Lesser siren

Species of amphibian

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.

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’s 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. 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

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 a process called “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.

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.

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, 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.

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. Siren intermedia make and responds to underwater sounds that may have specific communicational significance. Resting sirens are usually silent, but they may produce trains of pulsed sounds or “clicks”. Their tendency to click is greater in groups than in single specimens, which suggests that they are involved in intraspecific communication. These acoustic behaviors occur most often when other sirens are present. They are emitted at different pulse rates by specimens clicking simultaneously, and are associated with head jerking motions. Head jerking may offer visual reinforcement of an acoustically defended individual space, or it may be required for sound production. S. intermedia can produce clicks by moving the horny jaw coverings together rapidly, and head jerking may be associated with such movement when it is particularly forceful. This means of sound production is similar to the upper and lower teeth of humans, clicking with the mouth open.

Lesser sirens are filter feeders. They sift through pond bottoms and aquatic vegetation for prey items.

The lesser siren is nocturnal, spending its days hidden in the debris and mud at the bottom of slow moving bodies of water. Sirens are most active in the fall and spring seasons, when water temperatures are close to 15 °C and there is no danger of drought. During the summer months, the ponds and other habitats that the lesser siren live in go through periods of drought, which leads to low water levels and possibly dried up ponds. The lesser siren’s strategy to withstand the rigors of these dry seasons is something called “aestivation”, or summer dormancy, which can last up to 35 weeks, depending on the severity of the drought. During the sixteen week period from July to October, aquatic lesser sirens increase the osmotic concentration of their body fluids, and slow down bodily functions. They do this by burrowing themselves into the bottom of its drying pond in tube like channels about the length and width of their bodies. For the rest of the aestivating process, they then secrete a mucous cocoon. That is then followed by a significant drop in their oxygen consumption and heart rate. The gills slowly atrophy and the body shrinks, and as a result the fat is metabolized at one fifth the normal rate. As expected, large individuals store more fat and consume less oxygen per unit weight than small ones, and thus can survive much longer periods of aestivation. Some individuals, especially the smaller sirens during aestivation, die or suffer from dehydration as they are unable to store sufficient fat and the greater metabolic demand reduces their chance of survival. S. intermedia reduces the total densities of the anuran larvae, and by doing that, it allows the eastern newts to act as keystone predators over a broad range of prey densities.

Adult mole salamanders (Ambystoma talpoideum) and lesser sirens (Siren intermedia), are the top two predators in temporary ponds of the southeastern United States. Siren intermedia competes with and is an intraguild predator of A. talpoideum, limiting its growth and controlling its recruitment.