Pelagic Organism

a plant or animal that inhabits the open sea and the surface of a lake, sea, or ocean—the pelagic zone. Pelagic organisms are contrasted with benthic ones, that is, benthos. They are divided into organisms that passively float on the surface of the water (pleuston) or in its depths (plankton) and organisms that swim (nekton). A distinction is also made between holopelagic organisms, which inhabit the pelagic zone throughout their entire lifetime, and meropelagic organisms, which dwell in the zone only temporarily. The latter include the planktonic larvae of benthic animals and adult benthic animals that float up to the pelagic zone during reproduction.

Many pelagic organisms have adapted similarly to life in the pelagic zone. For example, they are characterized by adaptations that make possible buoyancy and motility. Buoyancy is made possible by the gas bubbles of algae, the gas chambers of siphono-phores, the swim bladder of fish, and the water-filled, gelatinous tissues of coelenterates and tunicates. An abundance of fat in the cells and tissues of pelagic organisms also provides for buoyancy. Motility is made possible by the cilia of protozoans and many larvae, the fins of fish and cephalopods, and the torpedolike body of many nektonic animals.

Pelagic plants (phytoplankton) are the principal producers of organic matter in bodies of water, providing food (directly or though food chains) for aquatic animals. The skeletal remains of pelagic organisms form part of the ocean’s benthic sediments (diatomaceous, radiolarian, foraminiferal, and pteropodous oozes).