The olm or proteus and is an aquatic salamander.
It is also known as the human fish, for its pale, pinkish skin.
Olms are entirely aquatic and only found in the waters of caves in the Dinaric Alps—that is, parts of Slovenia, Italy, Croatia, and Herzegovina.
They live in the subterranean, freshwater lakes and streams of limestone caves. The water in these caves is slightly acidic, contains high concentrations of oxygen, and ranges in temperature from 5 to 15 °C (41 to 59 °F).
Adapted to an aphotic environment, olms usually reside deep within cave systems. They are generally found over 300 meters (985 feet) below surface.
Olms can live for 100 years, far longer than any other amphibian; and scientists have no idea why.
The olm’s body is snakelike, 20 to 30 cm (8 to 12 in) long, with some specimens reaching up to 40 cm (16 in).
The trunk is cylindrical, uniformly thick, and segmented with regularly spaced furrows at the myomere borders.
The tail is relatively short, laterally flattened, and surrounded by a thin fin.
The limbs are small and thin, with a reduced number of digits compared to other amphibians: the front legs have three digits instead of the normal four, and the rear have two digits instead of five.
Its body is covered by a thin layer of skin, which contains very little of the pigment riboflavin, making it pale, pinkish in color.
The internal organs can be seen shining through on the abdominal part of the body.
Its pear-shaped head ends with a short, dorsoventrally flattened snout.
The mouth opening is small, with tiny teeth forming a sieve to keep larger particles inside the mouth.
The nostrils are so small as to be imperceptible, but are placed somewhat laterally near the end of the snout.
The regressed eyes are covered by a layer of skin. Although blind, the olm swims away from light. The eyes are regressed, but retain sensitivity.
The olm breathes with external gills that form two branched tufts at the back of the head. They are red in color because the oxygen-rich blood shows through the non-pigmented skin. The olm also has rudimentary lungs, but their role in respiration is only accessory, except during hypoxic conditions.
Because it inhabits permanently dark environments, the olm has developed non-visual sensory systems to better suit an aphotic lifestyle.
Highly sensitive chemoreceptors allow olms to detect extremely low concentrations of organic material in the water.
The olm also has the ability to register weak electric fields. Some behavioral experiments suggest that the olm may be able to use Earth‘s magnetic field to orient itself.
Amazingly, the olm finds it only necessary to eat occasionally as it can survive for up to ten years without eating anything. When it does become hungry and is in the good fortune of having some food available, the olm will feast on other small invertebrates. This includes: snails, worms, larvae and aquatic insects.
Olms are gregarious, and usually aggregate either under stones or in fissures.
Olms reproduce by either laying eggs or giving birth to live young. Temperature appears to be the factor that determines which reproductive strategy is used, with colder water triggering the bearing of live young. Adults commonly produce two live larvae using the live-bearing strategy, whereas they may produce up to 70 eggs using the egg-laying strategy.
On the IUCN Red List, the olm is listed as vulnerable because of its fragmented and limited distribution and ever-decreasing population.
The word “olm” is a German loanword that was incorporated into English in the late 19th century. The origin of the German original, Olm or Grottenolm ‘cave olm’, is unclear. It may be a variant of the word Molch “salamander.”
The olm 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.
The olm is a symbol of Slovenian natural heritage. The enthusiasm of scientists and the broader public about this inhabitant of Slovenian caves is still strong 300 years after its discovery. Postojna Cave is one of the birthplaces of speleobiology due to the olm and other rare cave inhabitants, such as the blind cave beetle.
The olm was also depicted on one of the Slovenian tolar coins, and was the namesake of Proteus, the oldest Slovenian popular science magazine, first published in 1933.