677.
Southern Dwarf Siren
(Pseudobranchus axanthus)
LC
Overview
Members of the family Sirenidae are paedomorphic, meaning that adults retain features of juvenile individuals which in other amphibian species are lost during metamorphosis; adult sirens lack eyelids, but have external gills. Paedomorphism has evolved independently in amphibians in a number of evolutionary lineages. This family is unique among salamanders as adults have no hind legs, and unlike their near relatives this group appear to have lost the ability for internal fertilization.
Urgent Conservation Actions
Monitoring of population size and distribution of the southern dwarf siren is required, as local populations have become extinct as a result of habitat loss from development.
Distribution
Florida peninsular, United States.
Evolutionary Distinctiveness
Order: Caudata
Family: Sirenidae
The family Sirenidae, which contains two genera, diverged from all other amphibians approximately 235 million years ago during the Triassic period, approximately 5 million years before the evolution of the dinosaurs. Members of this family are paedomorphic, meaning that adults retain features of juvenile individuals which in other amphibian species change during metamorphosis. Paedomorphism has evolved independently in amphibians in a number of lineages. Adult sirens have external gills but lack eyelids, like juveniles of the group. This family is unique among salamanders as adults have no hind legs, and unlike their near relatives this group appear to have lost the ability for internal fertilization.

Until recently only one species of dwarf siren was recognised, with five known sub-species, however in 1993 the genus Pseudobranchus was split into two full species based on chromosomal differences – the northern dwarf siren (Pseudobranchus striatus) with n = 24 chromosomes, and the southern dwarf siren (P. axanthus) with n = 32 chromosomes. Two of the five previously recognised sub-species of dwarf siren belong to the southern dwarf siren group, the narrow-striped dwarf siren (P. a. axanthus) and the Everglades dwarf siren (P. a. belli).

It is probable that the southern dwarf siren came about as a result of the isolation of the Florida peninsular from sea level rise. This part of the United States was cut off from the mainland as a result of elevated sea level on at least two occasions, the first in the Miocene epoch from approximately 16 to 10 million years ago, and the second during the Pliocene epoch from approximately 5 to 3 million years ago. It is thought that the southern dwarf siren speciated during the second of these events during the Pliocene, and colonised coastal lowlands when sea levels receded during the Pleistocene epoch.
Description
Members of the family Sirenidae are paedomorphic, meaning that adults retain features which are normally only seen in juveniles; external gills, which are normally lost when amphibians metamorphose, are present in adults of this family. Dwarf sirens (the collective name for the two species in the genus Pseudobranchus) are elongated and slim creatures with a single gill slit and bushy external gills. These amphibians lack hind limbs and have just three reduced toes on the front limbs. Members of the family Sirenidae have a horny beak on the upper and lower jaws and no eyelids.

Southern dwarf sirens are brownish black to pale grey with parallel yellow or tan stripes running from the head to the tip of the tail on the back and sides. Juveniles are like adults in colouration, and differ only in possessing a dorsal fin all the way down the back and in that the limbs are not yet fully developed. In adults the tail is approximately 40% of the total length, which can be as long we 25 cm. Hatchlings are approximately 16 mm total length.
Ecology
Southern dwarf sirens, which are a permanently aquatic species which feeds on invertebrates, take two years to reach sexual maturity. Although courtship and mating have not been observed in this species, it is assumed that fertilisation is external as dwarf sirens do not have the physiological adaptations seen in salamanders with internal fertilization. Single eggs are deposited among submerged aquatic vegetation from early November to March.

When pools dry dwarf sirens burrow into the sediment at the bottom and form a protective cocoon 10-30 cm underground where they aestivate. Individuals may remain buried for several months until the wetland refills.
Habitat
Southern dwarf sirens live and breed in open marshes, prairie ponds and shallow lakes with a lot of vegetation. This species is one which has benefited from the spread of the non-native water hyacinths; individuals can often be found in floating mats of water hyacinth.
Distribution
Population Estimate
Southern dwarf sirens may be locally common in areas of suitable habitat, however no estimate of the total population is known.
Population Trend
Declining.
Status
The southern dwarf siren was assessed as Least Concern on the 2007 IUCN Red List of Threatened Species.
Threats
Habitat loss and degradation, as a result of drainage for infrastructure development including urbanisation, agricultural and silvicultural development, and increased tourism, has cause local extinctions of the southern dwarf siren. The existing distribution and total population size of the southern dwarf siren is certainly smaller than it has been historically.
Conservation Underway
The southern dwarf siren occurs in a number of protected areas throughout its range, although these are mostly in the southern parts of Florida.
Conservation Proposed
Monitoring of population number and the range of southern dwarf sirens should be ongoing to ensure that the extent of further losses is known and appropriate action can be taken if required.
Links
References
AmphibiaWeb: Information on amphibian biology and conservation [web application]. 2006. Berkeley, California: AmphibiaWeb. Available: amphibiaweb. Accessed: 08 December 2006.

Conant, R. and Collins, J. T. 1998. A field Guide to Reptiles and Amphibians: Eastern and Central North America. Houghton Mifflin Company, Boston, New York.

Frost, Darrel R. 2006. Amphibian Species of the World: an Online Reference. Version 4 (17 August 2006). Electronic Database accessible at: . American Museum of Natural History, New York, USA.

Halliday, T. and Adler, C. (eds.). 2002. The new encyclopedia of reptiles and amphibians. Oxford University Press, Oxford.

IUCN, Conservation International and NatureServe. 2006. Global Amphibian Assessment. Global Amphibian Assessment. Accessed on 08 December 2006.

Moler, P. E. (Ed.) 1992. Rare and Endangered Biota of Florida. Volume III. Amphibians and Reptiles. University Press of Florida, Gainsville, Florida, USA.

Moler, P. E. and Kezer, J. 1993. Karyology and systematics of the salamander genus Pseudobranchus (Sirenidae). Copeia 1993 (1): 39-47.

Obst, F.J., Richter, K. and Jacob, U. 1984. The Completely Illustrated Atlas of Reptiles and Amphibians for the Terrarium. T.F.H. Publication Inc., N.J., U.S.A.

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