Clear Creek Gambusia

Gambusia heterochir

Description

The Clear Creek gambusia, Gambusia heterochir, is a stocky gambusia with a pronounced metallic sheen and distinctive, dark concentric markings on the back and sides. Females have a pronounced anal spot. Males are distinguished from other livebearers by a deep notch in the dorsal margin of the pectoral fin (behind the gills).

Behavior

The female Clear Creek gambusia may store sperm for several months after being inseminated by multiple males and is fertile for seven months of the year, from March through September. Males are semi-territorial during the breeding season, with the dominant males occupying territories with the lowest predation potential. Females abound in the preferred habitat, while less dominant males attract fewer receptive females. During the seven months in which the female is fertile, she is capable of producing about 50 young every 42 days, or six broods per year. After gestation of 40 days, eggs hatch inside the mother's body, and young emerge alive. Clutch size depends upon the size of the female, and up to 100 young may be produced per brood.

The primary food source, about 80%, for the Clear Creek gambusia is the amphipod, Hyalella texana, but it is an opportunistic feeder and will consume a wide variety of foods in the absence of amphipods. This gambusia is active during the day in all seasons.

Habitat

This gambusia is restricted to shallow springs and outflow streams at the base of a limestone cliff with clear, clean water, low pH (acid), and nearly constant, year-round temperatures. It prefers areas of profuse aquatic vegetation. Upper Clear Creek rises from the Wilkinson Springs, a group of limestone springs that discharges water from the Edwards Aquifer.

Clear Creek has an elevational difference of about 48 ft (15 m) from headwaters to confluence, and the four dams that have been constructed have caused pools to form. The Clear Creek gambusia occurs in these isolated spring-fed pools that maintain a fairly constant temperature of around 68ÞF (20ÞC). The surrounding shrub/brush rangeland is classified as Texas savannah and is used primarily for ranching.

Distribution

The Clear Creek gambusia is endemic to the source pools and headwaters of Clear Creek, a small tributary of the San Saba River in Menard County, central Texas. Its existence was first documented in 1953, and it has been known only from this location.

The population is restricted to the main pools of the Wilkinson Springs at the headwaters of Upper Clear Creek on the Clear Creek Ranch, situated 10 mi (16 km) west of Menard near Ft. McKavett. The headspring pool covers an area of about 2.2 acres (1 hectare). The natural course of Upper Clear Creek has been modified extensively to provide water for livestock, irrigation, and humans. About 250 ft (75 m) downstream from the headsprings, a series of four dams impounds the creek into pools.

Threats

The main threat to this species is simply its extremely limited distribution and dependence on a single group of springs. Should the water table fall because of groundwater pumping or water flows become diverted to support more intensive human uses, the population would be eliminated. The Clear Creek gambusia is also threatened by its ready ability to interbreed with other more common gambusia species, which are found in Clear Creek below the dams.

Conservation and Recovery

Of particular concern was the predatory mosquitofish (G. affinis ), which in the 1970s found its way into some of the dam pools through a deteriorated section of an aging, earthen-concrete dam. This dam was extensively repaired in 1979 by U. S. Fish and Wildlife Service (FWS) personnel, and mosquitofish were removed. Although the site is privately owned, the FWS has reached a conservation agreement with the landowner to manage the habitat pools. Since the repairs to the dam were completed, the primary goal of recovery has been to maintain the status quo. Should the habitat be further jeopardized, for example, by sale of the ranch to resort developers, the FWS anticipates a more aggressive intervention.

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

Hubbs, Clark. 1957. "G. heterochir, a New Poeciliid Fish from Texas with an Account of Its Hybridization with G. affinis. " Tulane Studies in Zoology 5:1-16.

Minckley, W. L. 1962. "Two New Species of Fishes of the Genus Gambusia (Poeciliidae) from Northeastern Mexico." Copeia 1962:391-396.

Rosen, D. E., and R. M. Bailey. 1963. "The Poeciliid Fishes (Cyprinodontiformes): Their Structure, Zoogeography, and Systematics." Bulletin of the American Museum of Natural History 126:1-176.

U.S. Fish and Wildlife Service. 1980. "Recovery Plan for Clear Creek Gambusia Gambusia heterochir. " U.S. Fish and Wildlife Service, Albuquerque.