The world's oceans are teeming with fish, ranging from the tiny Paedocypris progenetica at 1/4-inch long to the massive whale shark, which can measure up to 65 feet in length. Many fish live in an the "photic zone," an area about 30 to 100 feet from the surface where phytoplankton, a main food source, are found in abundance.
Marine mammals include whales, dolphins, seals, sea lions, walruses, sea otters and manatees. Like their land counterparts, marine mammals are warm-blooded, have body hair and give birth to live young. Some marine mammals, like sea lions and walruses, spend some of their time on land, gathering in large numbers on beaches and around harbors. Marine mammals often have large amounts of body fat, called blubber, to help insulate them from the ocean's cold temperatures. Whale blubber was once commonly used to create heating oil, while hunters of days past prized seal and otter hides for their warm fur.
Crustaceans comprise nearly 40,000 aquatic species, including lobsters, shrimp, crabs and barnacles. Crustaceans make up 19 percent of the species found in the world's oceans, according to the 2010 Census of Marine Life. Some, like the tiny krill, are a significant food source for other ocean animals like baleen whales. Others, like lobsters and crabs, have given rise to large commercial fishing operations around the world. Crustaceans generally have a hard shell, called an exoskeleton, jointed appendages and three body regions: the head, thorax and abdomen.
Coral reefs cover a small portion of the world's oceans and are often located in shallow tropical waters near large land masses, but they are home to nearly a quarter of the Earth's marine life. Reefs are built by individual corals, which are part of a class of simple animals called anthozoans. There are about 6,000 known anthozoan species, which include sea anenomes and sea fans, and about 2,500 corals. Corals and other anthozoans are largely immobile, and live in colonies that are attached to the sea floor. Other types of invertebrate sea life include mollusks like octopuses, squids, sea slugs and jellyfish.
