Sea otter
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Order: Carnivora
Family: Mustelidae

[Photo] Sea Otter (Enhydra lutris). Mother Sea Otter with pup at Morro Rock, Feb. 12, 2007 - photo by Mike Baird (www.bairdphotos.com). "A sea otter wraps itself in kelp in Morro Bay, California."

The sea otter (Enhydra lutris) is a marine mammal native to the coasts of the North Pacific, from northern Japan, the Kuril Islands and Kamchatka east across the Aleutian Islands and along the North American coast to Mexico.

Adult sea otters typically weigh between 14 and 45 kg (30 to 99 lb), making them the heaviest members of the weasel family, but among the smallest marine mammals. Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom with up to 400,000 hairs per cm2 (2.5 million per in2). It inhabits nearshore environments where it can quickly dive to the sea floor to forage. Although sea otters can walk on land, they are capable of spending their entire lives in the ocean.

The sea otter preys mostly upon marine invertebrates such as sea urchins, diverse mollusks and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects: Its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems. Kelp forests provide crucial food and habitat for a variety of marine organisms, and help contain coastal erosion. Finally, its diet includes prey species that are also valued by humans as seafood, which has led to conflicts with fisheries.

Sea otters, whose numbers were once estimated at 150,000–300,000, were hunted extensively for their fur between 1741 and 1911, and the world population fell to 1,000–2,000 individuals in a fraction of their historic range. A subsequent international ban on hunting, conservation efforts, and reintroduction programs into previously populated areas have all contributed to numbers rebounding in about two-thirds of the former range. The recovery of the sea otter is considered an important success in marine conservation, though populations in certain areas (the Aleutian Islands and California) are declining or have plateaued at depressed levels. For these reasons, as well as its particular vulnerability to oil spills, the sea otter remains classified as an endangered species.

Taxonomy
The first scientific description of the sea otter is contained in the field notes of Georg Steller from 1751, and the species was described by Linnaeus in his Systema Naturae of 1758. The generic name Enhydra, derives from the Ancient Greek en/εν 'in' and hydra/ύδρα 'water', meaning "in the water", and the Latin word lutris is meaning "otter." It was formerly sometimes referred to as the "sea beaver", although it is only distantly related to beavers. It is not to be confused with the marine otter, a rare otter species native to the west coast of South America.

Evolution
The sea otter is the heaviest member of the family Mustelidae, a diverse group that includes the thirteen otter species and terrestrial animals such as weasels, badgers and minks. It is unique among the mustelids in not making dens or burrows, in having no functional anal scent glands, and in being able to live its entire life without leaving the water. The only member of the genus Enhydra, the sea otter is so different from other mustelid species that as recently as 1982, some scientists believed it was more closely related to the earless seals. Genetic analysis indicates that the sea otter's closest relatives are the Eurasian otter, spotted-necked otter, African clawless otter and small-clawed otter.

The sea otter lineage diverged from the other Old World otters approximately 13 million years ago (mya). Fossil evidence indicates that the Enhydra lineage became isolated in the North Pacific approximately 2 mya, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter, Enhydra lutris. The sea otter evolved initially in northern Hokkaido and Russia, then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, who entered the water approximately 50 mya, 40 mya, and 20 mya, respectively, the sea otter is a relative newcomer to a marine existence. In some respects, however, the sea otter is more fully aquatically adapted than pinnipeds, which must haul out on land or ice to give birth.

Subspecies
There are three recognized subspecies, which vary in body size and in some skull and dental characteristics:

- The common sea otter, E. l. lutris (Linnaeus, 1758), ranges from the Kuril Islands to the Commander Islands in the western Pacific Ocean. It is also known as the Asian sea otter. It is the largest subspecies with a wide skull and short nasal bones.

- The southern sea otter, E. l. nereis (Merriam, 1904), is found off the coast of central California. It is also known as the Californian sea otter. It has a narrower skull with a long rostrum and small teeth.

- The northern sea otter, E. l. kenyoni (Wilson, 1991), also known as the Alaskan sea otter, is native to the Aleutian Islands and mainland Alaska, but has since been re-introduced to various locations from Alaska to Oregon. While intermediate between the other subspecies in most characteristics, it has longer mandible bones.

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