The oceans are the largest animal habitat on Earth, and most species are found along the continental margins. Although the continental shelves underlie only about 7.5 percent of the total area of the oceans, their shallow waters support more forms of life than any other area in the ocean—perhaps any other place on Earth. Scientists estimate that as many as 30,000,000 species of sea life may still be undiscovered. The types of animals found along the upper continental slope depend on whether the floor is sandy, muddy, or rocky. The lower slope and rise are home to fewer animals.

Animals that live in the sea have developed ways to cope with its high salinity. Naked (covered with thin "skins" or shells) animals maintain high levels of salt in their blood normally and do not need to expel any excess. Others, such as most fish, have special organs...