Sirens and dwarf sirens

(Sirenidae)

Class Amphibia

Order Caudata

Suborder None

Family Sirenidae

Thumbnail description
Small to large, elongate, aquatic salamanders with a larval-like form and lacking hind limbs

Size
4.7–38.5 in (12–98 cm)

Number of genera, species
2 genera; 4 species

Habitat
Swamp, lake, and wetland

Conservation status
Not classified by IUCN, but local populations of certain species may be threatened

Distribution
Southeastern United States, from Texas to southwestern Michigan to Maryland

Evolution and systematics

The oldest sirenid fossils are from the upper Cretaceous (165–65 million years ago [mya]) of Wyoming and Montana (e.g., Habrosaurus dilatus) and Sudan. The genus Siren, with four fossil species (Eocene [54–38 mya] of Wyoming; Miocene [28–5 mya] of Florida and Texas; Pliocene [5–1.8 mya] of Florida), is known from the Eocene to present. Pseudo-branchus, with two fossil species, is known from the Pliocene to present in the southeastern United States. Vertebrae are among the most common fossil elements collected.

In 1962 there was a proposal to place the sirenids in the order Trachystomata, separate from the order Caudata for the salamanders. This idea was quickly rebutted and never gained scientific acceptance.

A consensus of where the Sirenidae should be placed within a classification of the order Caudata has not been achieved. The extreme larval condition of these salamanders sometimes makes character evaluations difficult, and the lack of information on their fertilization mode adds to this dilemma. In various recent phylogenetic analyses of salamanders, the family usually is considered to be the sister group to all other salamanders, but information on the fertilization mode of these animals could change that arrangement.

The genus Siren (sirens) with the species S. lacertina was described by Linnaeus in 1767, and S. intermedia, now with three subspecies, was described by Barnes in 1826.

The genus Pseudobranchus (dwarf sirens) with the species P. striatus (Le Conte, 1824) was described by Gray in 1825, and several subspecies largely based on perceived differences in the muted, striped color pattern were subsequently named. That situation persisted until 1993 when the species was divided into P. axanthus, with two subspecies each with 32 chromosomes, and P. striatus, with three subspecies each with 24 chromosomes. Other than the chromosome morphology and range, the species are difficult to identify and all other information, presented mostly under the heading of P. striatus, applies equally well to both species.

No subfamilies are recognized.

Physical characteristics

All sirenids are paedomorphs (i.e., they attain sexual maturity but retain a larval morphology relative to their ancestors—no eyelids, external gills [main support structures branched], gill slits [one to three], teeth without attachment bases, and gill rakers present). They have a cylindrical body with a flattened tail and lack hind limbs and all pelvic girdle components; they have three to four fingers with cornified tips. The maxillary bone in the upper jaw is small to absent. There is a partial metamorphosis that is subtle and prolonged, but they never become terrestrial. These salamanders are refractory to extraneous hormone stimuli that promote metamorphosis in other salamanders and are permanently aquatic.

Because larviform salamanders seldom have external features representative of the sexes, it is usually impossible to sex an individual without internal examinations.

Hatchlings and small larvae have an intensely black ground color with contrasting marks in yellow, red, or silvery white; a band across the nose and one on the top of the head and stripes on the body are common. Clear, unmarked fins extending well onto the body become restricted to the tail and opaque in postlarvae.

Distribution

Sirens are distributed in the U.S. coastal plain and Mississippi River embayment from northeastern Tamaulipas, Mexico, to Maryland. Dwarf sirens occur in the coastal plain from the western Florida Panhandle to central South Carolina and throughout the Florida Peninsula.

Habitat

Sirenids occur in many types of still to slow-flowing, often swampy, sites with muddy substrates and often with floating and rooted vegetation.

Behavior

Sirens commonly find retreats in burrows in the bank during daylight hours and forage along the bottom and among vegetation at night. Dwarf sirens are most commonly caught among the fine roots of the exotic floating water hyacinth (Eichhornia crassipes). All sirenids swim by body and tail undulations but also move their rather small legs in walking motions when near the bottom.

