Color-enhanced micrograph of the crustacean zooplankton Bosmina longicornis (Christian Gautier/Photo Researchers, Inc.)
amount of air in their bodies, thus getting the nutrients they require and avoiding becoming food for predators. other plankton utilize their transparency or live in large schools of similar organisms to avoid being eaten.
Phytoplankton are microscopic floating photo-synthetic organisms that form an extremely important part of the biomass and food chain. Examples include diatoms, a type of algae that secretes walls of silica; cyanobacteria, photosynthetic bacteria that have been on the Earth for at least 3.8 billion years; and dinoflagellates, flagellate protists that exhibit characteristics of both plants and animals. Coccolithophores are one-celled, floating plants covered with an armor of small calcareous plates. silicoflagellates are similar but have plates made of silica.
Zooplankton comprises a huge variety of protozoans and small metazoans that exhibit a wide range of temperature and salinity tolerances. some zooplankton are holoplanktonic, meaning they remain free-floating throughout their lives. In addition to phytoplankton, this group includes zooplankton such as the extremely important foraminifera that produce calcium carbonate tests and radiolaria that produce silica tests, as well as tunicates, tiny jellyfish, and copepods. other zooplankton are considered meroplankton, meaning they spend only part of their lives as plankton, then they join either the benthic or nektonic realm. Meroplankton are common in coastal waters and include most fish eggs and the eggs and larvae of other marine animals such as marine worms or crustaceans (arthropods with stiff, chitinous outer shells or skeletons including lobsters, shrimp, crabs, and krill).
Gelatinous plankton such as jellyfish include the siphonophores that paralyze prey with stinging cells made of barbs attached to poison sacs. The sipho-nophores are colonies of animals that live together but function as a single animal. Ctenophores resemble jellyfish and have trailing tentacles, used to trap prey. They are carnivorous and may occur in large swarms, greatly reducing local populations of crustaceans and small or young fish. Tunicates are primitive planktonic creatures with backbones inside a barrel-shaped gelatinous structure.
Nekton are pelagic animals that move through the water primarily by swimming. Nektons are distinguished from other pelagic organisms (plankton) that float in the water. The most important nektons in the water today are the fish, whereas in the Paleozoic several other forms were common. The ammonoids of the Devonian were coiled cephalopod mollusks that evolved from earlier nautilids, and these existed with the free-swimming, scorpion-like eurypter-ids. Fish first appeared in the marine record in the Cambrian-Ordovician, and included the early bony-skinned fish known as ostracoderms, followed in the Late Silurian by the finned acanthodians. Heavily armored large-jawed fish known as placoderms are found in many Late Devonian deposits, as are lung fish, ray-finned fish, and lobe-finned fish that include the coelacanths, one species that survives to this day. Lobe-finned fish are the ancestors of all terrestrial vertebrates. sharks were very common in the marine realm by the Late Paleozoic.
See also benthic, benthos; biosphere; fossil; ocean basin; passive margin.
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