Many aspects of the behavior and life of the San Marcos Salamander are poorly known and are under research at this time.
The San Marcos Salamander has been listed as a threatened species since July 14, 1980. Agricultural and urban development around the springs it inhabits are thought to be the biggest threats. These problems, along with the fact that habitat is extremely limited, are becoming more serious as a rise in the human population in the area has also increased the demand for water in this semi-arid region. Constant groundwater pumping could dry up the San Marcos, Aquarena, and Comal springs that supply water to the San Marcos and Comal Rivers in a matter of years. To help control this growing problem, Hays County has been designated a critical habitat. Although threatened, the population seemed stable with a count of 17,000-21,000 recorded in 1984. (Mitchell 1990)
US Federal List: threatened
IUCN Red List of Threatened Species: vulnerable
Development - Life Cycle: metamorphosis
Little is known about the foraging behavior of Eurycea nana. It is carnivorous and has been found to feed mostly on amphipods, midge fly larvae, and some aquatic snails (Petranka 1998).
Animal Foods: insects; mollusks; aquatic crustaceans
Primary Diet: carnivore (Insectivore , Eats non-insect arthropods)
The San Marcos Salamander is endemic to the source of the San Marcos River in Hays County, Texas. Distribution for Eurycea nana is extremely limited, with populations being found in ranges of only several hundred feet along this river within close proximity to San Marcos Spring.
Biogeographic Regions: nearctic (Native )
This salamander is characteristically found in shallow, alkaline springs carved out of limestone with sand and gravel substrates. The thick moss and algae that cover the shallow pools around the springs provide and excellent habitat for Euryca nana to not only find food for itself, but also to keep it safe from predators. Water temperature in the San Marcos and Comal Rivers are remarkably stable with temperatures in close proximity to the springs being between 21.0 to 21.5 degrees Celsius. (Tupa and Davis 1976, Mitchell 1990)
Habitat Regions: temperate ; freshwater
Aquatic Biomes: rivers and streams
One of the lungless salamanders, Eurycea nana averages about 2 inches long, but may reach up to 3.25 inches. It is the smallest of the genus Eurycea. These salamanders are slender, with short limbs, 5 toes on the rear feet, and 4 and the front. Gill fringes are prominent behind the head and there are also 16 to 17 costal grooves. It is light brown on its back, yellowish white ventrally, with pale yellow flecks obvious on the midline. Distinctive large eyes have a dark ring around the lens. Males have more poorly defined mental and caudal hedonic glands than females. Developed, pigmented gills are maintained throughout adulthood, but gases are exchanged almost entirely through cutaneous respiration. The vents of males are lined with papillae, contrasting with the smooth folds in females. This species is voiceless and earless. (Mitchell 1990, Petranka 1998, Bartlett and Bartlett 1999, Tupa and Davis 1976, Herbeck and Larson 1998)
Other Physical Features: endothermic ; ectothermic ; heterothermic ; bilateral symmetry
Males are sexually mature at 19 to 23.5 mm and females when they are longer than 21 mm. There is only vague information on the reproduction of Eurycea nana and eggs have never been found in nature. Strongly acyclic oviposition and the presence of gravid females and very small larvae during every month of the year suggests that breeding occurs year round. There is not a pronounced peak in breeding. As has been observed in artificial habitats, the average egg clutch is 20 and the jelly-covered eggs are usually laid in standing pools with thick vegetation. After a 24-day period in the eggs, larvae-like tadpoles emerge. (Mitchell 1990, Petranka 1998, Tupa and Davis 1976)
Breeding season: Breeding seems to occur year-round.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); oviparous
The San Marcos salamander (Eurycea nana) is a small species of aquatic, lungless salamander native to the United States, endemic to Spring Lake and a small region of the headwaters of the San Marcos River near Aquarena Springs, in Hays County, Texas. It is one to two inches long, with a slender body and external gills, and is reddish-brown in color.
