San Marcos Salamander

Eurycea nana

Description

The slender-bodied San Marcos salamander, Eurycea nana, is about 2.4 in (6 cm) long and displays a prominent gill fringe behind the head. It is light brown above with a row of pale flecks on either side of the midline and yellowish white below. The large eyes have a dark ring around the lens. Limbs are short and slender with four toes on the forefeet and five on the hind feet. At first glance, it is similar to a lizard but lacks scales and claws. The specific name nana is from the Greek nanos, meaning "dwarf." This voiceless salamander is also earless.

Prominent external features of the small, slender salamander are moderately large eyes with a dark ring around the lens, well-developed and highly pigmented gills, relatively short, slender limbs with four toes on the forefeet and five on the hind feet, and a slender tail with well-developed dorsal fin. Compared to other neotenic Eurycea from Texas, the San Marcos salamander is smaller and more slender, different in coloration, has larger eyes relative to the size of its head, a greater number of costal grooves, and fewer pterygoid and premaxillary teeth.

Behavior

Salamanders lay jelly-covered eggs from which tiny fishlike larvae emerge and develop in the manner of tadpoles. The San Marcos salamander breeds and lays eggs in standing pools amid thick mats of aquatic vegetation. Eggs hatch in about 24 days. This species is carnivorous and feeds on amphipods, midge fly larvae, and aquatic snails. It remains stationary until prey pass closely and then abruptly snaps its head, taking the prey.

Salamanders in laboratory aquaria feed on amphipods and young brine shrimp. Stomach content analyses of 80 preserved specimens revealed the salamander's diet in its natural habitat included amphipods and midge fly larvae and pupae; other small insect pupae and naiads and small aquatic snails were found in lesser numbers. Small amounts of Lyngbya sp. and grains of sand occasionally were present, apparently as incidental items ingested along with principal food items. Feeding behavior observed in the laboratory indicated that the salamanders did not actively pursue their prey. Salamanders remained stationary until the prey items were near their head, then abruptly snapped forward while opening their mouths to engulf food items. This information suggests they respond either to visual or vibrational cues from living prey.

Male E. nana reach sexual maturity (possess at least one full darkly-pigmented lobe in each testis) after attaining a snout-vent length of 0.7 in (1.9 cm) or 1.4 in (3.5 cm) total length. All males with snout-vent lengths greater than 0.9 in (2.3 cm) or 1.6-1.8 in (4-4.5 cm) total length were mature, possessing darkly-pigmented testes with one to three lobes. Laboratory studies suggest that the salamanders breed in June and possibly again in the fall.

Salamanders had the following four classes of ova in the oviducts: very small clear ova, small opaque-white ova, small yellow ova, and large yellow ova. Females carrying large yellow ova are considered gravid and presumably ready for oviposition. Large yellow ova were present in females with snout-vent lengths greater than 0.8 in (2 cm) or 1.4 in (3.5 cm) total length. Females with a snout-vent length longer than 1 in (2.5 cm) carried one to 19 large yellow ova. Large yellow ova were present in some females in nearly every month of the year.

Courtship and egg deposition by E. nana has not been reported and no eggs have been collected from the habitat. However, courtship, oviposition, and hatching have been observed for the closely related Comal Springs salamander. Eggs of this species were deposited singly on plant material, stones, and the bottom of a glass bowl about 24 hours after courtship. The Comal Springs salamander has reproduced successfully several time in artificial spring upwellings at the Dallas Aquarium. Most, if not all, Eurycea breed in running water of brooks, caves, or springs. In most cases, adherent eggs are deposited singly on the bottom and sides of stones, or on aquatic vegetation.

The San Marcos salamander is capable of altering its dorsal coloration from light tan to dark brown in accord with the lightness or darkness of the substrate. This color change is accomplished by migration of pigment in melanophores, giving them these structures the appearance of expanding or shrinking.

The salamander's external gills expand and appear bright red from increased blood flow in cool water of low oxygen content. The bushy red gills are prominent on individuals when collected from the springs, but they show marked reduction, almost to the point of apparent resorption when specimens are kept in well-oxygenated aquaria.

Habitat

The San Marcos salamander is found in shallow alkaline springs carved out of limestone with sand and gravel substrates. Pools and streambeds are often punctuated with large limestone boulders. Aquatic vegetation is profuse, and the pool surfaces are covered with moss (Leptodictyium riparium ) and thick mats of coarse, blue-green algae. In Spring Lake it occurs where rocks are associated with spring openings, and in rocky areas up to 492 ft (150 m) downstream of the dams at Spring Lake.

The salamander is also found in shallow spring areas on the uppermost (northernmost) portion of Spring Lake on a limestone shelf in an area immediately in front of Aquarena Springs Hotel. The substrate in this area is sand and gravel interspersed with large limestone boulders. Concrete banks in front of the hotel and boulders in shallow (3.3-6.6 ft, or 1-2 m deep) water support a lush growth of an attached aquatic moss. Interspersed with the moss and blanketing the shallow sandy substrate are thick filamentous mats of a coarse, filamentous blue-green alga, the dark reddish-brown color of which almost perfectly matches the dark dorsal coloration of the San Marcos salamander.

