Amphibians (Caudata)

The olm (Proteus anguinus) is the only real cave amphibian in Europe and the only living representative of the genus Proteus. It is probably the most famous underground inhabitant in Croatia.

The olm lives in underground rivers and lakes of the Dinaric karst from northern Italy (Trieste area), across Slovenia and Croatia to Bosnia and Herzegovina. Recent research using environmental DNA has shown it probably lives in Montenegro as well.

In Croatia, the olm is recorded from Istria to Dubrovnik in three distinct populations that inhabit the areas of: Istria, Gorski kotar and Gacko polje (northern Lika), and Dalmatia (from the river Krka to Dubrovnik). It is interesting that it was never found in the area of Ličko polje, Velebit Mt and Zrmanja river, presumed due to hydrogeological barriers. With the new genetic researches, population of the olm in the Istria region has been separated into a separate subspecies – Istrian olm (Proteus anguinus ssp.n.).

The body of the olm is slender and elongated, with the total length of the body from 25 to 35 cm (up to 40 cm) from the tip of the muzzle to the tip of the tail. The skin is pale pink-white in colour, except in the black population of a subspecies P. a. parkelj in Slovenia. Legs of this species serve only for gentle movement during the search for food, while powerful tail swings are used for escape. The olm keeps the morphological characteristics of larva in its adult stage, which is called neoteny. Three pairs of pink to bright red gills stand out on its neck. In oxygen-rich waters it breathes with gills and skin, but it can also breathe with lungs in hypoxic conditions. In young individuals, the eyes are visible, while in adults they are atrophied, covered with thin skin and not visible.

The olm prefers clean, oxygen-rich water in the temperature range between 5 to 15 °C. It mainly inhabits deeper parts of the caves, but sometimes it can be seen in shallow underground lakes in search for food. During the spring floods, individuals are often thrown out from the underground through karst springs. The olm has a very slow development and reaches the adulthood around 14 to 18 years of age, and lives more than 60 years. It feeds on insect larvas, molluscs and crustaceans. It has been proven that, in exceptional conditions, it can survive from 18 to 96 months without food. Olm doesn’t have a lot of natural enemies, mostly fish that occasionally enter some underground systems.

The main cause of the endangerment of the olm is the degradation of karst underground habitats by pollution of groundwater and numerous interventions that alter hydrological groundwater regimes (e.g. during dam construction and accumulation), and introduction of non-native species into individual systems.

The only living representative of the genus Proteus is globally categorized as a sensitive species (VU), while at the national level it is on the list of endangered taxa (EN).