E. nana is an aquatic salamander. It does not leave the water to change into a terrestrial form, but rather matures in the water. As a neotenic form, E. nana retains its gills for its lifetime. The San Marcos salamander has a narrow head with a round snout, large eyes, and a dark iris. External gills are developed and pigmented. 2–6 palatopterygoid and 7–13 premaxillary teeth are present. The species is uniformly light brown in color, with a series of seven to nine irregular light spots present along its midline. The trunk is flattened above with a dorsal furrow that extends from head to tail. The venter, the bottom of a salamander, is white in color, and males have larger vents than females do. There are 16–17 costal grooves present, with 6–7 occurring between limbs. It has four toes on its forefeet and five on its hind feet.[4] The size of the species was measured to be 56 millimetres (2.2 in) total in length.[5]
The name nana is from the Greek nanos, meaning dwarf, as these adult salamanders are small in size. It is a member of the family Plethodontidae (lungless salamanders). Genetic variation of the species has not yet been observed.[6]
E. nana may be confused with E. neotenes, the only other species thus found to be in its genus; however, E. nana is smaller in size with a more slender form.[7]
San Marcos River, located in Hays County, Texas, is the only known location of the San Marcos salamander. The population estimate in 1973 was about 20,880 salamanders living in the uppermost reaches of Spring Lake.[8] An estimate in 1993 suggested about 30,451 salamanders across all ranges of Spring Lake and up to 150 metres (490 ft) downstream. The ratio of males to females was reported to be stable throughout the year.[5]
Clear, flowing spring water of the San Marcos River makes a well-delineated hydrologic system for the San Marcos Salamander. The springwater maintains a temperature of 21–22 °C (70–72 °F); the salamanders appeared to be stressed at waters over 30 °C (86 °F).[5] Critical thermal maximum temperatures of the species show a lower threshold for juveniles than adults. E. nana are often found along the river substrates, such as rocks and vegetation. The uppermost shallow portion of Spring Lake features sand, gravel, and large limestone boulders that provide habitat. Further down, concrete banks and boulders provide space for aquatic moss such as L. riparium and blue-green algae. A variety of aquatic macrophytes, including S. platyphylla, M. brasiliense, L. repens, and V. americana are also present. Substrates without vegetation and muddy slit areas are unsuitable for E. nana, as those serve as protective covers against predation from larger fish, turtles, and aquatic birds.[9]
The diet of E. nana includes amphipods, fly larvae, and aquatic snails.[5] San Marcos Salamanders typically follow a temporal diet, or a diet that varies with the availability of invertebrates present to them at a given time.[10] It relies on the schedule of invertebrate behavior and locations, and generally consumes many aquatic invertebrates.[10] The surrounding vegetation is a rich source of food.
E. nana displays predator avoidance responses relying on chemical cues from its native predators, such as Micropterus salmoides. Antipredator responses include freezing behavior to its fish predators.[11] Site tenacity, the pattern of returning to the same nest or breeding site dependent on the season, was observed in both males and females of E. nana. This suggests that there is a selection favoring aggregation and shelter use as a part of anti-predatory tactics.[12] Lab results show that predator-naïve salamanders showed a greater reduction in activity compared to predator-experienced salamanders, suggesting behavioral plasticity in avoidance responses.[13] E. nana are also known to show predator generalization in response to novel predators similar to native ones.[14] The E. nana response to their crayfish predators is unknown and studies to answer this question are currently ongoing.[15]
The feeding behavior of the salamander is rather passive as it waits for prey to pass and will abruptly snap forward to catch its food. This suggests this response is likely generated from visual or vibrational cues from prey.
The San Marcos salamander breeds and lays eggs in standing ponds in the middle of dense mats of aquatic vegetation. Eggs are jelly-covered and will hatch in about 24 days. Male E. nana reaches maturity with a snout-vent greater than 19 millimetres (0.75 in). Four classes of ova are present: small-clear ova, small-opaque ova, small-yellow ova, and large-yellow ova. Female E. nana with a snout-vent greater than 26 millimetres (1.0 in) carry large yellow ova, and are considered ready for oviposition. E. nana relies primarily on chemical cues rather than visual cues for association preference.[16] Both males and females exhibit sexual discrimination, suggesting the seeking out of potential mates—a rare behavior among salamanders. In addition, female and intersexual pairs are found cohabiting more than male pairs, suggesting selective aggressive behavior. While natural courtship and egg deposition have not been observed and documented as of yet, eggs of similar species of salamanders are known to be deposited on single plants and stones about 24 hours after courtship.[9]
Depending on how light or dark the substrate is, E. nana can change its dorsal coloring from light tan to dark brown by migrating pigments in melanophores. The color of its gills also changes in response to the oxygen content of the surrounding water, where it appears bright red from increased blood flow in low-oxygenated waters to the point of resorption when kept at highly-oxygenated areas.[9]
The San Marcos salamander has been federally listed as a threatened species since 1980.[2][3] Due to its extremely limited geographic range, it is threatened primarily by the contamination of groundwater sources and heavy dependence of central Texas cities upon the Edwards Aquifer for water. Other threats include drying of the spring and introduced species. As migration cannot introduce genetic variation due to limited geographic range, the loss of genetic diversity cannot recover naturally when lost.[6] Availability of food and sufficient prey also remain of concern for these salamanders, but less so than habitat loss or change.[10]
The San Marcos salamander and four other listed species are covered by the 1996 San Marcos/Comal (Revised) Recovery Plan,[11] which includes recovery goals such as preserving the integrity and function of the aquifer and developing strategies to address both local and broad regional issues related to recovery. Experiments at the Dallas Aquarium were initiated to develop captive breeding techniques in case the natural population was lost. E. nana in captivity showed a 30% hatching success rate, with a single female producing potentially 176 eggs per year.[17]
Kyphosis due to microsporidia, intracellular protistan parasites, has been reported in captivated E. nana.[18]
Batrachochytrium dendrobatidis, a global threat to amphibians, has also been detected in E. nana. It causes chytridiomycosis and death in salamanders, and has led to multiple extinctions since its discovery.[19]
The San Marcos salamander (Eurycea nana) is a small species of aquatic, lungless salamander native to the United States, endemic to Spring Lake and a small region of the headwaters of the San Marcos River near Aquarena Springs, in Hays County, Texas. It is one to two inches long, with a slender body and external gills, and is reddish-brown in color.
