678.
Northern Dwarf Siren
(Pseudobranchus striatus)
LC
Overview
Until recently there was only one recognised species of dwarf siren, but in 1993 genetic analysis split the genus into two species, the northern dwarf siren (further split into three sub-species) and the southern dwarf siren (with two sub-species). It is likely that the two species diverged when the Florida peninsular was isolated by sea level rise approximately three to five million years ago. Although listed as Least Concern by the IUCN, local population extinctions are known.
Urgent Conservation Actions
Monitoring of population size and distribution of the northern dwarf siren is required, as habitat loss from development has caused local extinctions.
Distribution
South-eastern 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. Three of the five previously recognised sub-species of dwarf siren belong to the northern dwarf siren group, the Gulf Hammock dwarf siren (P. s. lustricolus), slender dwarf siren (P. s. spheniscus), and broad-striped dwarf siren (P. s. striatus).

It is probable that the two species diverged 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 was probably during the second of these events during the Pliocene when the genus diverged into two distinct species.
Description
Members of the family Sirenidae are paedomorphic, meaning that adults retain features which are normally only seen in juveniles; external gills, which are usually 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.

Colouration of northern dwarf sirens varies between the sub-species, which have different characteristic striped patterns, however in general individuals are brownish to black with pale yellow, buff or tan stripes running in parallel from the head to the tip of the tail. The underside is paler than the top side. Juveniles are like adults but have a dorsal fin which runs down the length of the back and their limbs are not fully developed.
Ecology
Northern dwarf sirens, which feed on aquatic 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. This species may also hibernate, again by creating a burrow in sediment, during cold weather.
Habitat
Permanent and semi permanent cypress swamps and gum ponds are the preferred habitat of the northern dwarf siren, which are sometimes found in floating mats of vegetation such as frog’s-bit (Limnobium spongium), although this species is more typically an inhabitant of decaying vegetation and debris on wetland bottoms. This species is not found among floating mats of the invasive water hyacinth like its closest relative, the southern dwarf siren, probably because water hyacinths are less common in the acidic habitats preferred by the northern dwarf siren.
Distribution
Population Estimate
No population estimate is known, however northern dwarf sirens are reported as common in suitable habitats.
Population Trend
Declining.
Status
The northern dwarf siren was listed as Least Concern on the 2007 IUCN Red List of Threatened Species. However, the Gulf Hammock dwarf siren sub-species has not been collected since it was first described in 1951, when it was only reported from three locations. The status of this sub-species is unknown. Furthermore, northern dwarf sirens are considered Threatened in South Carolina.
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 northern dwarf siren. The existing total population size of the species is certainly smaller than it has been historically, although the range of this species has remained constant.
Conservation Underway
The northern dwarf siren occurs in a number of protected areas throughout its range.
Conservation Proposed
Monitoring of population number and the range of northern 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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