all taxa marked † are extinct) :<ref name="Carter 2011" />

{{col-begin}}
{{col-3}}

'''Grade [[Euprotobranchia]]'''
* Order [[Fordillida]]
:: 2 families (2†)
* Order [[Tuarangiida]]
:: 1 family (1†)

'''Subclass [[Heterodonta]]'''

'''Infraclass [[Archiheterodonta]]'''
* Order [[Carditida]]
:: 4 families

'''Infraclass [[Euheterodonta]]'''
* Unassigned Euheterodonta
:: 4 families
* Order [[Pholadomyida]] (=[[Anomalodesmata]])
:: 16 families
* Order [[Myida]]
:: 4 families
* Order [[Lucinida]]
:: 2 families
* Order [[Venerida]]
:: 30 families
{{col-3}}

'''Subclass [[Palaeoheterodonta]]'''
* Order [[Trigoniida]]
:: 16 families (15†)
* Order [[Unionida]]
:: 15 families (8†)
'''Subclass [[Protobranchia]]'''
* Order [[Nuculanida]]
:: 8 families
* Order [[Nuculida]]
:: 3 families (1†)
* Order [[Solemyida]]
:: 2 families
{{col-3}}

'''Subclass [[Pteriomorphia]]'''
* Order [[Arcida]]
:: 7 families

'''Infraclass [[Eupteriomorphia]]'''
* Order [[Ostreida]]
:: 2 families
* Suborder [[Pectinida]]
:: 7 families
* Suborder [[Limida]]
:: 1 family
* Suborder [[Mytilida]]
:: 1 family
* Suborder [[Pteriida]]
:: 4 families
{{col-end}}

== References ==

{{reflist
| colwidth = 30em
| refs =

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{{cite journal| doi = 10.1080/03115518008618934| last = Jell| first = Peter A.| year = 1980| title = Earliest known pelecypod on Earth&nbsp;— a new Early Cambrian genus from South Australia| journal = Alcheringa: An Australasian Journal of Palaeontology| volume = 4| issue = 3| pages = 233–239
| bibcode = 1980Alch....4..233J}}
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<ref name="Runnegar Bentley 1983">{{cite journal| last1 = Runnegar| first1 = B.| last2 = Bentley| first2 = C.| date = January 1983| title = Anatomy, ecology and affinities of the Australian early Cambrian bivalve ''Pojetaia runnegari'' Jell| journal = Journal of Paleontology| volume = 57| issue = 1| pages = 73–92| jstor = 1304610| url = http://jpaleontol.geoscienceworld.org/content/57/1_Part_I/73.abstract| access-date = 2015-12-01| archive-url = https://web.archive.org/web/20161110054330/http://jpaleontol.geoscienceworld.org/content/57/1_Part_I/73.abstract| archive-date = 2016-11-10| url-status = dead}}</ref>

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<ref name="Tutorial">{{cite web
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<ref name="Mineralogy">
{{cite journal
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| title = Environmental and biological controls on bivalve shell mineralogy
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| volume = 44
| issue = 4
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| doi = 10.1111/j.1469-185X.1969.tb00610.x
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{{cite book
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<ref name="EL">
{{cite web
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<ref name="Morton 2008">
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}}
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<ref name="Archerd">
{{cite web
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<ref name="FossilRecord">
{{cite web
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<ref name="Yonge">
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<ref name="BCO">
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<ref name="Harper">{{cite journal
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| year = 1990
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<ref name="Rollins">
{{cite journal
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| title = Growth increment and stable isotope analysis of marine bivalves: implications for the geoarchaeological record of El Niño
| journal = Geoarchaeology
| volume = 2
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| pages = 181–197
| doi = 10.1002/gea.3340020301
|bibcode=1987Gearc...2..181R }}
</ref>

