Zooplankton is found in almost all major fresh
and marine water bodies.Manyspecies, however,
are restricted to specific territories depending
on conditions of light, temperature, salinity,
chemical composition, and turbulence. Their
range also varies geographically and vertically
within the water column.
Zooplankton Types
The diversity of zooplankton makes classification
difficult. Holoplanktonic zooplankton are freefloating
their entire lives and are mostly invertebrates.
Flagellated, ciliated, and amoeboid protozoa
as well as several species from the animal
phyla Cnidaria, Ctenophora, Chaetognatha, Mollusca,
Annelida, Urochordata, Echinodermata,
and Arthropoda make up the majority of collected
organisms. Meroplanktonic zooplankton are the
free-floating forms of organisms having swimming
nekton or attached benthic stages as part of
their life cycle. They consist mostly of egg and larval
stages of marine benthic invertebrates that include
worms, snails, clams, barnacles, crabs, starfish,
and sea urchins, as well as many marine fish,
such as tuna. Body forms, life cycles, diet, and location
vary among species
Zooplankton are often divided according to
size. The most common divisions and sizes for
zooplankton are nanoplankton (2 to 20 micrometers),
microplankton (20 to 200 micrometers),
mesoplankton (0.2 to 20 millimeters), macroplankton
(2 to 20 centimeters), and megaplankton
(20 to 200 centimeters). Although generally small,
even jellyfish several meters in size are considered
zooplankton because they are unable to swim effectively
against the current.
Copepods are one of the most studied forms
and are classified as holoplanktonic zooplankton.
Copepods have a segmented body with three distinct
sections. Several pairs of legs and large antennae
are used to create feeding currents that
capture phytoplankton, especially diatoms, and
other small zooplankton. Copepods are classified
as mesoplankton and are nearly five millimeters
in size. They are the most important herbivore in
the ocean. Advanced collection techniques, however,
have uncovered the presence of smaller zooplankton
classifications that may be an important
ecological first link.
Ecological Importance
Zooplankton diet varies greatly among species.
There are carnivorous and herbivorous zooplankton, as well as those that feed on nonliving organic
material. Herbivorous forms feed on phytoplankton
through a process called grazing. This process
prevents damaging blooms of phytoplankton from
increasing in numbers. The link between phytoplankton
and zooplankton is also important in
passing the energy fixed by autotrophs to higher
trophic levels of the food chain. Herbivorous zooplankton
are eaten by carnivorous zooplankton,
other invertebrates, or fish. The food chain may
terminate in the top carnivores that include large
fish, birds, and mammals, including humans.
Adaptations
Because of their small size, zooplankton are
very sensitive to water conditions. Maintaining
position in the water column is an important adaptation
for many zooplankton species because
they lack the ability to swim strongly enough to
counteract the water current. Their density is often
slightly greater than that of sea water. In addition,
high salinity and low temperatures increase
the viscosity of surrounding water, making it
harder for zooplankton to move. Alarger surface
area and flattened body form covered by many
projections or spines counteracts these obstacles
and prevents the zooplankton from sinking. Several
zooplankton also increase their buoyancy by
storing low-density materials, such as certain oils.
Small body movements, including the action of
flagellae and cilia, also counteract the sinking motion.
Vertical Migration
The depth at which zooplankton exist is variable.
While some float at the surface of the water, others
may inhabit depths greater than four thousand
meters. The concentration, or biomass, of zooplankton
decreases with increasing depth. Many
zooplankton exhibit a daily rhythmic vertical
movement called diel vertical migration. They
may migrate downward from the water surface
during the day and upward at night. These movements
may avoid predation and may conserve energy
by slowing metabolism in colder deeper waters.
Seasonal migrations from deeper waters in
the winter months to surface waters in the spring
are common. This migration may decrease metabolism
and conserve energy throughout the winter
when there is a lack of food.
Classification:
Kingdom: Protista or Animalia
Phyla: Cnidaria, Ctenophora, Annelida, Nemertea,
Arthropoda, Chaetognatha, Mollusca,
Phoronida, Bryozoa, Echinodermata, Hemichordata,
Chordata, Urochordata
Geographical location: Large bodies of freshwater
and salt water
Habitat: Mainly in surface waters
Gestational period: Variable
Life span: Variable
Special anatomy: Highly variable between different
phyla; generally small with a flattened
body and many projections; zooplankton may
also store oils used for buoyancy
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