Feeding ecology and diet

Sirenids have rather small mouths but will eat any animal material small enough for them to swallow. They probe about with their snouts while foraging and likely detect prey by odor. Food items are sucked into the mouth by rapidly expanding the throat and opening the mouth so that the item is carried inside with the inrush of water; the item is retained by the gill rakers and the water is expelled out the gill slits. Their feeding activities can be quite gluttonous. These animals shake food items vigorously and swallow larger organisms in a series of gulps, but items are seldom broken into pieces.

Sirenids forage along the bottom and among floating and rooted vegetation, primarily at night. Older reports of at least sirens being herbivorous surely are based on the voluminous extraneous material that sometimes is captured while sucking appropriate animal-based food items into their mouth.

Reproductive biology

The mode of sperm transfer between males and females is not known for any species, but it is generally assumed that fertilization is external. Males lack glands that make a spermatophore (a gelatinous structure containing sperm) in other salamanders, and females lack a spermotheca (reproductive sac for sperm storage). The large sperm have elongate nuclei and two axial filaments, each with an undulating membrane.

The large eggs are laid singly, sometimes attached to vegetation, or in groups.

Conservation status

Members of this family generally are not threatened, and neither the IUCN nor CITES has listings for any of the species in this family. However, some species are in decline in parts of their distribution areas, mainly because of habitat loss.

Significance to humans

Dwarf sirens have been sold as fishing bait.

Species accounts

List of Species

Northern dwarf siren

Pseudobranchus striatus

taxonomy

Siren striata Le Conte, 1824, Riceborough, Liberty County, Georgia, United States. Three subspecies are recognized.

other common names

None known.

physical characteristics

In addition to the familial characteristics listed above, dwarf sirens have 34–36 costal grooves (successive vertical grooves along the sides marking embryological segments), three toes, one gill slit, and a total length up to 10 in (25 cm). The coloration usually involves a series of muted stripes that extend for most of the length of the slim, cylindrical body. The head is rather pointed. Species are separated reliably only by chromosome number: 24 in P. striatus and 32 in P. axanthus.

distribution

The species occurs in the northern third of the Florida Peninsula and in the Gulf and Atlantic coastal plains from the western Florida Panhandle to central South Carolina.

habitat

These salamanders are most often caught with nets or dredges that sample emergent or floating vegetation, especially water hyacinth (E. crassipes), that grow in swampy wetlands, sloughs, and lakes.

behavior

See the family behavior section.

feeding ecology and diet

Any small aquatic organism will be consumed; however, because of the small size of these salamanders, the average size of food items is smaller than for the species of Siren. Small

worms, insect larvae, crustaceans of various sizes, and small mollusks are common food items.

reproductive biology

The method of sperm transfer between male and female is not known. Eggs (lightly pigmented, about 100 per clutch, 0.1 in[2.5–2.7 mm] diameter, four jelly envelopes) with tough jelly coats and crystalline inclusions in the outer layers are laid singly and scattered and often are attached to the fibrous roots of water hyacinth and other aquatic vegetation. Larvae hatch in three to four weeks at about 0.6 in (15 mm) total length and are black with a silvery white stripe from the tip of the snout to the end of the body.

conservation status

This species is threatened in at least South Carolina, and destruction of wetland habitats usually is the reason.

significance to humans

None known, other than having been used for fish bait.

