The Human Fish (Proteus anguinus), also called the olm, olm salamander, or proteus, is an aquatic salamander in the family Proteidae, the only exclusively cave-dwelling chordate species found in Europe. In contrast to most amphibians, it is entirely aquatic; it eats, sleeps, and breeds underwater. Living in caves found in the Dinaric Alps, it is endemic to the waters that flow underground through the extensive limestone of the karst of Central and Southeastern Europe, specifically southern Slovenia, the basin of the Soča River (Italian: Isonzo) near Trieste, Italy, southwestern Croatia, and Bosnia and Herzegovina.
It is called the "human fish" by some locals because of its skin color, similar to that of Caucasian people as well as "cave salamander" or "white salamander".
This cave salamander is most notable for its adaptations to a life of complete darkness in its underground habitat. The olm's eyes are undeveloped, leaving it blind, while its other senses, particularly those of smell and hearing, are acutely developed. It also lacks any pigmentation in its skin. It has 3 toes on its forelimbs, but only 2 toes on its hind feet. It also exhibits neoteny, retaining larval characteristics like external gills into adulthood, like some American amphibians, the axolotl and the mudpuppies (Necturus). The olm is the only species in the genus Proteus and the only European species of the family Proteidae, whose other extant genus is Necturus.
The black olm (Proteus anguinus parkelj) is the only other recognized subspecies of the olm. It is endemic to the underground waters near Črnomelj, Slovenia, an area smaller than 100 square kilometres (39 sq mi). It was first found in 1986 by members of the Slovenian Karst Research Institute, who were exploring the water from Dobličica karst spring in the White Carniola region. It has several features separating it from the nominotypical subspecies (P. a. anguinus): darker skin; a longer body, shorter limbs, tail, and head with stronger jaw muscles; and more developed eyes.