Also see Sea Otter
Otters are aquatic or marine carnivorous mammals, members of the large and diverse family, Mustelidae, which also includes weasels, polecats, badgers and others. There are 13 species of otter in 7 genera, with a distribution that is almost worldwide.
Otters have a dense layer 1,000 hairs/mm² (~650,000 hairs/in²) of very soft underfur which, protected by their outer layer of long guard hairs, keeps them dry under water and traps a layer of air to keep them warm. Unlike most marine mammals (seals, for example, or whales), otters do not have a layer of insulating blubber, and even the marine Sea Otter must come ashore regularly to wash its coat in fresh water.
Fish is the primary item in the diet of most otters, supplemented by frogs, crayfish, and crabs; some have become expert at opening shellfish, and others will take any small mammals or birds that happen to be available. To survive in the cold waters where many otters live, the specialized fur is not enough: otters have very high metabolic rates and burn up energy at a profligate pace: Eurasian Otters, for example, must eat 15% of their body weight a day; Sea Otters, 20 to 25%, depending on the temperature. In consequence, otters are very vulnerable to prey depletion: in water as warm as 10°C an otter needs to catch 100 g of fish per hour: less than that and it cannot survive. Most species hunt for 3 to 5 hours a day; nursing mothers up to 8 hours a day.
All otters have long, slim, streamlined bodies of extraordinary grace and flexibility, and short limbs; in most cases the paws are webbed. Most have sharp claws to grasp prey but the Short-clawed Otter of southern Asia has just vestigal claws, and two closely related species of African otter have no claws at all: these species live in the often muddy rivers of Africa and Asia and locate their prey by touch.
The North American River Otter (Lutra canadensis) was one of the major animals hunted and trapped for fur in North America after contact with Europeans. They are playful and active, making them a popular exhibit in zoos and aquaria, but unwelcome on agricultural land because they alter river banks for access, sliding, and defense. River otters eat a variety of fish and shellfish, as well as small land mammals and birds. They are 3 to 4 feet (1 m) in length and weigh from 10 to 30 pounds (5 to 15 kg). They were once found all over North America, but are rare or extinct in most places, although flourishing in some locations.
The Sea Otter Enhydra lutris is found along the Pacific coast of North America. Their historic range included shallow waters of the Bering Strait and Kamchatka, and as far south as Japan. Sea otters have 1 million hairs per square inch of skin, a rich fur for which they were hunted almost to extinction. By the time they were protected under the 1911 Fur Seal Treaty, there were so few sea otters left that the fur trade had become unprofitable.
They eat shellfish and other invertebrates, and are frequently observed using rocks as crude tools to smash open shells. They are 2.5 to 6 feet (1 to 2 m) in length and weigh 25 to 60 pounds (30 kg). Although once near extinction, they have begun to spread again starting from the California coast.
Otters are also found in Europe.