Although most mammals are terrestrial, many species inhabit the world’s oceans and seas, including seals, otters, whales and dolphins. As air-breathing mammals, these creatures must return to the water's surface often to vent carbon dioxide and acquire more oxygen. Dolphins and whales have blowholes that facilitate this process, while seals and most other marine mammals breathe through their noses after surfacing.
Many marine mammals are gentle creatures that eat aquatic plants or filter small creatures from the water. For example, dugongs and manatees consume more than 60 plant species, including water hyacinth, sea grass and pickerel weed. The largest creature the world has ever seen, the blue whale, eats krill, which are among the smallest creatures in the ocean. Other marine mammals are aggressive predators, such as killer whales, which consume sharks, dolphins, fish and other whales, and leopard seals, which eat fish, penguins and other seals. Some marine mammals, such as California sea otters, use rocks to break open bivalve mollusks. The otters swim to the surface, roll over on their backs, place the clam on their chest and pound it with a rock that is held in their hands.
Because they are such excellent swimmers and spend so much of their life at sea, polar bears are considered by some scientists to be marine mammals. Polar bears primarily hunt seals, although they eat virtually any animal they can catch.