plankton

plankton are the tiny animals (zooplankton) and marine plants (phytoplankton) that drift freely in the waters of the oceans. The herbivorous zooplankton graze on the phytoplankton, either sieving the cells from the water, or trapping them on sheets of mucus, or feeding on individual cells. Carnivorous zooplankton feed on the herbivores. Zooplankton range in size from microscopic single-celled protozoans (ciliates, foraminifers, and radiolarians) to large crustaceans like copepods and krill. They are all consumed by larger predators, some of which, like fish larvae and arrow worms (Chaetognatha), are quite small, but others, like baleen whales (Balaenoptera spp.) and basking sharks (Cetorhinus maximus), are large. Many are permanently members of the plankton, but many are larvae that mature into larger adults and live in deeper water, or even on the seabed. Many of the larger planktonic species undertake extensive vertical migrations over 300 metres (1,000 ft) each day, commuting between the safety of deep water during the day and the food-rich upper layers at night. Larger planktonic species are sampled by towing fine-meshed nets, but the very small species are collected by filtering large volumes of water through fine membranes. There are very many fragile gelatinous species that are hard to sample and study, because they disintegrate either in the nets or when preserved.

The fossil remains of diatoms, microscopic marine plants, are used for keeping dynamite stable, for making luminous paint, and for filtering wine. See also phosphorescence.

Bibliography

Hardy, A. , The Open Sea. Its Natural History: The World of Plankton (1956).

M. V. Angel