Pelagic invertebrates are abundant and have an important key role in the marine food chain. Although some invertebrate animals living in the pelagic zone of oceans are microscopic, bigger species include jellyfish, and crustaceans, such as crabs, lobsters and prawns. Invertebrate animals that are not active swimmers and move by floating in the water are called zooplankton.
Fish are the most numerous group of pelagic animals, often choosing to live in lit areas close to the surface, where food is more abundant. However, some species have adapted over time to live in the darker high-pressured environment found at more than 700-foot depths, including the brownsnout spookfish (Dolichopteryx longipes) and the swordfish (Xiphias gladius). Highly adapted fish, such as the humpback anglerfish (Melanocetus johnsonii), live at about 4,000 feet deep.
Part of the order Cetacea, common pelagic mammals include dolphins, porpoises and whales. As other mammals, they depend on atmospheric oxygen to live, thus living close to the surface. Seven species of whale, including the North Atlantic right whale, the Western Pacific gray whale, the bowhead and the narwhal, are endangered today.
Pelagic reptiles include some species of sea snakes of the genus Laticauda and sea turtles of the family Cheloniidae, such as the endangered loggerhead (Caretta caretta) and green sea turtle (Chelonia mydas). Sea turtles have a salt gland at the eye or in the tongue, an adaptation that helps them to regulate the internal mineral balance.