<ref name="Odyssey">{{cite web
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<ref name="EFO">
{{cite web
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<ref name="PCO">
{{cite web
| url = http://www.fao.org/fishery/culturedspecies/Crassostrea_gigas/en
| title = Cultured Aquatic Species Information Programme: ''Crassostrea gigas'' (Thunberg, 1793)
| publisher = FAO Fisheries and Aquaculture Department
| access-date = 2012-05-19
}}
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<ref name="Trade">
{{cite web
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}}
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<ref name="outbreaks">
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<ref name="PSP">
{{cite journal
|author1=Wekell, John C. |author2=Hurst, John |author3=Lefebvre, Kathi A. | title = The origin of the regulatory limits for PSP and ASP toxins in shellfish
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<ref name="Shulkin Kavun 1995">
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| bibcode = 1995MarPB..31..330S}}
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<ref name="Hesse&Hesse">
{{cite book
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<ref name="Newell1969">
{{cite book
| last = Newell
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| year = 1969
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| editor-last = Moore
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}}
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<ref name="schneider2001">
{{cite journal
| author = Schneider, Jay A.
| year = 2001
| title = Bivalve systematics during the 20th century
| journal = Journal of Paleontology
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| issue = 6
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<ref name="Giribet & Wheeler 2002">
{{cite journal
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| last1 = Giribet
| first1 = G.
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| doi-access = free
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<ref name="Bieler & Mikkelsen 2006">
{{cite journal
| doi = 10.1111/j.1096-3642.2006.00255.x
| last1 = Bieler
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| last2 = Mikkelsen
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| title = Bivalvia&nbsp;– a look at the branches
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<ref name="Mikkelsen Bieler_ 2006">
{{cite journal
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| year = 2006
| title = Phylogeny of Veneroidea (Mollusca: Bivalvia) based on morphology and molecules
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| issue = 3
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<ref name="Taylor Williams_ 2007">
{{cite journal| doi = 10.1111/j.1463-6409.2007.00299.x| last1 = Taylor | first1 = J. D.| last2 = Williams | first2 = S. T.| last3 = Glover | first3 = E. A.| last4 = Dyal | first4 = P.| date=November 2007 | title = A molecular phylogeny of heterodont bivalves (Mollusca: Bivalvia: Heterodonta): New analyses of 18S and 28S rRNA genes| journal = Zoologica Scripta| volume = 36| issue = 6| pages = 587–606| s2cid = 84933022
}}
</ref>

<ref name="Taylor Williams 2009">
{{cite journal
| last1 = Taylor
| first1 = John D.
| last2 = Glover
| first2 = Emily A.
| last3 = Williams
| first3 = Suzanne T.
| year = 2009
| title = Phylogenetic position of the bivalve family Cyrenoididae&nbsp;– removal from (and further dismantling of) the superfamily Lucinoidea
| journal = Nautilus
| volume = 123
| issue = 1
| pages = 9–13
}}
</ref>

<ref name="Bouchet Rocroi_ 2010">
{{cite journal| doi = 10.4002/040.052.0201| last1 = Bouchet| first1 = Philippe| last2 = Rocroi| first2 = Jean-Pierre| last3 = Bieler| first3 = Rüdiger| last4 = Carter| first4 = Joseph G.| last5 = Coan| first5 = Eugene V.| title = Nomenclator of Bivalve Families with a Classification of Bivalve Families| journal = [[Malacologia]]| volume = 52| issue = 2| pages = 1–184| year = 2010| s2cid = 86546840
| url = https://www.biodiversitylibrary.org/part/386428}}
</ref>

<ref name="Tëmkin 2010">
{{cite journal
| doi = 10.1186/1471-2148-10-342
| last = Tëmkin
| first = I.
| year = 2010
| title = Molecular phylogeny of pearl oysters and their relatives (Mollusca, Bivalvia, Pterioidea) |doi-access=free
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| volume = 10
| issue = 1
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}}
</ref>

<ref name="Taylor Glover_ 2011">
{{cite journal
| last1 = Taylor
| first1 = John D.
| last2 = Glover
| first2 = Emily A.
| last3 = Smith
| first3 = Lisa
| last4 = Dyal
| first4 = Patricia
| last5 = Williams
| first5 = Suzanne T.
| date = September 2011
| title = Molecular phylogeny and classification of the chemosymbiotic bivalve family Lucinidae (Mollusca: Bivalvia)
| journal = Zoological Journal of the Linnean Society
| volume = 163
| issue = 1
| pages = 15–49
| doi = 10.1111/j.1096-3642.2011.00700.x
| doi-access = free
}} {{subscription required}}
</ref>

