September 2026
Potosí Pupfish
Cyprinodon alvarezi
Image © Barbara Nicca
The Potosí pupfish (Cyprinodon alvarezi) is a small desert fish endemic to the Potosí Springs near Galeana, Nuevo León, Mexico. It evolved in an isolated spring system at around 1,880 m above sea level, where it inhabited clear, spring-fed waters rich in aquatic vegetation. The species is the largest of the three Extinct in the Wild Mexican pupfishes, with adults typically reaching 50-60 mm in length, and males displaying a striking metallic blue coloration with a characteristic black tail bar. Like many pupfish, it was highly adapted to harsh, mineral-rich environments and fed primarily on filamentous algae, small crustaceans and aquatic insect larvae.
The species became Extinct in the Wild after decades of groundwater extraction for agriculture caused the complete desiccation of the Potosí Springs, leading to the loss of its habitat and the extinction of several other endemic aquatic species that shared the spring. The last confirmed wild record dates from 1994, and today the species survives only as ex-situ populations maintained by a small number of zoos and aquariums. Conservation efforts are now focused on increasing ex-situ populations, repatriating the species to Mexico, restoring suitable spring/semi-natural habitats, and ultimately re-establishing self-sustaining wild populations as part of the Extinct in the Wild Pupfish Conservation Action Plan 2024-2029.
Nomination: Alex Cliffe, The Zoological Society of London