The desert pupfish, a small cyprinodontid ranging from one to two inches in length, is an endangered species. But once there was an abundance of the desert pupfish in portions of southern Arizona and southeast California, northern Baja California and Sonora, the United States, and Mexico. The historical habitats varied in complexity, size, and character. The habitats included margins of larger lakes and rivers, streams, and lakes. Surprisingly the desert pupfish is adaptable to extreme environmental condition changes. The pupfish it is becoming extinct because it is losing its environment. Other factors for the desert pupfish going extinct are because of pollution, human modifications to their environment, and introduction of exotic fish.
Presently, the range of the desert pupfish is extremely diminished to just two springs in California, the Quitobaquito Springs in Arizona, and a few shoreline pools and irrigation drains of the Salton Sea. They can also be found in scattered locations in Mexico along the Rio Sonoyta, in the Lauguna Salada basin, and on the Colorado River Delta.
The body of the desert pupfish is chubby or markedly compressed laterally in adult males. The over all body structure is thick, the mouth is highly protractile and comes equipped with tricuspid jaw teeth. It has a smoothly rounded profile. The desert pupfish has spine-like projections that are characteristics of scale circuli. The background coloration in females and juveniles is silvery; the sides have narrow, vertical dark bars, laterally. It gives the appearance of a disjunct lateral band. The fins are pretty much colorless except for a dark ocellus in the dorsal and (maybe) a dark spot on the anal fin. Mature males are brightly colored, the caudal fin and the posterior portion of the caudal peduncle are orange or yellow, or they can sometimes be an intense orange-red. Other fins are usually dark colored. The body is an iridescent light-to-sky blue color, especially on the dorsal surface of the predorsal region and on the head.
Desert pupfish have the extraordinary ability to survive in very harsh conditions. They can survive in water that has three times more salt than the ocean; it can survive in high water temperatures. The desert pupfish can also survive in low dissolved oxygen concentration. Amazingly they can survive abrupt changes in the water salinity and temperature.
The desert pupfish can become sexually mature as early as six weeks of age if there is an abundance of food and suitable temperatures. Most do not breed until their second summer, although on rare occasions they will bread in their first summer. The male pupfish are highly aggressive during the mating season in which they establish, actively patrol, and defend territories that are customarily less than one meter deep. Usually, they defend an area of about one to two square meters, but this can change depending on their individual size. The male breeding behaviors are sometimes strange. But they usually resort to consort pairing in habitats with large populations.
The females usually swim in loose schools and forage inconspicuously. When a female in ready to spawn she leaves the school and then is attracted by a territorial male. As the two move toward one another, the female tilts, head-first toward the bottom of the river and then takes a small piece of substrate into her mouth. Once she resumes a horizontal position she spits out the material, this ritual may be repeated several times until she settles down at the bottom. The male then assumes a position parallel and against the female. Then the two contort together to form a 'S' shape. The male's anal fin cups around the vent region of the female, and then she releases an egg (about two millimeters), which is immediately fertilized. Spawning takes less than a minute, but can take longer depending on how many eggs the female produces. The eggs are randomly dispersed throughout the male's territory and there is no direct parental care. Incubation varies with water temperature, but is usually about ten days long.
Larva pupfish feed on tiny invertebrates usually within a few hours to a day after hatching. As they grow they become omnivores, consuming whatever algae, plants, small invertebrates, and detritus is available. Adult foods consist of crustaceans, insects, mollusks, pile worms, detritus or algae.