Desert pupfish

The desert pupfish (Cyprinodon macularius) is a rare species of bony fish in the family Cyprinodontidae. It is a small fish, typically less than 7.62 cm (3 in) in length. Males are generally larger than females, and have bright-blue coloration, while females and juveniles are silvery or tan. A notable attribute of the desert pupfish is their ability to survive in environments of extreme salinity, pH, and temperature, and low oxygen content. The desert pupfish mates in a characteristic fashion, wherein compatible males and females will come in contact and collectively jerk in an s-shape. Each jerk typically produces a single egg that is fertilized by the male and deposited in his territory. Breeding behavior includes aggressive arena-breeding and more docile consort-pair breeding.

The desert pupfish is a federally listed endangered species in the United States. The desert pupfish was once a common fish, but it is now extirpated from most of its historical natural range. The decrease in population has been a trend since the early 1900s due to habitat destruction and fragmentation. It has been, and continues to be, preyed upon and displaced by non-native fishes, such as tilapia. Currently, the desert pupfish occurs only in three drainage basins: the Salton Sea, California, the Colorado River Delta, Baja California, and in Sonora, Mexico. Historically, the desert pupfish occurred in the drainage basins of the Gila, Santa Cruz, San Pedro, and Salt, and lower Colorado River. The range stretched from Arizona to the Gulf of California and Sonora.

Cyprinodon macularius was first described by Spencer Baird and Charles Girard in 1853. Originally, it was considered to be made up of three subspecies: the nominal desert pupfish (Cyprinodon macularius macularius), the Quitobaquito pupfish (Cyprinodon macularius eremus) and the undescribed Monkey Spring pupfish (Cyprinodon sp.).

The study of mitochondrial DNA variation between different populations of pupfish traditionally considered as C. macularius provide the basis for the species distinction between C. macularius and C. eremus. The mtDNA haplotypes of the pupfish endemic to the Rio Sonoyta Basin/Quitobaquito Springs and the pupfish endemic to the Salton Sea/Lower Colorado River Basin suggest sustained, mutually exclusive evolutionary histories. Furthermore, the differences between the frequency of inherited alleles of each species are significant enough to suggest that these individuals became separate species about 100,000 years ago. As a result, the three subspecies have been reclassified as three distinct species: the desert pupfish (C. macularius), the Sonoyta (Quitobaquito) pupfish (C. eremus), and the Santa Cruz (Monkey Spring) pupfish (C. arcuatus), with the Santa Cruz pupfish being described in 2002.

The desert pupfish is a small fish that is typically less than 7.62 cm (3 in) long; males are larger than females and generally have more vivid markings, especially during breeding seasons. Females and juveniles typically have tan or olive backs and silvery sides with narrow, dark vertical bars situated laterally. These bars are often interrupted to give the impression of a disjunct, lateral band. During mating season, males become bright blue on the dorsal portion of the head and sides, and yellow or orange on the caudal fin and posterior caudal peduncle. The dorsal profile of the desert pupfish is smoothly rounded, while its body is thickened and laterally compressed. The desert pupfish has a protruding mouth equipped with tricuspid teeth.

The desert pupfish is found in shallow waters of desert springs, small streams, and marshes below 1,524 m (5,000 ft) in elevation. The species can tolerate high salinities, water temperatures, and lower oxygen content than most fish, and will occupy habitats inhospitable to invasive fish. Pupfish typically prefer clear waters with either rooted or unattached aquatic plants, restricted surface flow, or sand-silt substrates, and will do well if their habitats have little vegetation apart from mats of benthic algae.

The distribution of the desert pupfish historically ranged from the lower Colorado River in Arizona and California to the Gulf of Mexico and onto its delta in Sonora and Baja California. Specifically, the desert pupfish occurred the Salton Sink basin in California; the Gila River basin in Arizona and Sonora, including the Gila, Santa Cruz, San Pedro, and Salt Rivers; the Rio Sonoyta in Arizona and Sonora; Puerto Peñasco, Sonora; and the Laguna Salada basin in Baja California. A large population of desert pupfish is protected at the Cibola National Wildlife Refuge. Due to both habitat destruction and species reclassification, however, the desert pupfish has a smaller territory distribution than previously thought.^{[citation needed]}

As of 2010, naturally occurring populations of the desert pupfish are restricted in the United States to two tributaries and various shoreline pools and irrigation drains of the Salton Sea in California. In Mexico, the desert pupfish is scattered along the Colorado River Delta and the Laguna Salada basin. Collectively, there are 11 known populations of extant wild desert pupfish within the United States and Mexico. Attempted reintroductions have yielded 16 transplanted populations in Arizona, as well as 46 captive or refuge populations in Arizona, California, and Mexico.