By Heather Heying
Three extant species in two genera are currently recognized in this primitive family of large, fully aquatic salamanders. Geographic distribution is limited to the eastern United States (Cryptobranchus) and central China and Japan (Andrias).
Cryptobranchids are the largest salamanders, with recorded sizes for adult Andrias of up to 1.8 meters, while the hellbender (Cryptobranchus) can reach 750 mm. Members of the two Asian species of Andria are eaten by people. They undergo only partial metamorphosis, and live as paedomorphs (unlike their sister taxon, Hynobiidae). Their bodies and heads are dorsoventrally compressed, allowing these large, flat animals to wedge themselves under rocks. Cryptobranchids live exclusively in fast-flowing streams. Lacking gills as adults, the wholly aquatic cryptobranchids breathe primarily through their skin, which hangs loosely about them in folds. They have unusual specializations associated with cutaneous respiration, including the extreme vascularization of these skin folds, which allows water containing oxygen to come into contact with the folds. Cutaneous respiration is further aided by the side-to-side swaying of the body, especially in slower moving water. Cryptobranchids also use their lungs to breathe.
Although they lack gills, adult cryptobranchids retain one set of gill slits (open in Cryptobranchus, closed in Andrias). They have no eyelids, and all species have four fingers on each forelimb, five toes on each hind limb, and caudal fins. Cryptobranchids are unique in being able to employ asymmetrical suction feeding, wherein only one side of the mandible is depressed at a time. They are carnivorous, feeding on fish, other salamanders, worms, insects, crayfish and snails. Fertilization is external (as in hynobiids and probably sirenids, but unlike the advanced salamanders). Females lay eggs underwater in two long strings that attach to rocks. Males exhibit parental care by guarding fertilized eggs in underwater nests that they have excavated and defend. Both sexes appear to be territorial.
The suborder Cryptobranchoidea is comprised of the cryptobranchids and hynobiids. The cryptobranchids appear to be derived from a hynobiid-like ancestor through the retention of larval characters in the adults. These two families primarily share plesiomorphic character states, but three synapomorphies link them: 1) fusion of first hypobranchials and first ceratobranchials, 2) fusion of tibialis muscles, and 3) eggs enclosed in paired sacs. The suborder Cryptobranchoidea, often called the primitive salamanders, is sister to the Salamandroidea, the advanced salamanders.
Fossil cryptobranchids are known from North America (with the oldest known from the Paleocene), Europe (oldest known from the Oligocene) and Asia (from the Pleistocene). Some fossil Andrias are greater than two meters long.
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Contributors
Heather Heying (author).
