Smallest natural range for a vertebrate

The Devils Hole pupfish (Cyprinodon diabolis), a type of killifish that reaches no longer than 3 centimetres (1.2 inches), is endemic to a narrow, water-filled fissure known as the Devils Hole located in the Amargosa Desert region of Nevada, USA, to the east of Death Valley. The surface area of the spring-fed geothermal pool within the cavern is approximately 3.5 metres wide by 22 metres long (11 x 72 feet) and reaches depths of around 130 metres (430 feet), though the pupfish have been found no deeper than 24 metres (80 feet) from the surface.

The Devils Hole pupfish is critically endangered and one of the world's rarest fish species, having reached a record low of 35 individuals in 2013. Since regular counts began in 1972, the peak population size has been 548.

The species was first described in 1930 by US ichthyologist Joseph H Wales. It is most closely related to the Death Valley pupfish (C. salinus) and the Amargosa pupfish (C. nevadensis).

Within the subterranean pool, which remains at a balmy 32°C (89.6°F) throughout the year, the species' spawning ground is limited to a 2.6 x 6.1 m (8 ft 6 in x 20 ft) rocky shelf on the cavern's wall, just beneath the surface.

Attempts to protect the Devils Hole pupfish have included removing sediment from the rock shelf, adding supplementary food to the pull and building security fences around the site. There have been several attempts to relocate eggs in order to create further populations in other natural pools in the region as a precaution against extinction, but these have failed; however, a successful colony has been established in a refuge tank at the nearby Ash Meadows Fish Conservation Facility.

Another possible contender for this title is the widemouth blindcat (Satan eurystomus) restricted to five artesian wells that feed into the San Antonio Pool (part of the Edwards Aquifer) near San Antonio in Texas, USA.