Lesser siren, Eastern Lesser Siren, Western Lesser Siren
© 2008 César L. Barrio Amoros (1 of 21)
Siren intermedia Barnes, 1826
William T. Leja 1
1. Historical versus Current Distribution. Lesser sirens (Siren intermedia) inhabit the Atlantic and Gulf Coastal Plain from Virginia to Mexico, and range north in the Mississippi Valley to Illinois, Indiana, and southwestern Michigan (Conant and Collins, 1991). Their range extends south in Mexico to the state of Veracruz (Ramirez-Bautista et al., 1982) and north on the Atlantic Coastal Plain to Caroline County, Virginia (Roble, 1995). Geographical isolates occur in northern Indiana, southwestern Michigan, northeastern North Carolina, and Virginia (Petranka, 1998). Because lesser sirens thrive in pools of fish hatcheries (Hubbs, 1962; Brach, 1995), it is possible that their range has been expanded by fish stocking. Although rotenone, a pesticide used to eliminate undesired fish and amphibian populations, revealed the presence of sirens in Michigan lakes, it could presumably have eliminated them from the state as well (Williams, 1961; Harding, 1997). Several new species within the genus Siren will likely be erected following the completion of electrophoretic and karyological studies (P.E. Moler, personal communication). Because the name Siren intermedia is assigned to animals throughout most of the current range of the genus, the distribution of this nominal form may decrease as it is divided into newly recognized species.
2. Historical versus Current Abundance. Throughout their range, lesser sirens occur in scattered populations. With Euro-American settlement, these populations may have become further isolated by flood control programs (Petranka, 1998). The lesser siren populations of south Texas have been threatened by wetland drainage (Bury et al., 1980). In response, the south Texas lesser siren (large form; see Siren texana account, this volume) has been listed by the State of Texas as Endangered (Campbell, 1995). Within the Great Lakes drainage basin, based on the scarcity of recent records, lesser sirens are extremely rare and possibly even extirpated (Harding, 1997).
The former wet prairie of Illinois and Indiana, which covered about 20% of the ground surface in pre-settlement times, likely constituted ideal siren habitat. All the extensive intermittent prairie wetlands in the lower Midwest have been drained to yield prime farmland (Prince, 1997). However, the thousands of farm ponds constructed in the Midwest since the Dust Bowl years may have partially replaced these wetlands as lesser siren habitat.
Methods for capturing sirens have changed over time. Because of the relation of capture methods to qualitative assessments of abundance, they should be reviewed here. Early in this effort, Scroggin and Davis (1956) commented that sirens are highly secretive, and few herpetologists were equipped to capture them. They were successful in capturing them at night by using a seine weighted with a heavy chain. Later, Rossman (1960) noted that lesser sirens were among the most abundant amphibians in Pine Hills Swamp in southern Illinois when electroshocking was used to determine abundance, but that most collectors rarely found them. Electric fish shockers have proved effective at routing sirens from their hiding places during the day (Dundee and Rossman, 1989). Bennett and Taylor (1968) did not discover the presence of sirens in a lake until electrofishing was used, although a variety of netting techniques had been tried earlier. They suggested that seining was probably ineffective because the sirens are buried in the bottom mud during the day. Bennett and Taylor (1968) were also successful using rotenone, but sirens did not appear to be affected until they left their burrows after sunset. In a demonstration of the use of rotenone in fish eradication in a Texas farm pond, Davis and Knapp (1953) collected 209 lesser sirens the next morning. It is noteworthy that both of the two known Michigan populations were discovered when rotenone was applied to the shallow inlets of lakes for fish management purposes (Harding, 1997). More recently, minnow traps have been successfully used to trap lesser sirens (Gehlbach and Kennedy, 1978; Gibbons and Semlitsch, 1991; Raymond, 1991; Sever et al., 1996).
