Species of the month -OLM, Proteus anguinus (Linnaeus, 1766)

The olm is a tailed amphibian that inhabits flowing and standing waters in the underground karst caves that are rich in aquatic invertebrates that make up its diet. It never leaves these underground waters on its own, though occasionally high waters will bring them up to the surface following heavy rains or snow melt. It is unknown whether or not they can again find their way back into the caves.

Due to the exceptionally cold and sparse habitat in the caves, they achieve their sexual maturity while still in the larval phase, in which they remain their entire lives. This is evident by the very red colouration of the external gills, which is characteristic for the larval stage of most tailed amphibians. Considering that it does not complete its full metamorphosis into the adult stage, it does not ever leave the water to take on a terrestrial life, like other amphibians.

The basic body colour is white, with slight tones of yellow and pink that resemble human skin, making this species locally known as the "human fish". Due to its life in complete darkness, its eyes have withered away and are covered with a layer of skin. There is only one population of olm in western Slovenia where all the individuals are grey and still have functional eyes. This is thought to be an adaptation for occasionally visits to the surface areas and better lit parts of the caves. It has a long, slender body, usually from 20 to 25 cm in length. Considering that it never leaves the water, its legs are small, while its tail is very strong and serves for propulsion through the water.

During the courting stage, the male attracts a female by releasing a hormone into the water and waving its tail to direct the hormone towards her head. The males leave a packet of sperm on the substrate, which the female picks up and fertilization of the eggs is internal. The female lays up to 70 fertilized eggs under rocks, which hatch into a new generation of larvae.

The olm is endemic to the Dinaric karst and inhabits a narrow, coastal belt, from Slovenia to Montenegro. In Croatia, it inhabits caves and sinkholes of karst fields, from Istria to Dubrovnik. It is considered to be a threatened species and is listed in the Red Book of Cave Fauna in the category of vulnerable species (VU B1+2bc; C2a). The olm is protected under the Nature Protection Act.

The main causes of threat for this species are uncontrolled urbanization, changes in the regime of underground waters due to hydrotechnical works, from dam construction to water pumping, and groundwater pollution.

View the remaining species of the month HERE.