The coypu, or nutria (Myocastor coypus) is a large, herbivorous, semiaquatic rodent and the only member of the family Myocastoridae. Originally native to temperate South America, it has since been introduced to North America, Europe, Asia and Africa, primarily by fur ranchers. Although it is still valued for its fur in some regions, its destructive feeding and burrowing behaviors make this invasive species a pest throughout most of its range.
There are two commonly-used names in the English language for Myocastor coypus. The name nutria (or local derivatives such as "nutria- or nutra- rat") is generally used in North America and Asia; however, in Spanish-speaking countries, the word nutria refers to the otter. To avoid this ambiguity, the name coypu (derived from the Mapudungun word kóypu) is used in Latin America and Europe. In France, the coypu is known as a ragondin. In Dutch it is known as beverrat (beaver rat).
Four subspecies are generally recognized:
M. c. bonariensis, the subspecies present in the northernmost (subtropical) part of the coypu's range, is believed to be the type of coypu most commonly introduced to other continents.
The coypu somewhat resembles a very large rat in appearance. Adults are typically in weight, and in body length, with a tail. They have a coarse, darkish brown outer fur with a soft under-fur. Two distinguishing marks are the presence of a white patch on the muzzle, and webbed hind feet. They can also be identified by their bright orange-yellow incisor teeth (unlike rats, which have brownish yellow incisors). The nipples of female coypu are high on her flanks. This allows their young to feed while the female is in the water.
Coypu can also be mistaken for another widely dispersed semi-aquatic rodent that occupies the same wetland habitats, the muskrat. The muskrat, however, is smaller, more tolerant of cold climates, and has a laterally flattened tail that it uses to assist in swimming, whereas the tail of a coypu is round.
They are herbivorous, feeding on river plants, and live in burrows alongside stretches of water.
As demand for coypu fur declined, coypu have since become pests in many areas, destroying aquatic vegetation, irrigation systems, chewing through human-made items, such as tires and wooden house panelling in Louisiana, eroding river banks, and displacing native animals. Coypu were introduced to Louisiana in the 1930s for fur, for example, and nutria damage in Louisiana became so severe that in 2005, a bounty program was in effect to aid in controlling the animal. In the Chesapeake Bay region in Maryland, where they were introduced in the 1940s, coypu are believed to have destroyed of marshland in the Blackwater National Wildlife Refuge. In response, by 2003, a multi-million dollar eradication program was underway.
Coypu meat is lean and low in cholesterol. While there have been many attempts to establish markets for coypu meat, all documented cases have generally been unsuccessful. Unscrupulous entrepreneurs have promoted coypu and coypu farms for their value as "meat", "fur", or "aquatic weed control". In recent years they have done so in countries such as the United States, China, Taiwan and Thailand. In every documented case the entrepreneurs sell coypu "breeding stock" at very high prices. Would-be coypu farmers find that the markets for their products disappear after the promoter has dropped out of the picture.
In the former Soviet republics of Central Asia, specifically Kyrgyzstan and Uzbekistan, Nutria (Russian and local languages Нутриа) are farmed on private plots and sold in local markets as a poor man's meat.
In addition to direct environmental damage, coypu are the host for a nematode parasite (Strongyloides myopotami) that can infect the skin of humans. When this happens the condition is called "nutria itch."