By Heather Heying
Four extant species in two genera (Siren and Pseudobranchus) are currently recognized in this family of aquatic, eel-like salamanders. Geographic distribution for extant taxa is limited to southeastern United States and northeastern Mexico, with fossil representatives known from a wider area in North America.
Adults sirens are aquatic and neotenic, with lengths ranging from 4 -36 inches. Their stream-lined bodies entirely lack hindlimbs, their forelimbs are reduced, they have lidless eyes, and large external gills and gill slits. Primitive tetrapods and most salamanders have 4 pairs of larval gill slits, but this number is both reduced and variable in the sirenids; Siren has 3 pairs, Pseudobranchus has but 1. Furthermore, several autapomorphies distinguish the sirenids from other salamanders, including the lack of a pelvic girdle; the presence of an interventricular septum; and the presence of a horny beak, in lieu of teeth, over the edentate premaxilla and maxilla (absent in Pseudobranchus). Palatal teeth are present in large patches.
Sirens typically live in heavily vegetated, slow-moving water, such as swamps or lakes, where they prey actively on aquatic invertebrates by suction feeding. They also eat aquatic plants. Little is known of their reproductive biology, but the lack of both spermatheca in females, and cloacal glands in males, suggests external fertilization. Eggs are laid singly or in small clusters on submerged vegetation. Two of the three extant species are known to produce vocalizations, which is rare in salamanders. The function of these vocalizations is not understood. Sirens are known to aestivate when in habitats subject to drought. They burrow into the mud at the bottom of drying ponds and become entombed, living for up to several months in a cocoon produced either by layers of accumulated shed skin, or skin gland secretions. They continue to respire through their lungs (their mouths not being covered by cocoon), though their metabolic rate is greatly reduced.
Sirenidae is usually presented as the most basal of the extant salamander families. It is the sole family in the suborder Sirenoidae (and has been, in the past, put the separate order Meantes). Its relationship to other salamanders is problematic, however, due to 1) several primitive characters in common with the cryptobranchoids, 2) derived characters shared with more advanced salamanders, 3) numerous paedomorphic characters shared with other families displaying neoteny, and 4) a suite of characters unique among salamanders, some of which are likely primitive, others of which are probably derived. Several hypotheses of sirenid relationships within Caudata have been proposed, but when paedomorphic characters are excluded or down-weighted, it seems likely that sirenids are derived from post-kaurauroid (the most primitive extinct family of Caudata) salamanders.
Fossil sirenids are known from extant genera as far back as the Eocene in the early Cenozoic. An extinct genus, Habrosaurus, is represented in the fossil record in North America in the Upper Cretaceous and Paleocene.
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Contributors
Heather Heying (author).
