(Proteus anguinus)

The olm is Europe’s only cave adapted vertebrate, and has numerous adaptations for an underground life. Olms hunt their prey in the absolute dark and have developed a powerful sensory system of smell, taste, hearing and electrosensitivity. Olms are pale and sightless, although their skin-covered eyes are still light sensitive. They are an entirely aquatic species that can survive without food for up to 10 years and live to an age of 58 or more. Part of an ancient lineage of amphibians evolving independently for 190 million years, this species is now threatened by pollution and habitat disturbance. A small subpopulation of “black olms” may be a separate species requiring additional protection.

Urgent Conservation Actions
Research into the taxonomic status of the black olm; development of a Conservation Action Plan for both cave habitat and forests/streams above.
Slovenia, Bosnia and Herzegovina and Croatia. Introduced to the French Pyrenees and northeast Italy. Suspected in Serbia and Montenegro.
The olm is the only cave-adapted vertebrate in Europe. Current genetic research directed by Dr. Boris Sket of the University of Ljubljana, Slovenia suggests that “Proteus anguinus” is actually a complex of several species and it may therefore be incorrect to assume that the olm can only be divided into the subspecies Proteus anguinus anguinus and Proteus anguinus parkelj. There may be a number of different species of olm, rather than just one, across its known range.

The olm is a master of starvation. It can consume large amounts of food at once, and store nutrients as large deposits of fats and sugar (as glycogen) in the liver. When food is scarce, Olms reduce their activity and metabolic rate, and can reabsorb their own body tissues during severe food shortages. Captive experiements have shown that olms can survive for up to 10 years without food!

The olm is considered a national treasure by the government of Slovenia, which celebrated the species by depicting it on one of its coins. There is also the Proteus Postojna Speleobiological station, a museum about the olm and other cave fauna visited by thousands of Slovenian school children and tourists each year, and Postojna cave (which is inhabited by olms) is another major Slovenia tourist attraction. The olm was also depicted on one of the Slovenian Tolar coins, and the oldest Slovenian popular science magazine, Proteus (first published in 1933) was named in honour of the olm.

Olms are sometimes referred to as "human fish" because of the human-like semi-translucent skin of this species.

The olm was used by Charles Darwin to illustate the reduction of bodily structures through disuse in “The Origin of Species”:

Far from feeling surprise that some of the cave-animals should be very anomalous…as is the case with blind Proteus with reference to the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the scanty inhabitants of these dark abodes will have been exposed.

The first written mention of the olm is in Janez Vajkard Valvasor's 1689 publication “The Glory of the Duchy of Carniola” where the species is desrcribed as a baby dragon in reference to an old folk story.

Joseph Nicolai Laurenti was the first to briefly describe the olm in 1768, giving it its scientific name Proteus anguinus.

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Media from ARKive
ARKive image - Eggs of the cave salamander, attached to rock surface
ARKive video - Cave salamander - overview
ARKive image - Cave salamander
ARKive image - Cave salamander on rock
ARKive image - Cave salamander, head detail
ARKive image - Cave salamander moving over rocky bottom
ARKive image - Cave salamander on rocks
ARKive image - Cave salamander
ARKive image - Cave salamander
ARKive image - Cave salamander moving over rocky bottom
ARKive image - Cave salamander swimming, ventral view
ARKive image - Cave salamander swimming
ARKive image - Cave salamander, dorsal view
ARKive image - Cave salamanders
Evolutionary Distinctiveness
Order: Caudata
Family: Proteidae
Proteidae is an ancient family of salamanders comprising just six species, commonly referred to as the olm, mudpuppies and waterdogs. The lineage diverged from their closest relatives 190 million years ago in the early Jurassic period in the era of the dinosaurs. This predates the first fossil bird by 40 million years.

A highly unusual feature of the proteids is that metamorphosis is absent in this family. The young never transition into an adult form, instead retaining their larval characteristics throughout life, which include feathery gills, a tail fin and no eyelids, as appropriate to their permanently aquatic lifestyle. This is termed “neoteny” or “paedomorphosis” because these salamanders achieve reproductive maturity whilst still in the larval form. The family Proteidae is part of a suborder of salamanders that contains all of the internally-fertilising species (known as the “Salamandroidea”). Most salamanders reproduce via external ferlilsation, but in the proteids females take the spermatophone (or sperm packet) deposited by the male into their bodies, allowing fertisation of her eggs to occur within the body cavity.

