Trionychidae

Trionychidae is a family of turtles, commonly known as softshell turtles or simply softshells. The family was described by Leopold Fitzinger in 1826. Softshells include some of the world's largest freshwater turtles, though many can adapt to living in highly brackish waters. Members of this family occur in Africa, Asia, and North America, with extinct species known from Australia. Most species have traditionally been included in the genus Trionyx, but the vast majority have since been moved to other genera. Among these are the North American Apalone softshells that were placed in Trionyx until 1987.

Turtles of the family Trionychidae are called "softshell" because their carapaces lack horny scutes (scales), though the spiny softshell, Apalone spinifera, does have some scale-like projections, to which its common name refers. The carapace is leathery and pliable, particularly at the sides. The central part of the carapace has a layer of solid bone beneath it, as in other turtles, but this is absent at the outer edges. Some species also have dermal bones in the plastron, but these are not attached to the bones of the shell. The light and flexible shells of these turtles allow them to move more easily in open water or on muddy lake bottoms. Having a soft shell also allows them to move much faster on land than most turtles. Their feet are webbed and three-clawed, hence the family name Trionychidae, which means "three-clawed". The carapace color of each type of softshell turtle tends to match the sand or mud color of its geographical region, assisting in its "lie in wait" feeding methodology.

These turtles have many characteristics pertaining to their aquatic lifestyle. Many must be submerged to swallow their food. They have elongated, soft, snorkel-like nostrils. Their necks are disproportionately long in comparison to their body sizes, enabling them to breathe surface air while their bodies remain submerged in the substrate (mud or sand) a foot or more below the surface.

Females can grow up to several feet in carapace diameter, while males stay much smaller; this is their main exhibition of sexual dimorphism. Pelochelys cantorii, found in southeastern Asia, is the largest softshell turtle.

Most are strict carnivores, with diets consisting mainly of fish, aquatic crustaceans, snails, amphibians, and sometimes birds and small mammals.

Softshells are able to "breathe" underwater with rhythmic movements of their mouth cavity, which contains numerous processes copiously supplied with blood, acting similarly to gill filaments in fish. This enables them to stay under water for prolonged periods. Moreover, the Chinese softshell turtle has been shown to excrete urea while "breathing" underwater; this is an efficient solution when the animal does not have access to fresh water, e.g., in brackish-water environments.

According to Ditmars (1910): "The mandibles of many species form the outer border of powerful crushing processes—the alveolar surfaces of the jaws", which aids the ingestion of tough prey such as molluscs. These jaws make large turtles dangerous, as they are capable of amputating a person's finger, or possibly their hand.

Unlike the temperature-dependent sex determination of most turtles, trionychids have ZZ/ZW genetic sex determination; microchromosomes play a role in determining sex.