The lesser siren (Siren intermedia) is a species of aquatic salamander native to the eastern United States and northern Mexico. They are referred to by numerous common names, including two-legged eel, dwarf siren, and mud eel. The specific epithet intermedia denotes their intermediate size, between the greater siren, Siren lacertina, and the dwarf sirens, Pseudobranchus species.
Description Lesser sirens have elongated bodies possessing only two limbs, a pair of four-toed legs located behind the base of the head, and range in length from 7 to 27 inches (17 – 69 cm). Unlike greater sirens, lesser sirens have less than 35 costal grooves. Juvenile specimens have red to yellow banding on their heads and stripes running along the main length of the body, although these stripes are absent in Siren intermedia. Adults have plainer coloration; the dorsal side is typically olive green to blue-gray or black, while the ventral is usually light grey. Spotted patterns may also be present in adults. In salamanders, sexual size dimorphism is usually female-biased, but in Siren intermedia, it is the opposite. Males are the larger sex, and they also have significantly larger heads and enlarged masseter muscles than females. Lesser sirens are notably neotenic, and possess external gills throughout their lives.
Taxonomy Sources disagree on the number of subspecies within S. intermedia. The majority of the sources in the field agree there are at least two: an eastern and a western variety. Many sources also include a third subspecies, the Rio Grande lesser siren, S. i. texana, but researchers disagree whether the Rio Grande variety belongs as a lesser siren, within S. intermedia, or as a greater siren, within S. lacertina, and some others even consider it to be its own species, as S. texana.
Eastern lesser siren, S. i. intermedia Barnes, 1826 Western lesser siren, S. i. nettingi Goin, 1942 Rio Grande lesser siren, S. i. texana Goin, 1957
Habitat and distribution
Habitat Sirens are likely to be found in ponds near other intermittent wetlands. They usually inhabit swamps and ponds, and can survive in dry ponds for up to two years by aestivating. Sirens are considered poor overland dispersers because they rarely leave the water, so their geographical distribution is largely determined by pond connectivity and biotic interactions. Lesser sirens are aquatic salamanders, but their ability to withstand factors like dehydration allows them to have good survival skills in their ephemeral habitats. Sirens are able to osmoregulate and produce a mucoid cocoon during aestivation, which greatly reduces dehydration and electrolyte stress. Their small legs enable them to move on dry land for short periods of time.
Geographic distribution The lesser siren is found in the United States and Mexico, primarily from Virginia to Florida, west to Texas, ranging into northeastern Mexico as far as Veracruz, and north to Illinois and southwest Michigan. Multiple specimens have recently been rediscovered in Michigan after a 60-year absence. Lesser sirens only occur in permanent wetlands, whereas other salamander and frog species can occupy a range of dry and wetlands. Lesser sirens are top predators in permanent wetlands. They can quickly colonize and become dominant consumers in new ponds such as those constructed by beavers. Their ability to aestivate and burrow in soft sediments and live in dense aquatic vegetation in shallow waters facilitates their dominance. In addition, the species' high productivity, high fecundity, and rapid growth to early sexual maturity allows it reach a high density quickly and easily, and thus establish dominance in the habitat.
Conservation The lesser siren is quite common through most of its range, but rarely seen due to its secretive nature. Like almost all species of amphibian, their numbers are believed to be declining due to general reductions in water quality caused by agricultural pesticide and fertilizer runoff. They are frequently collected and used as bait for fishing. The species was once believed to be extirpated from Michigan, but has been recently rediscovered in limited numbers. The S. i. texana subspecies is listed as a threatened species in Texas. They are listed as Least concern on the IUCN Red List and a species with "Special Protection" in Mexico.
Physiology
Vision Western Lesser Sirens (Siren intermedia nettingi) have poor eyesight. They rely on their other senses to forage, hunt, and survive.
Chemical sense Sirens rely on chemical cues to detect prey and not on visual cues. The use of chemical stimuli in the detection of prey and predators are common in amphibians.
Respiration All species in the sirenidae family are paedomorphic, eel-shaped salamanders that live in aquatic environments. They respire through their gills, lungs and skin, and survive well in hypoxic environments low in oxygen. Siren intermedia is unique among amphibians in its ability to construct a mucoid cocoon that slows down dehydration during aestivation, and the lesser siren can withstand long periods of food deprivation without ill effects. During aestivation, respiration slows down significantly, and gills atrophy over the next 16 or more weeks.
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