[Photo: Texas Parks and Wildlife]

San Marcos Gambusia
Gambusia georgei

Description: The San Marcos gambusia is a small fish of the family Poeclilidae. The San Marcos gambusia is 1 of 3 species of Gambusia in the San Marcos River, although it has by far the most limited range. It is very similar in appearance to the western mosquitofish (Gambusia affinis). It is 1 inch (2.5 cm) long, with a dark stipe on the upper edges of its dorsal fin, and dark bars around the eyes. This fish has yellowish fins (yellowish-orange around the gonopodium of adult males) and a bluish sheen around the head in adult females.

Life History: The male San Marcos gambusia possesses a gonopodium (tube structure on anal fin) that is uses to transfer sperm to female during breeding. The San Marcos gambusia gives birth to live young and has been shown to produce clutch sizes from 12-60 in captivity. Relatively little is known about the food consumption of the San Marcos gambusia, although it is assumed to feed on insect larvae and other invertebrates.

Habitat: The San Marcos gambusia prefers partially shaded habitats with overhanging vegetation in shallow, sparsely vegetated, relatively still waters adjacent to fast flowing water. Constant water temperature (70-72 degrees F [21-22 deg C]) is extremely important to this fish and it occupies the upper portions of the San Marcos River and San Marcos Springs where temperature is constant. The San Marcos gambusia prefers muddy bottoms without much siltation.

Distribution: Gambusia georgei is distributed along the thermally constant sections of the San Marcos River which includes San Marcos Springs and the upper gravelly portion of the San Marcos River down to 0.5 miles below the Interstate 35 Bridge, in Hays County, Texas.

The map below depicts the critical habitat for this species, as designated by the US Fish and Wildlife Service pursuant to the Endangered Species Act.

NOTE: No warranty is given, expressed or implied, as to the accuracy, reliability, or completeness of these data. Data do no represent a legal description of the critical habitat boundary; refer to the textual description in the appropriate final rule for this species as published in the Federal Register.

Federally Designated Critical Habitat for the San Marcos Gambusia (Gambusia georgei) [Figure: Houston Advanced Research Center using US FWS data]

Status: The San Marcos gambusia is a state and federally listed (1980) endangered species. The San Marcos gambusia was last collected in 1983. This species is considered very rare, and may possibly be extinct, largely to reduced habitat and hybridization with the western mosquitofish. Slight changes in the San Marcos River channel, reduction in spring flow, or surface pollution could severely injure this population. The flow considered as "take" by the US FWS is 100 cubic feet per second.

Resources:

Hubbs, Clark and Alex E. Peden. 1969. Gambusia georgei sp. nov. from San Marcos, Texas. Copeia, Vol. 1969, No., pp. 357-364.

Texas Parks and Wildlife, Endangered Species of the Edwards Aquifer

Texas Parks and Wildlife, San Marcos Gambusia Profile

US Fish and Wildlife Service, Critical Habitat Listing

US Fish and Wildlife, Endangered Species Listing

US Fish and Wildlife Service, San Marcos Recovery Plan