Marine Fauna

animals that inhabit seas and oceans. There are about 160,000 species, including approximately 10,000 Protozoa (Foraminifera, Radiolaria, Flagellata, and Infusoria), about 5,000 Porifera, about 9,000 Coelenterata, more than 7,000 Polychaeta and other worms, more than 4,000 Brachiopoda and Bryozoa, more than 80,000 Mollusca, more than 20,000 Crustacea, 6,000 Echinodermata, approximately 1,000 Tunicata, about 16,000 fishes, and about 150 species of mammals and reptiles. Of the 60 classes of extant nonparasitic animals, representatives of only three classes—Onychophora, Myriapoda, and Amphibia—are not found in the seas.

The origins of all animal phyla can be traced back to the sea. Some marine animals subsequently transferred to life in fresh water and on dry land, giving rise to freshwater and terrestrial fauna. Some vertebrates that returned to the marine environment have retained their ties to land, leaving the sea for reproduction (pinnipeds and sea turtles). Some birds, such as penguins and albatrosses, are permanently bound to the ocean. The most diverse marine fauna is that of tropical shallows, particularly near coral reefs, which serve as habitats for numerous mollusks, crabs, echinoderms, and fishes. As depth increases, marine fauna grows sparser. Only a few dozen invertebrate species have adapted to life at maximum depths (over 9–10 km). The marine fauna of shallow coastal regions of temperate and cold waters are characterized by the greatest biomass.

According to habitat and way of life, marine animals are classified as being either pelagic (plankton and nekton) or benthic (benthos). Characteristic representatives of marine zoo-plankton are some foraminifers, some radiolarians, some tintinnids, siphonophores, medusae, ctenophores, copepods, euphausids, pteropods, salpids, and the larvae of many pelagic and benthic animals. The principal mass of nekton is composed of fishes and cephalopod mollusks; cetaceans are less numerous. Special communities of animals that swim on the surface of the sea, or pleustons, are distributed primarily in the tropics; such surface animals include the siphonophore Velella, goose barnacles, and organisms that live among floating algae (especially Sargassum). In polar seas a unique community, or cryopelagic biocenosis, develops near the undersurface of marine ice; this community includes diatomaceous algae, amphipods, and fish fry.

The animals that predominate among the benthic population are foraminifers, poriferans, hydroids, pennatularians, various corals, polychaete worms, acorn barnacles, amphipods, isopod and decapod crustaceans, gastropod and bivalve mollusks, echinoderms, Pogonophora, ascidians, and fishes. Benthic marine animals are classified as inhabitants of the littoral, sublittoral (200 m), bathyal (2,000–3,000 m), abyssal (6,000–7,000 m), and abyssalbenthic (7,000–11,000 m) zones. The vertical zonation of marine fauna that inhabit the pelagic zone is more difficult to pinpoint because many pelagic animals migrate vertical distances up to several hundred meters (sometimes more than 1,000 m). The pelagic zone may be divided into surface (to 200 m), intermediate (200–750 m or 200–1,000 m), and deep-sea areas.

Large, rapidly swimming animals, such as whales, pinnipeds, many fishes, and squid, are able to travel distances of many hundreds or thousands of kilometers and to make regular horizontal migrations, primarily from their feeding places to reproductive sites and back. Some fishes spend most of their lives in the sea and migrate to rivers for spawning (anadromous migrations); others migrate from rivers to the sea for breeding (catadromous migrations). Because marine animals are generally unable to tolerate substantial decreases in salinity, they are less diverse in seas with low salinity, such as the Black, Azov, and Baltic seas, than in oceans and seas with normal salinity (about 3.5 percent). The adaptations of animals to life in the sea are very diverse. They include development from a free-floating plank-tonic larva to ensure distribution, adaptations for soaring in the water in many planktonic organisms and for rapid swimming in many ocean fishes, squid, and cetaceans, to the development of luminescent organs in many deep-sea inhabitants.

The economic significance of marine fauna is extremely great. In 1970, the world catch of marine animals was more than 60 million tons, including more than 53 million tons of fish, 3 million tons of mollusks (oysters, mussels, scallops, squid), and 1.5 million tons of crustaceans (crabs, lobsters, shrimp). Some marine animals are destructive because they adhere to the bottoms of ships and to underwater structures.