nutria koiˈpo͞o [key], aquatic rodent, Myocastor coypus, of South America, introduced in the S United States for its fur, which is similar to that of beaver but not as thick or durable. The nutria resembles a small beaver with a ratlike tail. It is up to 25 in. (64 cm) long, excluding the 15-in. (38-cm) sparsely haired, round tail; it has large reddish incisor teeth and partially webbed hind feet. The outer fur is long, coarse, and brown; it is the soft, gray undercoat that is valued commercially. Descendants of nutrias escaped or released from fur farms are now found in much of the United States, especially in swampy regions. They build burrows in banks, with the entrances above water level, and feed on aquatic vegetation, competing with the native muskrat for food. They have seriously damaged marshland ecosystems in southern Louisiana and, to a lesser degree, around the Chesapeake Bay. Nutrias have also established themselves successfully in Europe. They are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Rodentia, family Capromyidae.

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