Olm

Olms (German: [ɔlm] ) are the nine species in the genus Proteus. They are aquatic salamanders, and the only exclusively cave-dwelling chordate genus found in Europe. The genus is assigned to the family Proteidae, along with Necturus. In contrast to most amphibians, this genus is entirely aquatic; eating, sleeping, and breeding underwater. Living in caves found in the Dinaric Alps, the genus is endemic to the waters that flow underground through the extensive limestone bedrock of the karst of Central and Southeastern Europe in the basin of the Soča River (Italian: Isonzo) near Trieste, Italy, southern Slovenia, southwestern Croatia, and Bosnia and Herzegovina. Introduced populations are found near Vicenza, Italy, and Kranj, Slovenia. It was first mentioned in 1689 by the local naturalist Valvasor in his Glory of the Duchy of Carniola, who reported that, after heavy rains, the olms were washed up from the underground waters and were believed by local people to be a cave dragon's offspring.

This genus of cave salamander is most notable for its adaptations to a life of complete darkness in its underground habitat. Olms have eyes that are undeveloped, leaving most species blind, while its other senses, particularly those of smell and hearing, are acutely developed. Most species also lack any pigmentation in their skin. Olms have three toes on their forelimbs and two toes on their hind feet. The genus exhibits neoteny, retaining larval characteristics such as external gills into adulthood, like some American amphibians, the axolotl and the mudpuppies (Necturus). Like the glass frog, olms have uniquely transparent skin, leaving their internal organs visible.

Originally, Proteus was thought to be a monotypic genus, containing only the species Proteus anguinus. In 2026, two independent research groups determined that Proteus comprises nine distinct species with narrow distributions, isolated in separate cave systems within the total distribution range.

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