Amphiumidae
Amphiumas, Congo Eels

Extant members of this small family, which contains but three species in a single extant genus, were once known colloquially as congo eels, for their long, slippery, almost limbless bodies. They are known only from the southeastern United States.

Amphiumas are elongate, paedomorphic, aquatic salamanders that superficially resemble the primitive sirens (Sirenidae). Unlike sirens, amphiumas retain both pairs of girdles and limbs, though both are reduced in size, and the limbs are often both difficult to discern and of decreased function in adults. In the larval stage, however, the limbs are larger relative to body size, and are used for walking. Amphiumas also lack external gills (while retaining one of three pairs of larval gill slits), and their teeth are pedicellate. Amphiumas lack eyelids and tongues, but do have lungs. Diploid number is 28.

The three species of extant amphiumas are named for the number of toes on each limb: the three-toed and two-toed species are large, attaining lengths of more than one meter, and capable of delivering a painful bite. The one-toed species (Amphiuma pholeter) is much smaller, with a maximum size of only 35 cm. All species are aquatic, inhabiting slow streams and swamp rivers, although Amphiuma means (two-toed) has been observed foraging on land during rainy nights. It has been reported that males either court several females simultaneously, or that females vie for the attention of a single male, but given that females in other salamandroid families tend to be passive or coy during courtship, these observations need to be confirmed. After males deposit spermatophores onto the cloaca of females, up to 150 eggs are laid on mud near water. Eggs are attended by the female until hatching, about 20 weeks later. Hatchlings must find their way to water, often doing so while it rains. Adults eat a wide variety of animals, including reptiles, other amphibians, fish, snails, crayfish and insects.

Amphiumids are members of the suborder Salamandroidea, the "advanced salamanders" that include all internally-fertilizing salamanders. They have previously been placed in their own suborder, Amphiumoidea, but most current analyses reject this clade. There appear to be no extant salamanders that are closely related to the amphiumas. Paedomorphic characters, in combination with several unique derived characters, keep the phylogenetic placement of amphiumids in doubt. They are probably sister to the plethodontids, but are nevertheless rather distantly related to that diverse family of lungless salamanders.

Three species of fossil amphiumas were widely distributed throughout North America from the upper Cretaceous through the upper Miocene. One extinct genus, Proamphiuma, is known from the Cretaceous. In the Pleistocene, fossil amphiumas were restricted to the southeastern United States.

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