The pelage of this opossum is unique among didelphids. The short, dense fur is marbled grey and black. The muzzle, eyestripe, and crown are black, and a light band runs across the forehead anterior to the ears. The tail, furred and black at the base, is yellow or white at its end. The ears are naked and rounded. There are sensory facial bristles in tufts above each eye as well as whiskers. The pisiform bone is enlarged on the wrists of the forefeet and resembels a sixth finger. Head and body length ranges from 11 to 16 inches and tail length from 12 to 17 inches. In addition to dense water-repellent fur, the water opossum has several other adaptations for its aquatic lifestyle. Its hindfeet are webbed and are used for propulsion through water. The elongated hallux makes the hindfoot symmetrical, which distributes force equally along both borders of the webbing; this increases the efficiency of the water opossum's movement through the water. The forefeet are unwebbed, with long, hairless, tactile fingers and reduced claws.

Both sexes have a well-developed pouch. In females a sphincter muscle closes the pouch to create an airtight (and watertight) compartment for the young, who are able to endure several minutes without oxygen. In males the sphincter is not as watertight as that of females. However, it serves to hold the scrotum which is drawn up into it when the animal swims or runs fast.

Chironectes is found in tropical and subtropical areas along fresh-water streams and lakes. It has been reported at high elevations.

Biomes: tropical rainforest, tropical deciduous forest

Neotropical: Chironectes minimus , the only member of the genus, ranges from southermost Mexico through Central America, Colombia, Venezuela, Guianas, Ecuador, Peru, Paraguay, eastern Brazil, and northeastern Argentina.

Females of Chironectes minimus are polyestrous. Precopulatory behavior includes close association between male and female. The male circles or follows the female and oral-genital contact has been observed. The male pulls the female toward himself when he mounts her. In Brazil, partruition occurs in December and January. One opossum with five young was found in February. Litters generally contain one to five young; the average is 3-4.

Hair appears on the young 22 days after birth, body pigmentation 28 days after birth, and color bands 34 days after birth. The eyes open on day 38 and by day 40 the young are too big to fit into the pouch; their heads remain inside and their bodies protrude. Forty-eight days after partruition the young begin to detach from the nipples, but they still nurse, sleep next to their mother, and sometimes climb onto her back.

The water opossum eats primarily crayfish, shrimp, fish, frogs, and possibly aquatic vegetation and fruit. It propels its streamlined body through the water with its hind feet and uses its sensitive, tactile hands to catch its animal prey underwater. Adults of this species show agonistic behavior when they feed.

Individuals of this species are solitary and antisocial. They are sporadically active at night, showing no apparent circadian rhythm. Generally the water opossum lives in a subterranean den enterred through an opening just above the water line of a stream bank. An excavated den revealed an entrance 4 inches above the water level, a tunnel that descended 1.9 feet at a 45 degree angle, and a terminal enlarged nest chamber. During the day water opossums use an external ground nest made of grass or leaves. They gather nesting materials with their forefeet, push the grasses under their bodies, and grasp the load with their tails to transport it back to the den.

This animal is not listed as endangered. It is considered rare but it is unclear whether this notion is simply a result of its nocturnal lifestyle and relatively inaccessible habitats or if, in fact, Chironectes is uncommon. Grimwood reported in 1969 that the skins of this animal were acquiring monetary value in Peru; he suggested protection measures.

Holocene subfossil fragments of Chironectes have been discovered in Sao Paulo, Brazil. Also, there are specimens from late Pleistocene-Recent cave deposits in Minas Gerais, Brazil and from the late Pliocene from Entre Rios Province, Argentina.

Nowak, Ronald. Walker's Mammals Of The World. Baltimore: Johns Hopkins University Press, 1999