Lesser siren

Siren intermedia

taxonomy

Siren intermedia Barnes, 1826, Liberty County, Georgia, United States. Three subspecies are recognized, although the status of these names is debated (e.g., S. i. texana).

other common names

German: Mittlerer Armmolch; Spanish: Sirena menor.

physical characteristics

In addition to the familial characteristics listed above, this species has 31–37 costal grooves, four toes, three gill slits, and a total length up to 27 in (69 cm). The head is broadly rounded as viewed from the top. Larvae (about 0.4 in [10 mm] total length at hatching) have pronounced red bands across the tip of the snout, across the head, and on the body; postlarvae may keep a pale snout band, but the other markings disappear. The adult pattern appears to be geographically variable, but with a greenish to gray ground color with variable amounts of iridophore speckling.

distribution

The species inhabits the Atlantic coastal plain and Mississippi River embayment from northeastern Tamaulipas, Mexico, to southeastern Virginia.

habitat

Lesser sirens occur in many types of slowly flowing or still, often swampy water.

behavior

These aquatic salamanders can be extremely numerous; they spend daylight hours burrowed into the bottom debris or the bank. Sound production is uncommon in salamanders, but a bitten individual or one routed from a hiding spot by another salamander often yelps; an individual placed in unfamiliar surroundings may make several types of sounds. If oxygen concentrations become low, these salamanders come to the surface to gulp air.

If the habitat dries out, lesser sirens move into burrows in the bottom and estivate (condition of torpor analogous to hibernation but in response to hot or dry conditions). Because gills do not work effectively when not submerged, the gills reduce to small nubs under these conditions and the salamanders respire by their lungs. They produce a water-impervious cocoon by shedding their skin several times.

feeding ecology and diet

Almost any appropriately sized aquatic animal will be consumed; large individuals eat larger items than smaller salamanders, but even larger specimens eat considerable numbers of small crustaceans. Snails, worms, insect larvae, and small fishes are consumed. Individuals forage mostly along the bottom at night, and all items are sucked into the mouth and swallowed whole.

reproductive biology

The method of fertilization or methods of courtship are not known, but during the presumed breeding season, most individuals large enough to be sexually mature have a number of bite marks on them that match the dimensions of the mouth. These bites are presumed to reflect either courtship or territorial disputes by males, or perhaps both. A group of up to 1,500 single, unpigmented eggs is laid in a localized spot (nest) on the bottom and is seemingly guarded by a parent. The eggs are 0.25 in [6.0–6.5 mm] in diameter with four jelly envelopes and have crystalline inclusions in the outer jelly layer. Hatching occurs in 45–75 days at about 0.4 in (10 mm) total length, and the larvae are densely black with red markings on the snout, head, and body. The clear, unmarked dorsal fin originating on the back eventually is opaque and restricted to the tail in larger individuals. Most of the red markings are lost, but the nose band may persist in muted form.

conservation status

This species is threatened in Texas, probably because of the drainage of wetlands, but seems abundant in most places with the proper habitat.

significance to humans

None known, although some people fear them because they confuse them with the larger amphiuma (Amphiuma), which can produce a dangerous bite.

Greater siren

Siren lacertina

taxonomy

Siren lacertina Linnaeus, 1767, Charleston, South Carolina, United States. No subspecies are recognized, but considerable variations from throughout the range are known.

other common names

French: Sirène lacertine; German: Grosser Armmolch; Spanish: Sirena grande.

physical characteristics

In addition to the familial characteristics listed above, this species has 36–40 costal grooves, four toes, three gill slits, and a total length up to 36 in (91 cm). Larvae have pronounced yellow bands across the tip of the snout and along the body; postlarvae may retain the snout band, but the other markings disappear. The adult pattern appears to be geographically variable, but with a greenish to gray ground color with variable amounts of lighter speckling.

distribution

The species occurs throughout the Atlantic coastal plain from western Alabama to Maryland and in all of the Florida Peninsula.

habitat

Usually they inhabit larger wetlands, lakes, sloughs, and ponds with soft substrates and considerable vegetation.

behavior

Same behavior as the lesser siren. The greater sirens are also hosts to parasitic copepods.

feeding ecology and diet

Their feeding habits are similar to those of the lesser siren, although more, larger objects are common in the diet.

reproductive biology

Eggs are laid singly and attached to vegetation. The larvae are similar to those of the lesser siren, but the markings are yellow and there are lateral and ventrolateral stripes on the body.

