The Devils Hole pupfish (Cyprinodon diabolis) is a critically endangered species of the family Cyprinodontidae (pupfishes) found only in Devils Hole, a water-filled cavern in the US state of Nevada. It was first described as a species in 1930 and is most closely related to C. nevadensis and the Death Valley pupfish (C. salinus). The age of the species is unknown, with differing analyses offering ranges between one thousand and sixty thousand years. It is a small fish, with maximum lengths of up to . Individuals vary in coloration based on age and sex: males are bright metallic blue while females and juveniles are more yellow. A defining trait of this species is its lack of pelvic fins. The pupfish consumes nearly every available food resource at Devils Hole, including beetles, snails, algae, and freshwater crustaceans, with diet varying throughout the year. It is preyed on by the predaceous diving beetle species Neoclypeodytes cinctellus, which was first observed in Devils Hole in 1999 or 2000. Reproduction occurs year-round, with spikes in the spring and fall. Females produce few eggs and the survivorship from egg to adult is low. Individuals live 10–14 months.
Devils Hole is more than deep, though pupfish are only found in the upper . The water is a constant temperature of and dissolved oxygen levels are low. A small, shallowly submerged rock shelf provides critical feeding and spawning habitat for the pupfish. Nearby agricultural irrigation in the 1960s and 1970s caused the water to drop in Devils Hole, resulting in less and less of the shelf remaining submerged. Several court cases ensued, resulting in the Supreme Court case Cappaert v. United States, which determined that the preservation of Devils Hole as a National Monument in 1952 implicitly included preservation of adequate groundwater to maintain the scientific value of the pool and its fauna. Other threats faced by the species include flash floods, earthquakes, and vandalism.
As its entire native range is a single locality, efforts to create other populations have proceeded since the 1960s and 1970s, most of which have failed. Three refugia were created in 1972, 1973, and 1990, though all were closed by 2007 as a result of maintenance failures, hybridization, and small founder populations. In the early 2010s, an exact replica of the uppermost of Devils Hole was constructed at Ash Meadows Fish Conservation Facility, which was populated with eggs taken from Devils Hole in winter months when development into adults is unlikely. Efforts to conserve the wild population have included removing sediment from the shallow shelf, adding supplemental food, and installing fences and security cameras to keep unauthorized people away. Conservation efforts have been costly and divisive. During the legal battle over ground water in the 1960s and 1970s, bumper stickers were distributed that read "Kill the Pupfish" or "Save the Pupfish". Some have argued that the species should be allowed to go extinct, while others have said this would be akin to "bombing the Louvre to make way for a parking lot".
Population counts are conducted twice a year, in the spring and fall, with the fall population usually much larger. Since 1972, population counts have peaked at around 550 individuals. The April 2013 count showed only 35 remaining in the wild, but the numbers recovered, and by September 2022, the count showed a total of 263 observed wild pupfish. However, after winter earthquakes swept algae off of the spawning shelf, the Spring 2025 count showed only 38 fish remaining. The Devils Hole pupfish has been listed as endangered by the US federal government since 1967 and critically endangered by the International Union for Conservation of Nature since 2014.
Taxonomy and evolution
The Devils Hole pupfish was described as a new species in 1930 by American ichthyologist Joseph H. Wales. The holotype had been collected by Wales and George S. Myers in March 1930 at Devils Hole in the US state of Nevada. The species name "diabolis" was chosen to allude to the type locality of Devils Hole. Both the species name and the location spell "Devils Hole" without an apostrophe, a relic of the "quirks of government cartographers and scientists". According to genetic analysis, the sister taxon of the Devils Hole pupfish is Cyprinodon nevadensis. Along with the Death Valley pupfish (C. salinus), the three Amargosa River basin species form a clade.
The age of the species is subject to considerable debate, with analyses recovering starkly different figures. Devils Hole was formed around 60,000 years ago, with some researchers assuming the pupfish has existed in isolation for 10,000–20,000 years. How it colonized Devils Hole is unknown; hypotheses include arriving via subterranean waters or over dry land. As Native Americans used pupfish species as food, it has been speculated that they introduced the pupfish to Devils Hole, intentionally or not. Its divergence from a common ancestor with C. nevadensis mionectes was estimated at 217–2530 years in one study. Two studies, each based on independent genetic datasets, estimated that this species may have first colonized the hole within the past 1,000 years, but another suggested the species is as old as 60,000 years. These estimates depend heavily on knowledge of the mutation rate in this species, which is unknown, but is predicted to be one of the highest for any vertebrate due to its small population size.
Description
The Devils Hole pupfish is the smallest pupfish species in the genus Cyprinodon, with lengths up to . The average length is . Males and females differ in coloration. Males are overall dark brown with metallic blue on their sides. The margins of all fins are black, and the back has golden iridescence. Iridescence is particularly pronounced on the opercles (gill covers) which have a violet sheen on their posterior side. The iris is blue and also iridescent. Females and young are more yellow in color than the males. Females have yellowish-brown backs, and the margins of their pectoral and caudal fins are yellow, not black. The dorsal fin has a black margin like the males, however. Females' opercles are metallic green, and their eyes are tinted metallic blue. The young are overall colored as the females, though they have a faint vertical bar on their sides.