<ref name="ponder_lindberg2008">
{{cite book
|author1=Ponder, W. F. |author2=Lindberg, David R. | year = 2008
| title = Phylogeny and Evolution of the Mollusca
| page = 117
| publisher = University of California Press
| isbn = 978-0-520-25092-5
}}
</ref>

<ref name="Harper Dreyer_ 2006">
{{cite journal
| doi = 10.1111/j.1096-3642.2006.00260.x
| last1 = Harper
| first1 = E. M.
| last2 = Dreyer
| first2 = H.
| last3 = Steiner
| first3 = G.
| year = 2006
| title = Reconstructing the Anomalodesmata (Mollusca: Bivalvia): morphology and molecules
| journal = Zoological Journal of the Linnean Society
| volume = 148
| issue = 3
| pages = 395–420
| doi-access = free
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</ref>

<ref name="Carter 2011">
{{cite journal
|author1=Carter, J. G. |author2=Altaba, C. R. |author3=Anderson, L. C. |author4=Araujo, R. |author5=Biakov, A. S. |author6=Bogan, A. E. |author7=Campbell, D. C. |author8=Campbell, M. |author9=Chen, J. |author10=Cope, J. C. W. |author11=Delvene. G. |author12=Dijkstra, H. H. |author13=Fang, Z. |author14=Gardner, R. N. |author15=Gavrilova, V. A. |author16=Goncharova, I. A. |author17=Harries, P. J. |author18=Hartman, J. H. |author19=Hautmann, M. |author20=Hoeh, W. R. |author21=Hylleberg, J. |author22=Jiang, B. |author23=Johnston, P. |author24=Kirkendale, L. |author25=Kleemann, K. |author26=Koppka, J. |author27=Kříž, J. |author28=Machado, D. |author29=Malchus, N. |author30=Márquez-Aliaga, A. |author31=Masse, J-P. |author32=McRoberts, C. A. |author33=Middelfart, P. U. |author34=Mitchell, S. |author35=Nevesskaja, L. A. |author36=Özer, S. |author37=Pojeta, J. Jr. |author38=Polubotko, I. V. |author39=Pons, J. M. |author40=Popov, S. |author41=Sánchez, T. |author42=Sartori, A. F. |author43=Scott, R. W. |author44=Sey, I. I. |author45=Signorelli, J. H. |author46=Silantiev, V. V. |author47=Skelton, P. W. |author48=Steuber, T. |author49=Waterhouse, J. B. |author50=Wingard, G. L. |author51=Yancey, T. | year = 2011
| title = A synoptical classification of the Bivalvia (Mollusca)
| journal = Paleontological Contributions
| volume = 4
| pages = 1–47
| url = http://kuscholarworks.ku.edu/dspace/bitstream/1808/8287/3/Carter%20ms.pdf
}}
</ref>
}}

== Further reading ==
{{EB1911 Poster|Lamellibranchia}}
* {{cite journal
  | doi = 10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2
  | last = Schneider
  | first = Jay A.
  | year = 2001
  | title = Bivalve systematics during the 20th century
  | volume = 75
  | issue = 6
  | pages = 1119–1127
  | journal = Journal of Paleontology
  | bibcode = 2001JPal...75.1119S
 | issn = 0022-3360
  }}
* {{cite book
  | last1 = Poutiers
  | first1 = J.-M.
  | last2 = Bernard
  | first2 = F. R.
  | year = 1995
  | chapter = Carnivorous bivalve molluscs (Anomalodesmata) from the tropical western Pacific Ocean, with a proposed classification and a catalogue of recent species
  | editor-last = Bouchet
  | editor-first =  P.
  | title = Résultats des Campagnes Musorstom
  | series = Mémoires Muséum National d'Histoire Naturelle
  | volume = 167
  | pages = 107–188
  }}
* {{cite book
  | last = Vaught
  | first = K. C.
  | year = 1989
  | title = A Classification of the Living Mollusca
  | publisher = American Malacologists
  | isbn = 978-0-915826-22-3
  }}

{{Mollusc}}
{{Taxonbar|from=Q25368}}
{{Authority control}}

[[Category:Bivalves| ]]
[[Category:Extant Cambrian first appearances]]
[[Category:Mollusc classes]]
[[Category:Taxa named by Carl Linnaeus]]