Olms () 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 () 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 its forelimbs, but only 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, allowing others to see their internal organs. Olms have not been represented frequently in media, and are only now experiencing a surge of representation, such as in Amphibia, a TV series released from 2019-2022.
Currently, the taxonomic status of the species is undergoing a revision, with two independent research groups determining that the genus Proteus comprises 9 distinct species with much narrower distributions, isolated in separate cave systems within the total distribution range.
Etymology The word olm is a German loanword that was incorporated into English in the late 19th century. The origin of the German or 'cave olm' is unclear. It may be a variant of the word 'newt'.
Common names The animal is called the "human fish" by locals because of its fleshy skin color (translated literally from , , , , ), as well as "cave salamander" or "white salamander". In Slovenia, it is also called močeril (from *močerъ 'earthworm, damp creepy-crawly'; moča 'dampness').
Description External appearance
The olm's body is snakelike, long, with some specimens reaching up to , which makes them some of the largest cave-dwelling animals in the world.Weber A. (2000). Fish and amphibia. In: Culver D.C. et al. (ed.): Ecosystems of the world: Subterranean Ecosystems, pp. 109–132. Amsterdam: Elsevier The average length is between 23 and 25 cm. Females grow larger than males, but otherwise the primary external difference between the sexes is in the cloaca region (shape and size) when breeding. 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 yellowish-white or pink in color.
The white skin color of the olm retains the ability to produce melanin, and will gradually turn dark when exposed to light; in some cases the larvae are also colored. One population, the black olm, is always pigmented and dark brownish to blackish when adult. The olm's 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. 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.
Sensory organs Cave-dwelling animals have been prompted, among other adaptations, to develop and improve non-visual sensory systems in order to orient in and adapt to permanently dark habitats. The olm's sensory system is also adapted to life in the subterranean aquatic environment. Unable to use vision for orientation, the olm compensates with other senses, which are better developed than in amphibians living on the surface. It retains larval proportions, like a long, slender body and a large, flattened head, and is thus able to carry a larger number of sensory receptors.
Photoreceptors Although blind, the olm swims away from light. The eyes are regressed, but retain sensitivity. They lie deep below the dermis of the skin and are rarely visible except in some younger adults. Larvae have normal eyes, but development soon stops and they start regressing, finally atrophying after four months of development. The pineal body also has photoreceptive cells which, though regressed, retain visual pigment like the photoreceptive cells of the regressed eye. The pineal gland in Proteus probably possesses some control over the physiological processes.Langecker T.G. (2000). The effects of continuous darkness on cave ecology and cavernicolous evolution. In: Culver D.C. et al. (eds.): Ecosystems of the world: Subterranean Ecosystems, pp. 135–157. Amsterdam: Elsevier Behavioral experiments revealed that the skin itself is also sensitive to light. Photosensitivity of the integument is due to the pigment melanopsin inside specialized cells called melanophores. Preliminary immunocytochemical analyses support the existence of photosensitive pigment also in the animal's integument.Kos M. (2000). (Immunocitochemical analysis of the visual pigments in the sensory cells of the eye and the pineal organ of the olm (Proteus anguinus, Amphibia, Urodela).) PhD thesis. Ljubljana: University of Ljubljana.
Chemoreceptors
The olm is capable of sensing very low concentrations of organic compounds in the water. They are better at sensing both the quantity and quality of prey by smell than related amphibians.Hüpop K. (2000). How do cave animals cope with the food scarcity in caves?. In: Culver D.C. et al. (ed.): Ecosystems of the world: Subterranean Ecosystems, pp. 159–188. Amsterdam: Elsevier The nasal epithelium, located on the inner surface of the nasal cavity and in the Jacobson's organ, is thicker than in other amphibians. The taste buds are in the mucous epithelium of the mouth, most of them on the upper side of the tongue and on the entrance to the gill cavities. Those in the oral cavity are used for tasting food, where those near the gills probably sense chemicals in the surrounding water.
Mechano- and electroreceptors The sensory epithelia of the inner ear are very specifically differentiated, enabling the olm to receive sound waves in the water, as well as vibrations from the ground. The complex functional-morphological orientation of the sensory cells enables the animal to register the sound sources.Bulog B. (1990). (Sense organs of the octavolateral system in proteus Proteus anguinus (Urodela, Amphibia). I. Otic labyrinth). Biološki vestnik 38: 1–16 As this animal stays neotenic throughout its long life span, it is only occasionally exposed to normal adult hearing in air, which is probably also possible for Proteus as in most salamanders. Hence, it would be of adaptive value in caves, with no vision available, to profit from underwater hearing by recognizing particular sounds and eventual localization of prey or other sound sources, i.e. acoustical orientation in general. Behavioural (ethological) tests have shown that its sensitivity for detecting underwater sound waves reaches into the area of frequencies of sound waves between 10 and more than 12,000 Hz, while the greatest sensitivity is reached between 1,500 and 2,000 Hz. The ethological experiments indicate that the best hearing sensitivity of Proteus is between 10 Hz and up to 12,000 Hz.Bulog B. & Schlegel P. (2000). Functional morphology of the inner ear and underwater audiograms of Proteus anguinus (Amphibia, Urodela). Pflügers Arch 439(3), suppl., pp. R165–R167. The lateral line supplements inner ear sensitivity by registering low-frequency nearby water displacements.
A new type of electroreception sensory organ has been analyzed on the head of Proteus, utilizing light and electron microscopy. These new organs have been described as ampullary organs.
Like some other lower vertebrates, the olm 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. In 2002, Proteus anguinus was found to align itself with natural and artificially modified magnetic fields.Bulog B., Schlegel P. et al. (2002). Non-visual orientation and light-sensitivity in the blind cave salamander, Proteus anguinus (Amphibia, Caudata). In: Latella L., Mezzanotte E., Tarocco M. (eds.). 16th international symposium of biospeleology; 2002 Sep 8–15; Verona: Societé Internationale de Biospéologie, pp. 31–32.
Ecology and life history
The olm lives in well-oxygenated underground waters with a typical, very stable temperature of , infrequently as warm as . There have also been observations in northeastern Italy where they swim to the surface in springs outside the caves, even in daylight, where they occasionally feed on earthworms. The black olm may occur in surface waters that are somewhat warmer.
The olm swims by eel-like twisting of its body, assisted only slightly by its poorly developed legs. It is a predatory animal, feeding on small crustaceans (for example, Troglocaris shrimp, Niphargus, Asellus, and Synurella amphipods, and Oniscus asellus), snails (for example, Belgrandiella), and occasionally insects and insect larvae (for example, Trichoptera, Ephemeroptera, Plecoptera, and Diptera). It does not chew its food, instead swallowing it whole. The olm is resistant to long-term starvation, an adaptation to its underground habitat. It can consume large amounts of food at once, and store nutrients as large deposits of lipids and glycogen in the liver. When food is scarce, it reduces its activity and metabolic rate, and can also reabsorb its own tissues in severe cases. Controlled experiments have shown that an olm can survive up to 10 years without food.
Olms are gregarious, and usually aggregate either under stones or in fissures. Sexually active males are an exception, establishing and defending territories where they attract females. The scarcity of food makes fighting energetically costly, so encounters between males usually only involve display. This is a behavioral adaptation to life underground.
Breeding and longevity