Leon Springs pupfish are found in spring-fed waters, and are tolerant of a wide range of salinities and temperatures. Historically, they occupied a large area with spring-fed marshes, streams, and pools, but are now restricted to a narrow area due to decliningand drying springs. They evolved to be the only pupfish species in a given habitat; the presence of sheepshead minnow (Cyprinodon variegatus) is a threat to their persistence due to competition and hybridization.

Population size can be limited by total available habitat suitable for territories. Measured territories range from 0.4–2.5 square meters (4–27 square feet). Hybridization between Leon Springs pupfish and sheepshead minnow is possible. Instances of introgression of sheepshead minnow into the Leon Springs pupfish genome have not been as extensive or rapid as for Pecos pupfish (Cyprinodon pecosensis), but were still significant threats to Leon Springs pupfish. Restoration treatments to remove all Leon Springs pupfish and sheepshead minnows from portions of the Diamond Y Preserve using intensive seining and ichthyotoxins took place in the mid-1970s and the late 1990s to early 2000s. 

Critical Habitat

Critical habitat for Leon Springs pupfish was finalized in 1980 (45 FR 54678–54680). At the time, the entire known range of the species was designated due to its extreme restriction. The critical habitat consists of the Diamond Y Spring and pool, its outflow channel to the confluence with Leon Creek, the area of the confluence, and Leon Creek and its riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian area to about 1.6 km (1.0 mi) past the intersection of Leon Creek and Texas State Highway 18. It is contained almost entirely within The Nature Conservancy’s Diamond Y Preserve.

The critical habitat rule for Leon Springs pupfish does not identify specific physical and biological features (PBFs) used to develop critical habitat, but it does describe features needed by the species to persist (45 FR 54678–54680).For example, the rule states that the species inhabits saline, quiet waters near the edges of shallow pools with minimal growth of vegetation. It also notes that Leon Springs pupfish do much of their feeding from the bottom mud or from vegetation-consuming invertebrates, detritus, diatoms, and vascular plants. Therefore, physical and biological features needed by Leon Springs pupfish would include: spring flows from a mix of aquifers, predominantly the Rustler Aquifer; calm and shallow waters; limited aquatic emergent vegetation; algae and aquatic invertebrates for food; hard substrate or algal mats in shallow water for spawning.

Leon Springs pupfish are omnivorous, feeding primarily on detritus, algae, and small animals found on the substrate.

Territorial males do not guard eggs specifically, but by chasing intruders provide indirect protection. Males exhibit three different behavior techniques around mating based on size: large males maintain and defend territories, average-sized males use satellite positioning around the periphery of territories, and small males “sneak” into territories to quickly mate with females before escaping.

Baird and Girard described Leon Springs pupfish based on material from “Leon’s Spring,” Pecos County, Texas. Leon Springs pupfish are a blackish-brown above and yellowish green beneath. Males do not have vertical dark bars, and females have broken. During breeding season, males have yellow on their dorsal and caudal fins. The maximum size is approximately 50 millimeters (1.97 inches) in standard length.

Leon Springs pupfish typically live up to 20–23 months in the wild. 

Reproduction in Leon Springs pupfish is possible year-round. The peak breeding period was recorded as July in 1977, but more recently was determined to be late May and early June. Females become receptive to males and spawn primarily in the water temperature range of 24–29 °C (75–84 °F). Sexually mature females are at least 29 mm (1.14 in) in standard length. Growth rates have not been reported. Most individuals participate in one spawning season, while a few may participate in two. Spawning is typical of pupfish species. Female Leon Springs pupfish lay eggs one at a time within male territories.