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

Duellman, William E., and Linda Trueb. Biology of Amphibians. New York: McGraw Hill, 1986.

Estes, Richard. Gymnophiona, Caudata. Vol. 2 of Handbuch der Paläoherpetologie, edited by Peter Wellnhofer. Stuttgart, Germany: Gustav Fischer Verlag, 1981.

Periodicals

Altig, Ronald. "Food of Siren intermedia nettingi in a Spring-fed Swamp in Southern Illinois." American Midland Naturalist 77, no. 1 (1967): 239–241.

Aresco, Matthew J. "Siren lacertina (Greater Siren). Aestivation Chamber." Herpetological Review 32, no. 1 (2001): 32–33.

Asquith, Adam, and Ronald Altig. "Osmoregulation of the Lesser Siren, Siren intermedia (Caudata: Amphibia)." Comparative Biochemistry and Physiology 84, no. 4 (1986): 683–685.

——. "Phototaxis and Activity Patterns of Siren intermedia." Southwestern Naturalist 32, no. 1 (1987): 146–148.

Duke, Jeffrey T., and Gordon R. Ultsch. "Metabolic Oxygen Regulation and Conformity during Submergence in the Salamanders Siren lacertina, Amphiuma means, and Amphiuma tridactylum, and a Comparison with Other Giant Salamanders." Oecologia 84, no. 1 (1990): 16–23.

Gehlbach, Frederick R., Roxanne Gordon, and Judy B. Jordan. "Aestivation of the Salamander, Siren intermedia." American Midland Naturalist 89, no. 2 (1973): 455–463.

Gehlbach, Frederick R., and Braz Walker. "Acoustic Behavior of the Aquatic Salamander Siren intermedia." BioScience 20 (1970): 1107–1108.

Godley, J. Steven. "Observations on the Courtship, Nests, and Young of Siren intermedia in Southern Florida." American Midland Naturalist 110, no. 1 (1983): 215–219.

Goin, Coleman J. "Notes on the Eggs and Early Larvae of Three Florida Salamanders." Natural History Miscellanea 10 (1947): 1–4.

Moler, Paul E., and James Kezer. "Karyology and Systematics of the Salamander Genus Pseudobranchus (Sirenidae)." Copeia 1 (1993): 39–47.

Neill, Wilfred T. "Juveniles of Siren lacertina and S. i. intermedia." Herpetologica 5, no. 1 (1949): 19–20.

Raymond, Larry R. "Seasonal Activity of Siren intermedia in Northwestern Louisiana (Amphibia: Sirenidae)." Southwestern Naturalist 36, no. 1 (1991): 144–147.

Reilly, Stephen M., and Ronald Altig. "Cranial Ontogeny in Siren intermedia (Caudata: Sirenidae): Paedomorphic, Metamorphic, and Novel Patterns of Heterochrony." Copeia 1 (1996): 29–41.

Reno, Harley W., Frederick R. Gehlbach, and Robert A. Turner. "Skin and Aestivational Cocoon of the Aquatic Amphibian, Siren intermedia Le Conte." Copeia 4 (1972): 625–631.

Sever, David M., Lisa C. Rania, and John D. Krenz. "Reproduction of the Salamander Siren intermedia Le Conte with Especial Reference to Oviducal Anatomy and Mode of Fertilization." Journal of Morphology 227, no. 3 (1996): 335–348.

Sullivan, Aaron M., Paul W. Frese, and Alicia Mathis. "Does the Aquatic Salamander, Siren intermedia, Respond to Chemical Cues from Prey?" Journal of Herpetology 34, no. 4 (2000): 607–611.

Ultsch, Gordon R. "Observations on the Life History of Siren lacertina." Herpetologica 29, no. 4 (1973): 304–305.

——. "The Relationship of Dissolved Carbon Dioxide and Oxygen to Microhabitat Selection in Pseudobranchus striatus." Copeia 2 (1971): 247–252.

Ronald Altig, PhD