The marine realm

While seals and possibly sirenians may be included in descriptions of the conventional biogeographic regions, this is not usually the case with cetaceans. It seems worthwhile to consider all the above in a marine realm for the sake of completeness. Marine mammals are distributed from the Arctic Ocean to the Antarctic and occur in the deep ocean, coastal waters, estuaries, and in a few cases in rivers and lakes. Two orders are fully aquatic, Cetacea (whales and dolphins) and Sirenia (manatees and dugongs). The third order, the Pinni-pedia, is sometimes classified as part of the Carnivora. It consists of three families, seals, sea lions and fur seals, and walruses. They are mainly aquatic but haul out onto land or ice to rest, mate, and give birth.

The 76 species of whales and dolphins collectively have a worldwide distribution. Several species have extensive individual ranges in all the major oceans. Freshwater species of dolphins live in the Amazon, Yangtze, Indus, and Ganges Rivers. The four living species of sirenians are found in coastal waters, estuaries, and rivers mainly in the tropical and subtropical zones. Steller's sea cow (Hydrodamalis gigas), the only cold-water adapted species, formerly lived in the Bering Sea but was hunted to extinction by 1768.

There are 33 extant species of pinnipeds, mainly distributed in temperate and polar waters but a few species are found in the tropics. The 14 species of fur seals and sea lions are mainly distributed in the Pacific and southern oceans. One species is endemic to the Galápagos Islands, and two more have restricted distributions on Juan Fernandez and nearby islands and islands off California. The walrus (Odobenus ros-marus), the only member of the family Odobenidae is an inhabitant of the Arctic. The 19 species of so-called true seals, Phocidae, are predominantly distributed in northern and southern waters. Six species occur in Arctic and subarctic waters and four in the Antarctic. The Hawaiian monk seal (Monachus schauinslandi) is endemic to those islands. Isolated species occur in the Caspian Sea (Phoca caspica) and in the freshwater of Lake Baikal in Siberia (P. sibirica).

In addition, the polar bear (Ursus maritimus) and the sea otter (Enhydra lutris) of western North America also spend much of their lives in a marine environment. Taken together, marine mammals amount to only about 2.5% of all mammal species, despite the fact that seas and oceans cover a greater proportion of the surface of the earth than land does. This reflects the relative homogeneity of the environment and a lack of natural barriers that allow species to evolve in isolation.

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