Cyprinodon

Cyprinodon is a genus of pupfishes found in waters that range from fresh to hypersaline. The genus is primarily found in Mexico, the Caribbean Islands and southern United States (Arizona, California, Florida, Nevada, New Mexico, Oklahoma and Texas), but C. variegatus occurs as far north as Massachusetts and along the entire Gulf of Mexico coastline, and C. dearborni and C. variegatus are found in northern South America. Many species have tiny ranges and are highly threatened, in some cases already extinct. Cyprinodon are small; the largest reaches 10 cm (3.9 in) in length and most other species only reach about half that size.

Based on phylogenetic evidence, Cyprinodon diverged from its closest relative, the recently extinct Megupsilon, during the Late Miocene, and saw a rapid evolutionary radiation afterwards. The only known fossil species from the genus is C. breviradius from the late Miocene or early Pliocene-aged sediments of Death Valley National Park (originally considered part of the Late Eocene-aged Titus Canyon Formation).

A few Cyprinodon species have quite large ranges, notably C. variegatus, but the vast majority have small ranges, typically restricted to one or two Mexican states or U.S. states, Hispaniola, or a Bahaman island. C. longidorsalis and C. diabolis have both been said to have the smallest native range of any vertebrate species, with the former being restricted to a spring pool that covers about 10 m² (110 sq ft) and the latter to an 18 m² (190 sq ft) shelf in a spring pool, but C. longidorsalis is now only found in captivity as its habitat has disappeared. While most Cyprinodon species have separate distributions, seven (C. beltrani, C. esconditus, C. labiosus, C. maya, C. simus, C. suavium and C. verecundus) are endemic in Lake Chichancanab in Quintana Roo, Mexico, and three (the endemic C. brontotheroides and C. desquamator, and the widespread C. variegatus) live in the hypersaline lakes on San Salvador Island in the Bahamas. In a few other cases separate species do come into contact towards the edges of their distributions where they often hybridize, notably C. eximius with C. pachycephalus and C. atrorus with C. bifasciatus, but also C. variegatus in places where it has been introduced by humans into the ranges of other Cyprinodon species.

Although the individual Cyprinodon species often have a highly specific habitat, overall the genus occurs in a remarkable range of places, such as springs (including those isolated in deserts), pools, lakes, coastal lagoons, creeks, streams and rivers. Their salinity and temperature range is very broad, like those living in hot springs (taken to the extreme in C. julimes in water up to 46 °C or 115 °F, and C. pachycephalus up to 49 °C or 120 °F), and those of hypersaline habitats where the salinity far exceeds that of sea water. Certain species may even experience very large variations in the temperature and salinity over a relatively short period. For example, some populations of C. variegatus live in water where the temperature has been known to change from 15 to −1.8 °C (59 to 29 °F) in less than 24 hours (in the coldest temperatures they bury into the substrate). Some populations of C. nevadensis tolerate water temperatures between 2 and 44 °C (36–111 °F), and C. salinus live in waters where the temperature may change by as much as 19 °C (34 °F) in a day and 40 °C (72 °F) in a season. Furthermore, the salinity of C. salinus' habitat may vary from less than one-third of that of sea water to almost five times as much as sea water in a season.

Most species in the genus are seriously threatened. C. arcuatus, C. ceciliae, C. inmemoriam, C. nevadensis calidae and an undescribed species popularly known as the "Perrito de Sandia" are already extinct. C. arcuatus was restricted to springs in the US state of Arizona and probably also in the Mexican state of Sonora, while all the others were restricted to spring systems in Mexico.

Three species from southwestern Nuevo León, C. alvarezi, C. longidorsalis and C. veronicae, have become extinct in the wild, only surviving in captivity. A few other Mexican species still considered endangered or vulnerable by the IUCN, including at least C. maya, C. simus and C. verecundus of Lake Chichancanab, also appear to only survive in captivity. Several others have very small remaining populations in the wild. Among the species that survive in the wild, the rarest is perhaps C. diabolis from the tiny Devils Hole in Nevada; in recent decades its population has fluctuated between a few tens and a few hundred individuals. Primary threats to pupfish are habitat loss due to water extraction, drought and pollution, and introduced species.

Most Cyprinodon species feed on algae, cyanobacteria and detritus, but may also supplement their diet with small crustaceans and aquatic insect larvae. Some species mainly feed on small animals like aquatic insects. C. variegatus, a species that otherwise has a diet typical of pupfish, will clean other fish by feeding on parasites on their body.

In the two places where several species live together they have diverged into different niches, including the fish-eating C. maya (Lake Chichancanab), zooplankton-eating C. simus (Lake Chichancanab), amphipod- and bivalve-eating C. labiosus and C. verecundus (Lake Chichancanab), scale-eating C. desquamator (San Salvador Island lakes), and ostracod- and gastropod-eating C. brontotheroides (San Salvador Island lakes).