River Otters can be thought of - and in a very real sense are - semi-aquatic weasels. Like fishers, martens, and mink, they have long, slender bodies, short limbs, and a short face, plus a set of adaptations for their aquatic lifestyle: an oily, waterproof coat, webbed toes, and small external ears. River Otters are good swimmers and divers, able to stay underwater for up to eight minutes. They feed on crayfish, crabs, fish, birds, small mammals, and some aquatic plants. They once lived in streams, rivers, lakes, swamps, and coastal areas throughout Canada and the United States. Now they are gone from the central and eastern United States, and extinct or rare in Arizona, Colorado, Indiana, Iowa, Kansas, Kentucky, Nebraska, New Mexico, North Dakota, Ohio, Oklahoma, South Dakota, Tennessee, Utah, and West Virginia. Scientific studies have shown them to be sensitive to pollution. Still these animals are commercially harvested: 20,000 - 30,000 are taken annually for their lustrous fur.

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account