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.
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.
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.
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.
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.
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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