San Marcos Gambusia

Gambusia georgei

Description

Gambusia is well-defined and mature males may be distinguished from related genera by their thickened upper pectoral fin rays. Only a limited number of Gambusia are native to the United States and of this subset, San Marcos gambusia, G. georgei, has one of the most restricted ranges.

The San Marcos gambusia ranges in length from 1-1.6 in (2.5-4 cm). It is subtly different from the western mosquitofish, G. affinis. Scales tend to be strongly crosshatched in contrast to the less-distinct markings on the scales of G. affinis. In addition, San Marcos gambusia tend to have a prominent dark pigment stripe across the distal edges of their dorsal fins. A diffuse mid-lateral stripe extending posteriorly from the base of the pectoral fin to the caudal peduncle is also often present, especially in dominant individuals. As in G. affinis, a dark subocular bar is visible and is elicited easily from frightened fish. Compared to G. affinis, G. georgei has fewer spots and dusky pigmented regions on the caudal fin. The median fins (i.e., unpaired fins: dorsal, caudal, and anal fins) of wild-caught specimens of San Marcos gambusia tend to be lemon yellow under certain behavioral patterns (when they are not under stress). In a dominant or high male, this color can approach a bright yellowish-orange, especially around the gonopodium. A bluish sheen is evident in more darkly pigmented individuals, especially near the anterior dorsolateral surfaces of adult females.

Behavior

The San Marcos gambusia is a livebearer, which means eggs hatch inside the female's body and emerge alive. The female is capable of bearing up to 60 young in a single brood. This gambusia feeds on insect larvae and other invertebrates in slow-moving waters that are shaded by overhanging trees or bridges.

Hybridization between G. georgei and G. affinis was first noted in 1969 and the production of hybrid individuals between them has continued for many years without obvious introgression of genetic material into either of the parental species. Given the history of hybridization between these two species, this factor was not thought to be of primary importance in considerations of the status ofG. georgei. It was thought that so long as the proportion of hybrids remained relatively low compared to the abundance of pure G. georgei, few problems associated with genetic swamping or introgression would occur. However, the series of collections taken during 1981-1983 indicate that hybrid individuals may have become many times more abundant than the pure G. georgei. It is possible that hybrid individuals may now be competing with G. georgei, placing an additional stress on the small native population.

Habitat

The San Marcos gambusia apparently prefers quiet waters adjacent to sections of moving water, but seemingly of greatest importance, thermally constant waters. G. georgei is found mostly over muddy substrates but generally not silted habitats, and shade from over-hanging vegetation or bridge structures is a factor common to all sites along the upper San Marcos River where apparently suitable habitats for this species occur. Introduced elephant ears have been noted in previously recorded localities for the species. Although the exact nature of the relationship between the occurrence and abundance of elephant ears and the disappearance of G. georgei is unknown, some investigators believe these non-native plants may have modified essential aspects of the gambusia's habitat.

Compared to G. georgei, G. affinis tends to show similar preferences for shallow, still waters, but differs strikingly from G. georgei in ability to colonize environments with greater temperature fluctuation. These environments include the partially isolated sloughs, intermittent creeks, and drainage ditches found in the upper San Marcos River, and in the nearby Blanco River and lower San Marcos River, as well.

The San Marcos gambusia apparently requires: 1) thermally constant water; 2) quiet, shallow, open water adjacent to sections of moving water; 3) muddy substrates without appreciable quantities of silt; 4) partial shading; 5) clean and clear water; and 6) food supply of living organisms.

Distribution

The gambusia's entire known range is restricted to the San Marcos River near the city of San Marcos (Hays County) in south-central Texas. Historically, the San Marcos gambusia populations have been extremely sparse.

The San Marcos gambusia is currently restricted to a 0.6-mi (1-km) section of the San Marcos River. Most specimens have been found between the Interstate Highway 35 crossing and Thompson's Island. This gambusia is extremely rare as determined by surveys conducted in 1978 and 1979 in the San Marcos River. Biologists netted more than 20,000 Gambusia specimens but counted only 18 San Marcos gambusia among them.

Intensive searches conducted in May, July, and September 1990 were unsuccessful in locating any pure San Marcos gambusia. More than 15,450 Gambusia were identified during the searches. One individual collected during the search—an immature fish with plain coloration—was visually identified as a possible backcross (hybrid) of the San Marcos gambusia G. affinis. According to the 1996 Recovery Plan for the species in the Comal Springs and San Marcos habitat, the decrease in San Marcos abundance along with their hybrids suggests the extinction of the species.

Threats

The San Marcos gambusia's very restricted distribution in the river and its absence from the head-waters at Spring Lake indicate very specific habitat requirements. It is extremely sensitive to any alteration of its habitat. Changes in water turbidity caused by runoff from land clearing and construction, an increase in water temperatures caused by lowered water flows, and excessive pumping of groundwater from the Edwards Aquifer, which supplies water to the city of San Antonio, could easily eliminate the species. The entire known range has been designated as habitat critical to the survival of the San Marcos gambusia.

The Texas Department of Water Resources forecast that groundwater pumping for human uses in the region will continue to increase well into the twenty-first century. At the current rate of increase, scientists predict that the Edwards Aquifer will be so depleted that flow from the San Marcos Springs will cease around 2000. Without the cooperation of all state and local agencies that manage use of the aquifer, recovery of the San Marcos gambusia and other endemic wildlife is considered a remote possibility.

Conservation and Recovery

The 1996 San Marcos/Comal (Revised) Recovery Plan, which covers the San Marcos gambusia 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 gambusia) 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. Though downlisting is considered a possibility for several of the species, the plan notes that, because the San Marcos gambusia is teetering on the brink of extinction (indeed, it may have gone over the edge), it is unlikely the species could be reclassified from Endangered to Threatened in the foreseeable future. Interim conservation measures are recommended to measure progress toward preventing its extinction. Academic researchers, Texas Parks and Wildlife Department scientists and the U. S. Fish and Wildlife Service continue to search for the gambusia during all collection and research with fishes that is done on the San Marcos River.

Contact

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

References

Brune, G. 1981. Springs of Texas. Branch-Smith, Fort Worth.

Hubbs, C., and A. E. Peden. 1969. "Gambusia georgei from San Marcos, Texas." Copeia 1969 (2):357-364.

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