Lesser sirens can reach high densities in favorable habitats. In an east Texas beaver pond in preferred habitat (shallow water, dense aquatic vegetation, deep sediments) there were 1.3 sirens/m2; the standing crop was 56.6 grams/m2 (Gehlbach and Kennedy, 1978). Gehlbach and Kennedy (1978) attributed this extreme density in a 5-yr-old aquatic habitat to high fecundity, rapid growth, and successful aestivation during dry periods.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. Sirens lack an obvious overland dispersal stage in their life cycle (Petranka, 1998), but aquatic migrations to specialized breeding sites are possible.
ii. Breeding habitat. A subset of the adult habitat.
Mating behavior of sirens has not been reported (Zug, 1993; Petranka, 1998). Numerous attempts to breed sirens in captivity have failed (Godley, 1983). However, Brach (1995) stated that "sirens will breed in captivity providing there is sufficient vegetation or debris in which to build their nests." Fertilization is concurrent with oviposition (Sever et al., 1996). In South Carolina, Sever et al. (1996) found that oviposition occurred primarily in February–March, but also possibly in January and April. Oviposition occurred from late December to March in south-central Florida (Godley, 1983). A nest of eggs accompanied by an adult lesser siren was found in early April in Arkansas (Noble and Marshall, 1932).
i. Egg deposition sites. Egg masses accompanied by females or adults of undetermined sex have been found at the base of rooted macrophytes, in fibrous mats of water hyacinth roots, and in muddy depressions or plant debris in pond bottoms (Noble and Marshall, 1932; Hubbs, 1962; Godley, 1983).
ii. Clutch size. The number of eggs produced in a season is highly variable, ranging from about 200–500 or more (Harding, 1997). The incubation period lasts 1.5–2.5 mo, based on the first appearance of small larvae in seasonal samples (Petranka, 1998).
C. Larvae/Metamorphosis. The larvae are about 11 mm long upon hatching. Sexual maturity is reached in about 2 yr after hatching (Pope, 1947; Davis and Knapp, 1953; Harding, 1997; Petranka, 1998).
D. Juvenile Habitat. Juveniles are most active at night and live in burrows or in thick mats of aquatic vegetation, where they forage on small invertebrates (Petranka, 1998). Juvenile sirens feed mostly on zooplankton but also eat larger prey, such as amphipods, craneflies, and lumbriculid worms (Carr, 1940a). Even at night, larval sirens in experimental ponds spend most of the time hidden in leaf litter (Fauth et al., 1990).
E. Adult Habitat. Shallow, warm, quiet water of ponds and sloughs where aquatic vegetation is plentiful (Smith and Minton, 1957). Permanent or semipermanent habitats, including marshes, swamps, farm ponds, ditches, canals, sloughs, and sluggish, vegetation-choked creeks (e.g., Neill, 1949b; see also Petranka, 1998). Temporary floodplain pools and shallow, heavily vegetated sections of ponds with deep sediments provide burrowing sites (Funderburg and Lee, 1967; Gehlbach and Kennedy, 1978). In areas of sympatry, lesser sirens and greater sirens (S. lacertina) are partitioned by habitat preferences—lesser sirens tend to inhabit more acidic pH waters, while greater sirens are found in aquatic sites with circumneutral pH (P.E. Moler, personal communication, in McAllister et al., 1994). This appears to be consistent with Carr's (1940) observations in Florida that lesser sirens are characteristic of pine flatwoods ponds and bayheads (a normally acidic environment), while greater sirens are characteristic of sloughs, canals, and drainage ditches, and frequent in lakes. Lesser sirens are restricted to wetlands that hold water for at least 6 mo (Snodgrass et al., 1999).
Low intrinsic metabolic rate, efficient lungs, the ability to withstand prolonged inanition and to undergo facultative anaerobiosis, along with the ability to extract oxygen from waters of low oxygen tension, have allowed sirens to populate warm, shallow, and occasionally hypoxic and hypercarbic waters (Guimond and Hutchison, 1976). Sirens are often abundant in water hyacinth communities, which are, from the viewpoint of gas exchange, one of the most restrictive environments for aquatic organisms (Ultsch, 1976).
F. Home Range Size. In a Texas pond where lesser sirens are abundant, no individual among 60 recaptures moved > 12 m from the first capture point. Forty-six (77%) of them traveled fewer than 6 m (Gehlbach and Kennedy, 1978).
