DEFINITION OF PLANKTON

Aquatic organisms that float, drift freely, or

swim lightly. These include a large variety of species in the

marine realm, including bacteria, phytoplankton (one-celled

plants), and zooplankton that are animals such as jellyfish,

invertebrates, as well as numerous non-marine aquatic

species. Planktonic species are contrasted with nektonic

organisms, which are strong swimmers, and benthic organisms

that are bottom-dwellers.

Planktonic species tend to be small and lack strong skeletons,

and they utilize the density of surrounding water to support

their dominantly water-filled bodies. Many types sink or

float to specific depths or levels where light and salinity characteristics

meet their needs. They move vertically by changing

the amount of air in their bodies, thus getting the nutrients

they require, and avoiding becoming food for predators.

Other plankton utilize their transparency to avoid being

eaten or live in large schools of similar organisms.

Phytoplankton are dominantly microscopic one-celled

floating plants that form an extremely important part of the

biomass and food chain. Diatoms are the most abundant,

secreting walls of silica, and dinoflagellates exhibit characteristics

of both plants and animals. Coccolithopores are onecelled

floating plants covered with an armor of small

calcareous plates, whereas silicoflagellates are similar but

have plates made of silica.

Zooplankton have a wide range of temperature and

salinity tolerances and include a huge variety of species.

Holoplankton are those that remain free-floating throughout

their life stages, whereas meroplankton include the larval

stages of dominantly benthic organisms. Holoplankton

include the extremely important foraminifera and radiolaria

that live throughout the oceans, producing calcium carbonate

and silica tests, respectively. The Crustacean (insects of the

sea) include arthropods with stiff chitinous outer shells. They

include cocopods and euphausiids (krill) that form the dominant

food for many fish and whale species. Meroplankton are

common in coastal waters and include the eggs and sperm of

many benthic animals and fish larvae and tend to be freefloating

for a few weeks.

Gelatinous plankton such as jellyfish include the siphonophores

that paralyze prey with stinging cells made of barbs

attached to poison sacs. The siphonophores 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.

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