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Did you know that the olm is the largest subterranean aquatic animal of the Dinaric karst, and the only vertebrate adapted to living underground?

The olm (Proteus anguinus) is endemic to the Dinaric karst. It is distributed from the Soča River basin to the Trebišnjica River Basin, i.e. in Slovenia, Croatia, Bosnia and Herzegovina and Italy (only near Trieste). It was discovered in Krka National Park in the Miljacka II cave in 1989, and today it is known to inhabit four cave structures within the park. It lives in calm, cold, underground waters that are rich in oxygen, to depths of 120 m, and is a true troglobiont.

Some individuals have been reported on land in wet, underground structures (likely in search of food), and during torrential periods, they can be expelled out of the karst springs. It has a fragile, long body that is white without any pigment, though it gives off a pale pink tone due to the capillaries under the skin. The head is elongated with a rounded snout, and the eyes are undeveloped and hidden under the skin. It has short, thin legs; with three toes on the fore legs and two poorly developed toes on the hind legs. It has a flat tail that is much shorter than the body, and has a thin fin. It breathes with its lungs, but like neotic animals (those that retain larval characteristics), it retains three pairs of branching, external, bright red gills and two pairs of tufts. Its internal organs are visible through its skin. It is 25 to 30 cm long, though some individuals 40 cm long have been reported. Males are smaller than females, and during the mating season can be distinguished from the females due to their long, extruding cloaca. They feed on aquatic invertebrates, especially insect larvae and small aquatic crabs. They find their prey with their well developed chemical and mechanical receptors. Due to their slow metabolism, they can go without food for several years during the dormant period.

During mating, the males protect the area from other males. The male courts the female by waving his tail towards her head, to which she responds by touching his cloaca with her snout, and following him as he releases the spermatophore. When she accepts the spermatophore with her own cloaca, the female leaves the male and lays up to 70 eggs on rock about 2 or 3 days later. They eggs hatch after 182 days at a temperature of 8ºC, after 140 days at 10ºC, after 123 days at 11ºC and after 86 days at 15ºC. In colder water, the female can retain the eggs inside her body and give birth to live young. The olm sexually matures at the age of 14 years. They are long-lived, with a life span of up to 70 years, as there are no predators threatening them in the underground waters.