G. Territories. Lesser sirens produce trains of pulsed sounds or clicks, often accompanied by head-jerking movements, that may be used in defense of individual space (Gehlbach and Walker, 1970). Social interactions will preclude group formation if sufficient shelter (e.g., deep, soft sediment or debris) is available (Asquith and Altig, 1987).
H. Aestivation/Avoiding Dessication. Lesser sirens are able to survive drought and the drying of their habitat by retreating into crayfish tunnels to a depth of ≥ 1 m (Cagle, 1942) or by burrowing into the mud (Harding, 1997). However, the lack of valves on the external nares precludes the possibility of extensive burrowing (Noble, 1929b). If the surrounding mud begins to dry, sirens conserve water by producing a protective cocoon, which covers its body except for the mouth (Reno et al., 1972). During aestivation, the gills atrophy and the body shrinks as fat is metabolized; oxygen consumption and heart rate are reduced (Gehlbach et al., 1973). Aestivation can continue for several weeks to > 1 yr (Gehlbach et al., 1973; Gibbons and Semlitsch, 1991). In Indiana in June, under the moist soil of a pond bottom that had been drained the previous fall, Blatchley (1899) found numerous sirens at a depth of 8–10 cm while the land was being plowed for row crops. By remaining in the basin of a pond when it dries, sirens are able to prey on the eggs and larvae of even the earliest colonists after the return of water, and are thus likely to have a profound effect on community structure (Fauth and Resetarits, 1991).
I. Seasonal Migrations. Lesser sirens are primarily aquatic, but they have been found on land beneath brush piles and under logs and can move overland on occasion, as they will colonize artificial ponds that have never had a direct connection with natural habitats (Minton, 1972, 2001). Although lesser sirens survive droughts by aestivation in an underground cocoon, they may also migrate to other water bodies. During an unprecedented drought in Louisiana, a lesser siren was collected under oak leaves in flat, mixed woodland about 600 m from the fringe of marshes and cypress swamps (its normal habitat) which border Lake Pontchartrain (Viosca, 1924b). Further, it is possible that lesser sirens forage on land or migrate to other habitats during rains. Viosca (1924b) found a lesser siren 450 m from the edge of a marsh under a plank in a wet field after heavy rains. Lesser sirens will occasionally leave aquatic habitats in mid winter: after a January cold snap following a warm December in Arkansas, large numbers of lesser sirens were found frozen within or above the ice (Sugg et al., 1988). Also, after a winter cold snap near St. Louis, Missouri, during which the temperature dropped to -26 ˚C, hundreds of lesser sirens were found frozen in the ice (Hurter, 1911). Possibly they left the water during the warm rains that often precede winter cold spells. Noble (1929b) commented that the hypertrophy of the Jacobson's organ in sirens may have developed in connection with terrestrial as well as aquatic feeding habits. Lesser sirens have an apparent ability to climb. Gaines (1895) kept a captured lesser siren for 8 mo in a barrel containing some mud and water. On one occasion he placed two laths in the barrel; on the next day he found the animal squirming about on the floor. Scroggin and Davis (1956) found that lesser sirens consumed a large number of terrestrial organisms during rainy periods. They assumed this food was carried in by runoff or by a rising water level, but it is also possible that lesser sirens sometimes feed terrestrially at night during rainy periods. Snodgrass et al. (1999) found lesser sirens in wetlands approximately 0.6 km distant from other aquatic habitats, suggesting that they possess substantial dispersal capabilities.
Lesser sirens demonstrate seasonal activity patterns. In a temporary pond, Raymond (1991) found that adults were considerably more active in fall and winter. He believed that the intensified activity was due either to increased foraging or to reproductive behavior. In contrast, in a permanent pond with deep organic sediments and a high density of lesser sirens, Gehlbach and Kennedy (1978), using the same trapping technique, observed no significant difference between spring and fall catches. In South Carolina, Sever et al. (1996) were able to trap females and males from January–April, but from May–October, males were almost exclusively caught.
