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.Lesser siren (siren intermedia). Species Profile: Lesser Siren (Siren intermedia) | SREL Herpetology. (n.d.). Retrieved April 28, 2022, from https://srelherp.uga.edu/salamanders/sirint.htm 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.Caudata Culture Species Entry – Siren intermedia. Caudata.org. Retrieved on 2019-12-23. 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.Reinhard, Sandy, et al. "External Fertilisation and Paternal Care in the Paedomorphic Salamander Siren Intermedia Barnes, 1826 (Urodela: Sirenidae)". Zoologischer Anzeiger – a Journal of Comparative Zoology, vol. 253, no. 1, August 2013, pp. 1–5, 10.1016/j.jcz.2013.06.002. Accessed 25 March 2022.
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.Fauth, John E. "Interactions between Branchiate Mole Salamanders (Ambystoma Talpoideum) and Lesser Sirens (Siren Intermedia): Asymmetrical Competition and Intraguild Predation". Amphibia-Reptilia, vol. 20, no. 2, 1 May 1999, pp. 119–132, 10.1163/156853899506942. Accessed 30 October 2020.
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.Asquith, Adam, and Ronald Altig. "Osmoregulation of the Lesser Siren, Siren Intermedia (Caudata: Amphibia)". Comparative Biochemistry and Physiology Part A: Physiology, vol. 84, no. 4, January 1986, pp. 683–685, 10.1016/0300-9629(86)90387-7. Accessed 1 May 2021. 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.Anderson, Thomas L., et al. "Functional Responses of Larval Marbled Salamanders (Ambystoma Opacum) and Adult Lesser Sirens (Siren Intermedia) on Anuran Tadpole Prey." Copeia, vol. 108, no. 2, 19 May 2020, p. 341, 10.1643/ce-19-212. Accessed 20 February 2022. 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.Gehlbach, Frederick R., and Stephen E. Kennedy. "Population Ecology of a Highly Productive Aquatic Salamander (Siren Intermedia)." The Southwestern Naturalist, vol. 23, no. 3, 10 August 1978, p. 423, 10.2307/3670250. Accessed 14 May 2021.
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.• MARTIN, B. T., GOODDING, D. D., FORD, N. B., & PLACYK, J. . J. S. (2013). Sensory Mediation of Foraging Behavior in the Western Lesser Siren (Siren intermedia nettingi). Journal of Herpetology, 47(1), 75–77. https://doi.org/10.1670/11-209
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.
Acoustic behavior The lesser siren is vocal, unlike most salamanders. The lesser siren is known to emit a series of clicks when it approaches others of its species, or a short screeching sound if handled. Acoustic behavior serves a functional service in S. intermedia, especially since it is nocturnal and it burrows in sediments, swims and crawls through densely vegetated waters. However, its visual and olfactory senses are very limited in this habitat, and thus the possibility of predation and other risks are high in this case. Nevertheless, acoustic behavior in lesser sirens is still infrequent, and becomes even less frequent with undisturbed habitat residency, perhaps because of their familiarity with the environment.Gehlbach, Frederick R., and Braz Walker. "Acoustic Behavior of the Aquatic Salamander, Siren Intermedia." BioScience, vol. 20, no. 20, 15 October 1970, pp. 1107–1108, 10.2307/1295417. Accessed 11 March 2021.
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.
Another frequently produced sound ("yelp") is associated with cases of distress, or alarm. When S. intermedia butt or bite each other in their habitat, the bitten or injured individuals may swim away quickly, emitting yelps at frequencies of 880 Hz. These yelp sounds could have communicational significance if they prevent further attack or signal to other individuals that one is injured.
Osmoregulation Animals that inhabit freshwater habitats have high-affinity sodium uptake systems. Therefore, the better an animal is adapted to freshwater, the lower the rate of sodium loss and uptake. Sirens have a high sodium affinity (around 0.2 peq/g per day), and thus are able to resist the harmful effects of low-sodium environments.
Hormones Insulin has been isolated from the pancreas of lesser sirens that is mostly conserved in sequence as compared to other amphibian orders. However, it does have some substitution differences that indicate that sirens diverged early from other salamanders.
Diet Lesser sirens are filter feeders. They sift through pond bottoms and aquatic vegetation for prey items. They have heavily keratinized beaks and vomerine dental arrays that can inflict substantial wounds. The diet of the lesser siren includes at least 10 different taxa, which includes primarily aquatic invertebrates, including tadpoles and snails. They also eat several insects and their larvae, crayfish, mollusks, amphibians, siren eggs, and algae, although the plant material may be an incidental result of their gape-and-suck feeding style.Hampton, Paul M. "Ecology of the Lesser Siren, Siren Intermedia, in an Isolated Eastern Texas Pond". Journal of Herpetology, vol. 43, no. 4, December 2009, pp. 704–709, 10.1670/08-168.1. Accessed 25 March 2022.
Reproduction and life cycle
Annual seasonal behavior changes