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Study Species

San Marcos salamander (Eurycea nana)


Species: Federally threatened. Adult size ranges are about 1-2 in. Fully aquatic with external gills. Once found in algal mats near the surface of the river, though currently they are most often found along the substrate under rocks and logs. In captivity, individuals of E. nana often aggregate under shelters during the day and are active out of shelter at night.




Habitat: This species is endemic to the headwaters of the San Marcos River (Spring Lake) and up to 50 meters downstream. The San Marcos River (Hays County, Texas) is thermostable, fluctuating only 2 degrees year-round, and is fed from underground springs originating in the Edward’s Aquifer. Like other aquifer-fed springs in the region, this habitat houses numerous endemic species with ranges often restricted to the unique characteristics of only one or two springs. Because of the high level of endemism, numerous species in Spring Lake have been federally listed as threatened or endangered and this habitat is currently protected as critical habitat. Because of increasing water demands and other threats of human encroachment in the region, the San Marcos National Fish Hatchery and Technology Center maintains refugium populations of E. nana and other protected, regional endemics (e.g. Texas blind salamander, Barton springs salamander, fountain darter, Texas wild rice, Comal springs riffle beetle).


Predators: There are numerous native and introduced predators in the habitat of E. nanaincluding sport fish (bass, sunfish, perch), cichlids, crayfish, and potentially birds in shallower parts. I have shown that E. nana exhibits the ability to innately recognize and respond to the chemical cues of native fish predators and their introduced congeners and that it exhibits threat-sensitive avoidance. My work also suggests that predation pressure in the natural habitat may be resulting in serious constraints on time management for this species.



Texas blind salamander (Eurycea rathbuni)

Species: Federally endangered. Cave-adapted and fully aquatic with reduced eyes, complete vision loss, almost no pigmentation, and external gills.


Habitat: Endemic to the underground, water-filled caves of the Edward’s Aquifer below the San Marcos River in Hays County, Texas. A captive population is maintained at the San Marcos National Fish Hatchery and Technology Center.




Amazon & sailfin mollies (Poecilia formosa & P. latipinna)


Species: Members of the family of live-bearing fishes (Poecilidae) with internal fertilization. After ~28 days, females give birth to live young. The Amazon molly (P. formosa) resulted from a hybridization event between sailfin (P. latipinna) and shortfin (P. mexicana) mollies about 100,000 years ago. Amazon mollies reproduce by gynogenesis, a form of asexual reproduction in which females must mate with males of a closely related sexual species (usually a parental species) to start egg development, but the paternal genome is not incorporated in the offspring. Because males gain no fitness benefits from mating with gynogens, Amazon mollies are considered sexual parasites of their host, P. latipinna.


Habitat: Brackish water systems along the gulf coast. Sailfin mollies occur from northern Mexico to South Carolina. Amazon mollies occur from south-central Mexico to southern Texas and are sympatric with sailfins in northern Mexico and southern Texas.


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