Eubanks et al. (2002) found western lesser sirens in Tennessee migrating upstream from one pond to another through a large flooded culvert. They note that the timing of this migration was coincident with the published timing of reproductive activities and suggest these animals were seeking oviposition sites.
J. Torpor (Hibernation). During excavation of a drainage ditch in mid winter in the dry bed of a slough in southern Illinois, Cockrum (1941) found 60–70 lesser sirens at a depth of 46–102 cm below the ground surface in what appeared to be crayfish burrows. The ground was frozen to a depth of 15 cm and covered with 8 cm of snow. Lesser sirens removed from the ground were active despite near-freezing air temperatures. Cagle and Smith (1939) found what they believed was a hibernating aggregation of 138 lesser sirens in shallow, ice-free water connected to two frozen ponds in early January in southern Illinois. The animals were comparatively inactive and easily secured. Barbour (1971) stated that although lesser sirens remain active most of the winter, individuals spend at least some time in dormancy.
K. Interspecific Associations/Exclusions. In large, temporary ponds of the southeastern United States, lesser sirens and eastern newts (Notophthalmus viridescens) are two of the most abundant vertebrate predators. Their larvae are generalized predators that utilize similar types of invertebrate prey, competing as equals in these ephemeral larval environments (Fauth et al., 1990). Until recently, lesser sirens have been overlooked with regard to their role as top predators in pond amphibian communities (Fauth, 1999a). By nonselectively reducing the density of anuran larvae, lesser sirens act to support eastern newts as keystone predators of larval anuran communities in North Carolina ponds (Fauth, 1999a). Eastern newts function as keystone predators by selectively feeding on dominant larval anuran species, thus allowing a greater number of weakly competing anuran species to survive (Fauth and Resetarits, 1991). However, in ostensibly similar ponds in the Francis Marion National Forest in South Carolina, eastern newts and lesser sirens do not function as keystone predators (Fauth, 1999b). Instead, mole salamanders (Ambystoma talpoideum; which are not present in the North Carolina ponds) act as top predators (Fauth, 1999b). Lesser sirens affect mole salamanders by limiting their growth and recruitment (Fauth, 1999a).
L. Age/Size at Reproductive Maturity. Lesser sirens become reproductively mature during their second year of life, when males average 18 cm SVL, females 15 cm.
M. Longevity. The record for longevity is held by a specimen at the Rio Grande Zoo in New Mexico that lived for almost 7 yr (Snider and Bowler, 1992).
N. Feeding Behavior. Feeding is primarily nocturnal (Noble and Marshall, 1932). Lesser sirens consume a variety of invertebrate prey, including small crustaceans, insect larvae, snails, and annelid worms (Scroggin and Davis, 1956). Davis and Knapp (1967) found that small crustaceans accounted for up to 87%, and snails and sphaeriid clams accounted for ~10%, of the total number of food items eaten. Altig (1967) concluded that lesser sirens can apparently obtain food by filter feeding—sifting through bottom material and aquatic vegetation. Lesser siren eggs have been noted in the stomachs of adults (Scroggin and Davis, 1956; Collette and Gehlbach, 1961). Cranial anatomy appears to facilitate feeding within the confines of a burrow (Reilly and Altig, 1996). Lesser sirens will feed on tadpoles (Fauth et al., 1990), including those of the genus Bufo (Lefcourt, 1998), and larval salamanders (Fauth and Resitarits, 1991; Fauth, 1999a).
Hurter (1911) stated that lesser sirens feed on worms and minnows. Lesser sirens have been caught by anglers using minnows for bait (Goin, 1957; Scroggin and Davis, 1956), and fish scales have been found in their guts (Altig, 1967). McAllister and McDaniel (1992) believe that third-stage larval anisakid nematodes (Contracaecum sp.) infected lesser sirens through ingestion of parasitized fish. However, Pope (1947) questioned Hurter's (1911) statement that lesser sirens feed on minnows. Pope's contention was based on Carr's (1940) observation of the relatively poorly developed prey capturing skills of greater sirens in Florida. Davis and Knapp (1953) considered lesser sirens to be bottom feeders incapable of active pursuit of larger animals.
