Nekton, the assemblage of pelagic animals that swim freely, independent of water motion or wind. Only three phyla are represented by adult forms. Chordate nekton include numerous species of bony fishes, the cartilaginous fishes such as the sharks, several species of reptiles (turtles, snakes, and saltwater crocodiles), and mammals such as the whales, porpoises, and seals. Molluscan nekton include the squids and octopods. The only arthropod nekton are decapods, including shrimps, crabs, and lobsters.
Herbivorous nekton are not very common, although a few nearshore and shallow-water species subsist by grazing on plants. Of the nektonic feeding types, zooplankton feeders are the most abundant and include, in addition to many bony fishes, such as the sardines and mackerel, some of the largest nekton, the baleen whales. The molluscans, sharks, and many of the larger bony fishes consume animals bigger than zooplankton. Other fishes and most of the crustaceans are scavengers.
Nektonic species are limited in their areal and vertical distributions by the barriers of temperature, salinity, nutrient supply, and type of sea bottom. The number of nektonic species and individuals decreases with increasing depth in the ocean.