Individuals lack pelvic fins, though have been observed to grow them when raised in lower temperatures in captivity. Its dorsal fin has twelve rays, while each pectoral fin has seventeen rays. The caudal fin is convex in appearance and has twenty-eight rays, curving outward at the margin. Its lateral series (the number of scales from the back end of the opercle to the beginning of the tail) is twenty-seven scales. The scales are ctenoid, or toothed, on the outer margin.
Biology and ecology
The Devils Hole pupfish consumes a variety of food items representing nearly all possible food resources in Devils Hole. Its food resources include inorganic particulate matter; the algae Spirogyra and diatoms; the freshwater crustaceans Hyalella azteca and ostracods; protozoa; the beetle Stenelmis calida; the flatworm Girardia dorotocephala; and the freshwater snails Tryonia. The consumption of the various food resources varies seasonally, though inorganic particulate matter had a high frequency of occurrence in stomach contents year-round in one study. The inorganic particulate matter consists of primarily travertine, a form of limestone. The three most common food items for each season by frequency of occurrence were:
Spring (March through May): inorganic particulate matter (83%), diatoms (75%), and Spirogyra (58%) Summer (June through August): inorganic particulate matter (79%), Spirogyra (46%), and diatoms (46%) Autumn (September through November): inorganic particulate matter (95%), Spirogyra (74%), and Hyalella azteca (33%) Winter (December through February): inorganic particulate matter (100%), diatoms (91%), and ostracods (45%)
As Spirogyra was mostly found undigested in the stomach, the authors hypothesized that it was not important as a food resource, but rather as a foraging substrate. The inorganic particulate matter was thought to be incidentally consumed as well as a result of the fish's foraging strategy of bottom feeding and surface feeding.
Predators of the Devils Hole pupfish include the diving beetle species Neoclypeodytes cinctellus, which consumes its eggs and juveniles. N. cinctellus likely also preys on some of the same invertebrates as does the Devils Hole pupfish, meaning that it is a competitor as well as a predator. The diving beetle only recently became part of the ecosystem, and was first documented at Devils Hole in 1999 or 2000.
Although spawning year-round, spawning peaks from mid-February to mid-May with a smaller peak from July to September. Devils Hole pupfish females have low fecundity, which is the capacity to create offspring. The average female may only produce four or five mature ova (egg cells) each breeding season. Mature ova represent 10–20% of the total number of ova produced. During each spawn, a mature female is thought to produce only a single egg, which are only in diameter. In addition to its low fecundity, eggs have low hatching success and juveniles have low rates of survival. Individuals have a lifespan of 10–14 months.
Because the rock shelf upon which the fish feed and breed is susceptible to seismic activity, specialized behavior mitigates the impact of earthquakes. When a disturbance such as an earthquake occurs, it causes the fish to flee en masse into the depths, and begin a spawning event that may be out of season. During spawning brought upon by a disturbance, several males chase lone females until they become receptive, at which point the female allows one of the males to swim next to her. The female then lays an egg that the male immediately fertilizes.
The Devils Hole pupfish has daily and seasonal movement within the Hole. Around midday, when incoming sunlight is at its maximum intensity on the shallow shelf, the number of fish on the shelf decreases. This tendency to leave the shelf at midday is most pronounced April through September. From December to March, when the shallow shelf receives little if any direct sunlight at midday, the number of fish on the shelf increases as the day advances. In the summer months when the shelf receives the most sunlight, fish are overall less likely to use the shallow shelf.
Having adapted to an environment with low oxygen saturation, the Devils Hole pupfish has developed a behavior known as "paradoxical anaerobism". The fish enters a state of torpor, and has been known to forego breathing oxygen for up to two hours. As a byproduct of this alternate respiration method, the fish produces ethanol.
Habitat
Devils Hole and the pupfish are located in the Amargosa Desert ecosystem, in the Amargosa Valley, of southwestern Nevada, US, east of Death Valley and the Funeral Mountains and Amargosa Range. The Amargosa River is part of Devils Hole and the region's aquifer hydrology. Devils Hole is a water-filled cavern extending into a hillside. It is at an elevation of above sea level and the water is a constant temperature of . The surface area of Devils Hole is about 22 m long by 3.5 m wide (72 ft long by 11.5 ft wide). Its depth is at least . Devils Hole "may be the smallest habitat in the world containing the entire population of a vertebrate species". Approximately deep on one end of Devils Hole is a small rock shelf of . The dissolved oxygen of the water is 2.5–3.0 ppm up to around in depth, though the shallow shelf can have dissolved oxygen levels as high as 6.0–7.0 ppm in June and July.