O. Predators. Natural predators are poorly documented but undoubtedly include water snakes, fishes, alligators, and wading birds (Petranka, 1998). In Hickman County, Kentucky, lesser sirens make up about 33% of the total food of cottonmouths (Agkistrodon piscivorus; Barbour, 1971). Marvel (1972) observed a yellow-bellied water snake (Nerodia erythrogaster flavigaster) eating a lesser siren. Predation of lesser sirens by Mississippi green watersnakes (Nerodia cyclopion) has been reported in Illinois (Garton et al., 1970). Buck (1946) removed a lesser siren from the stomach of a mud snake (Farancia abacura). A lesser siren was removed from the stomach of a largemouth bass (Micropterus salmoides) in Louisiana (Walker, 1963).
P. Anti-Predator Mechanisms. The nocturnal behavior of lesser sirens is presumed to be an anti-predator behavior that minimizes predation risk from diurnal predators such as fish and wading birds (Petranka, 1998). A lesser siren captured by a water snake in the wild emitted several shrill distress cries (Marvel, 1972). Distress cries of immature frogs captured by water snakes have been suggested to be an anti-predator behavior because they attract large American bullfrogs (Rana catesbeiana) and sometimes allow escape of the victim in the ensuing confusion (Smith, 1977). Chemically mediated fright responses of southern leopard frog (Rana sphenocephala) and southern toad (Bufo terrestris) tadpoles to lesser sirens have been examined in the laboratory (Lefcourt, 1996, 1998, respectively).
Q. Diseases. Unknown.
R. Parasites. McAllister et al. (1994) present a summary table of the known helminth fauna of lesser sirens. The following nine helminths have been found in lesser sirens: the trematodes Allassostomoides louisianensis (Brooks and Buckner, 1976), Progorgodera foliata (Brooks and Buckner, 1976) and Diplostomum sp. (McAllister et al., 1994); the cestoid Proteocephalus sireni (Brooks and Buckner, 1976; Brooks, 1978); the acanthocephalans Fessisentis fessus (Landewe, 1963; Nickol, 1972; Dunagan and Miller, 1973; Buckner and Nickol, 1979) and Neoechinorhynchus sp. (Miller and Dunagan, 1971); and the nematodes Falcaustra chabaudi (Dyer, 1973; McAllister et al., 1994), Capillaria sp. (McAllister et al., 1994), and Contracaecum sp. (McAllister and McDaniel, 1992).
McAllister et al. (1994) comment that there is some degree of host specificity among helminths from different siren taxa, particularly among trematodes of lesser and greater sirens. The two species in the genus Siren appear to be partitioned by habitat preferences (P.E. Moler, personal communication, in McAllister et al., 1994); intermediate hosts may be partitioned in the same manner (McAllister et al., 1994).
Cystacanths of the acanthocephalan parasite Fessisentis fessus occur in two aquatic isopods, Asellus forbesi and Lirceus lineatus, in Jackson County, Illinois, where lesser sirens are the usual definitive host; laboratory infections have confirmed the life cycle of the parasite (Buckner and Nickol, 1979). Older lesser sirens harbor more and larger specimens of the acanthocephalan Fessisentis fessus than do younger animals (Nickol, 1972).
4. Conservation. Throughout their range, lesser sirens occur in scattered populations. With Euro-American settlement, these populations became further isolated by flood control programs and wetland drainage (Bury et al., 1980). Within the Great Lakes drainage basin, lesser sirens are possibly extirpated (Harding, 1997). Lesser sirens are considered Threatened in Michigan (Stearns and Lindsley, 1977). The Kentucky Department of Fish and Wildlife Resources lists lesser sirens as Rare and Endangered (Babcock, 1977).
One problem in determining the conservation status of sirens is that they are highly secretive, and methods for capturing them often do not work. Rossman (1960) noted that lesser sirens were among the most abundant amphibians in Pine Hills Swamp in southern Illinois when electroshocking was used to determine abundance, but that most collectors rarely found them.
Acknowledgments. Thanks to Russ Hendricks for providing additional literature sources.
1William T. Leja
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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