The Julimes pupfish (Cyprinodon julimes) () is a species of killifish in the family Cyprinodontidae. This pupfish is endemic to El Pandeño, a hot spring in Julimes, Chihuahua, Mexico, and it is adapted to life in water that reaches temperatures as high as . Consequently, it has been referred to as the "hottest fish in the world", although the closely related Cyprinodon pachycephalus can occur in somewhat warmer waters. Cyprinodon julimes was scientifically described in 2009 as similar to Cyprinodon eximius, but it has a bigger head, nearly one-third of its standard length. Appearance Its body is deep, the dorsal and ventral profile is convex and the lower jaw exceeds the premaxilla. The dorsal fin is placed forward with respect to the pelvic fin. Females and young have reticulate patterns of dark and light silver-brownish bands of varying lengths and thickness over the flanks, as well as a conspicuous black spot or ocellus on the distal edge of the dorsal fin. Dominant males are bluish-green in color and show a black bar along the distal edge of the caudal fin. Holotype.- UANL 18721 Collected by Ma.de Lourdes Lozano Vilano, Mauricio De la Maza Benignos, Ma. Elena García Ramírez, and the WWF group. February 25, 2007. Mature male 31.8 mm SL Paratypes.- UANL 18721 (39 specimens); TNHC39729 (2 specimens); USNM 391634 (2 specimens); UMMZ 248729 (2 specimens). Same data as holotype. Distribution The Julimes pupfish inhabits sections of "El Pandeño" hot spring system in the municipality of Julimes, Chihuahua. The system is located within the Middle Rio Conchos Basin and consists of a geothermal spring and a series of divergent canals. Until year 2011 C. julimes was restricted to an area of approximately that is part of the original system and a contiguous man-made blind canal, with depth ranging from and constant water temperatures ranging between . Biology and behavior Females and young group in small schools. They feed on detritus, perifitum (organic matter that collects on the surface of aquatic plants), and invertebrates — including the microendemic cochliopid Julimes springsnail Tryonia julimensis — and were often seen feeding on floating grass seeds. Dominant males establish territories of about in diameter around sunken branches, rocks or pieces of travertine from where they court passing females. Displays of aggression dissuade other males from entering their territories. Females that are ready to spawn cautiously enter the dominant male's territory, showing a courtship display