The water opossum (Chironectes minimus), also locally known as the yapok (), is a marsupial of the family Didelphidae. It is the only monotypic species of its genus, Chironectes. This semiaquatic creature is found in and near freshwater streams and lakes from Mexico through Central and South America to Argentina and is the most aquatic living marsupial (the lutrine opossum also has semiaquatic habits). It is also the only extant marsupial species in which both sexes have a pouch. The now extinct thylacine, commonly referred to as the Tasmanian tiger, also exhibited this trait.
The local name for the water opossum, "yapok", may come from the name of the Oyapock River in French Guiana.
Physical appearance
thumb|Skull of a water opossum
The water opossum is small, measuring long, with a long tail. The fur is in a marbled grey and black pattern, while the muzzle, eyestripe, and crown are all black. A light band runs across the forehead anterior to the ears, which are rounded and hairless. There are sensory facial bristles in tufts above each eye, as well as whiskers. The water opossum's tail, furred and black at the base, is yellow or white at its end. The hind feet of the water opossum are webbed, while the forefeet ("hands") are not. The forefeet can be used to feel for and grab prey as the animal swims, propelled by its tail and webbed back feet. Unlike other didelphids, the water opossum does not have a cloaca. thumb|Drawing of a water opossum
Aquatic adaptations
The water opossum has several adaptations for its watery lifestyle. It has short, dense fur, which is water-repellent. The broad hind feet are webbed and are used for propulsion through water, moving with alternate strokes. They are symmetrical as well, which distributes force equally along both borders of the webbing; this increases the efficiency of the water opossum's movement through the water. The water opossum's long tail also aids in swimming.
Being a marsupial and at the same time an aquatic animal, the water opossum has evolved a way to protect its young while swimming. A strong ring of muscle makes the pouch (which opens to the rear) watertight, so the young remain dry, even when the mother is totally immersed in water.
Holocene subfossil fragments of Chironectes have been discovered in São Paulo, Brazil. Also, there are fossil specimens from the late Pleistocene–recent cave deposits in Minas Gerais, Brazil, as well as from the late Pliocene in Entre Ríos Province, Argentina.
Subspecies
- Chironectes minimus argyrodytes <small>Dickey, 1928</small> - southern Mexico, Belize, Guatemala, El Salvador, Honduras, and northern Nicaragua