Spirogyra sp. and a few other larger filamentous green algae species, as well as the carnivorous angiosperm known as bladderwort (Utricularia gibba ), are present in small amounts in the aquatic moss. A wide variety of rooted aquatic macrophytes occur on the periphery of the salamander habitat at 3-10-ft (0.9-3.1-m) depths. The macrophytes include arrowhead (Sagittaria platyphylla ), parrot's feather (Myriophyllum brasiliense ), water primrose (Ludwigia reens ), and wild celery (Vallisneria americana ). In deeper water, Carolina fanwort (Cabomba caroliniana ), Hydrilla (Hydrilla verticillata ), and elodea (Egeria densa ) become the dominant macrophytes of the mud and detritus-laden benthic region.

The salamanders are abundant within the wiry mesh of the aquatic moss and the filamentous mats of Lyngbya sp. in the shallow headwaters area. Sandy substrates devoid of vegetation and muddy silt or detritus-laden substrates with or without vegetation are apparently unsuitable habitats for E. nana. Specimens occasionally are collected from beneath stones in predominantly sand and gravel areas. In view of the abundance of predators (primarily larger fish, but also crayfish, turtles, and aquatic birds) in the immediate vicinity of the springs, protective cover such as that afforded by the moss and cyanophycean bacteria (=blue-green algae) is essential to the survival of the salamander. This vegetation also supports a plentiful food supply for the salamander.

Flowing water is apparently a prerequisite for suitable E. nana habitat, as no specimens were found in still water areas of the lake or river. The flowing spring waters in the principal habitat are slightly alkaline, stenothermal (narrow range of temperatures) at 69.8-71.6°F (21-22°C), and clear.

In summary, the San Marcos salamander apparently requires: (1) thermally constant waters; (2) flowing water; (3) clean and clear water; (4) sand, gravel, and rock substrates with little mud or detritus; (5) vegetation for cover; and (6) anadequate food supply.

Distribution

The limited range of the San Marcos salamander comprises the San Marcos Springs, Spring Lake, and a few hundred feet (1 ft=0.3 m) of the San Marcos River in Hays County of Texas. In 1976, population numbers in the floating algal mats at the uppermost portion of Spring Lake to be between about 17,000 and 21,000 individuals. Following the same procedure, a survey published in 1993 estimated that the mats were inhabited by about 23,000 salamanders. Also published that year, an additional search of rocky susbtrates around the spring openings throughout Spring Lake located an estimated 25,000 salamanders. At the time, estimates of the population below Spring Lake associated with the rocky substrate were calculated to be approximately 5,000 individuals. The total population estimates from these combined studies stood at 53,200 for Spring Lake in 1993, and some of these estimates, notably the rocky substrate figures, were thought to be low.

Threats

Although the population appears relatively stable for the moment, the salamander is threatened by potential degradation or modification of its very limited habitat. This region, which is halfway between San Antonio and Austin, has experienced an upsurge in residential and agricultural development. The rising demand for water for human use and irrigation may well cause the spring sources to dry up in a very few years. The endangered Texas wild-rice (Zizania texana ), found further downstream near the town of San Marcos, has suffered from generally lower water levels in recent years.

Conservation and Recovery

The owner of Spring Lake has taken care to safeguard the spring sources and has cooperated closely with biologists to ensure that wildlife populations are protected. The key to preserving the San Marcos salamander is controlling the amount of water that is pumped out of the ground—a divisive issue in semi-arid south-central Texas. Critical Habitat was designated for the salamander to include its entire known range in Hays County.

Experiments are underway at the Dallas Aquarium to develop captive breeding techniques for the salamander in the event that the natural population at San Marcos Spings is lost, using techniques patterned after those used for the breeding the Comal salamander. Efforts to induce propagation at another facility in simulated spring environments were unsuccessful.

The 1996 San Marcos/Comal (Revised) Recovery Plan, which covers the San Marcos salamander and four other listed species, notes that recovery goals for the habitat's species include the survival of these species in their native ecosystems; the development of an ecosystem approach using strategies to address both local, site-specific and broad regional issues related to recovery; and the conservation of the integrity and function of the aquifer and spring-fed ecosystems that these species inhabit.

Delisting is considered unattainable for all five species (including the San Marcos salamander) due to the potential for extinction from catastrophic events. Consequently, the revised Recovery Plan calls for the establishment and continued maintenance of refugia capability for all five species in case of a catastrophic event.

Contact

Regional Office of Endangered Species
U. S. Fish and Wildlife Service
P. O. Box 1306
Albuquerque, New Mexico 87103
http://southwest.fws.gov/

References

Bishop, S. C. 1943. Handbook of Salamanders. Com-stock Publishing, Ithaca, New York.

Tupa, D. D., and W. K. Davis. 1976. "Population Dynamics of the San Marcos Salamander, Eurycea nana Bishop." Texas Journal of Science 32: 179-195.

U. S. Fish and Wildlife Service. 1980. "Determination of the San Marcos salamander (Eurycea nana ) as Threatened." Federal Register 45: 47355-47364.

U. S. Fish and Wildlife Service. 1984. "San Marcos River Recovery Plan." U. S. Fish and Wildlife Service, Albuquerque.

U. S. Fish and Wildlife Service. 1987. "Endangered and Threatened Species of Texas and Oklahoma (with 1988 Addendum)." U. S. Fish and Wildlife Service, Albuquerque.

U. S. Fish and Wildlife Service. 1996. "San Marcos and Comal Springs and Associated Aquatic Ecosystems (Revised) Recovery Plan." U. S. Fish and Wildlife Service, Albuquerque.