water opossum (Chironectes minimus), also called yapok or yapock, a semiaquatic, web-footed marsupial (family Didelphidae, subfamily Didelphinae) found along tropical rivers, streams, and lakes from Mexico to Argentina. Adults average 70 cm (28 inches) in total length and weigh up to 790 grams (1.7 pounds). A pouch is present in both sexes, but only in the female can it be closed to keep the young dry. The fur is short and dense with a few intermixed longer hairs. The colour of the back is silvery gray overlaid by dark brown to black markings on the shoulders, back, hips, and rump, interconnected by a narrow black stripe running from the back of the head to the tail. The entire ventral surface is silvery white. The face and ears are blackish brown, except for the silvery white cheek and spot above each eye. The tail is a little longer than the head and body, densely furred at the base and scaly, nearly naked, and dark brown or black for most of its length except at the tip, which is yellowish white.
Water opossums eat fish, crabs, frogs, freshwater shrimp, and other small animals found in and along watercourses. It is not known when the breeding season is, but females have been found with two to five young in the pouch in July, December, January, February, and March. The life span in the wild is unknown, but one water opossum lived 2 years 11 months in captivity.