The proteids comprise two geographically distant genera: Proteus (the European olm) and Necturus (the mudpuppies and waterdogs of eastern North America). The olm is Europe’s only cave-dwelling vertebrate species and is highly adapted for life in the subterranean darkness. They have depigmented skin, degenerate eyes, an extremely powerful sense of smell, and electrosensitivity, allowing them to hunt with a bizarre sixth sense that detects electric fields in a similar way to sharks. Their closest relatives, the mudpuppies and waterdogs, are not cave dwellers and have normal skin pigmentation and eyesight. However, a dark-skinned subspecies of the olm with normally developed eyes was discovered in 1986 by members of the Slovenian Karst Research Institute who were exploring the water from Doblicice karst spring. It was names the black olm (Proteus anguinus parkelj) and is only found in a 100 km sq. area in the Bela krajina region of Slovenia. The Proteus and Necturus lineages diverged 140 million years ago in the late Jurassic period. In terms of mammal groups, this makes them about as dissimilar from each other as wombats are from humans. Current genetic research under the direction of Dr. Boris Sket of the University of Ljubljana, Slovenia suggests that the olm is actually a complex of several species.
The olm is an elongated and slender salamander with small, thin limbs with three toes present on the fore limbs and two toes on the hind limbs. The head is lengthened with a rounded snout and the tail is flattened and markedly shorter than the body. Olms possess poorly developed eyes, which are covered with skin in one of the subspecies (P. anguinus anguinus), and have three pink external gills on each side of the head. They have whitish skin with a pink tinge due to the presence of blood capillaries near the surface. Since the skin is translucent, it is also possible to see the form of an olm’s internal organs, making it easy to determine the sex of adults. Also, during breeding activity males have a larger and more elongated swollen cloaca (urinogenital opening) than the females. Juveniles sometimes show a faded spotting, and dark pigmentation can be induced by exposure to light. The effect of light on an olm’s skin indicates that these amphibians do not display albinism, as has been commonly thought, because they still have the ability to produce melanin. The average total length of an olm is 230-250 mm, with males being slightly smaller than females. The subspecies P. anguinus parkelj has a permanent dark pigmentation of the skin, and functional eyes. It also has a shorter head than P. anguinus anguinus (the "while olm").

The olm is the only cave-adapted vertebrate with a range restricted to Europe and has numerous adaptation to its entirely aquatic (or water-dwelling) underground life. This species inhabits a world without light, and as a consequence has poorly developed eyes and no skin pigmentation. Although the eyes are regressed, they retain sensitivity to light despite lying deep below the skin and rarely being visible except in some younger adults. Larvae have normal eyes, but development soon stops and the eyes start to diminish, finally atrophying after about four months. Other light-sensitive areas of the olm’s body include a “pineal body” in the centre of the head, and even the skin registers light through the proposed presence of the photosensitive pigment melanopsin inside specialised cells called melanophores. However, since no light penetrates into the cave system, this light sensitivity is seldom, if ever, used.

In place of sight, the olm has developed an acute sensory system for hunting in the dark. The front part of the olm's head carries sensitive chemo-, mechano-, and electroreceptors. Olms have one of the best senses of smell of any amphibian, and are capable of sensing very low concentrations of organic compounds in the water through both smell and taste. The ear is specialised to receive sound waves in the water, as well as vibrations from the ground. Experiments indicate that the optimal hearing sensitivity of this species between 10 Hz-15,000 Hz. A pressure-senistive sensory system running along the body (common in fish) called the lateral line system supplements inner ear sensitivity by registering tiny nearby water displacements. Furthermore, olms have the ability to detect weak electric fields through sensors called “ampullary organs” in the head. This means they have a electrosensitve sixth sense, similar to that employed by sharks. Also, some behavioral experiments suggest that the olm may be able to use the earth's magnetic field to orient itself, as has recently been found to align itself with natural and artificially modified magnetic fields.

The species generally occurs in large subterranean aquatic karst systems formed in limestone and dolomite rocks, and may be found in cave entrances (especially during episodes of high rainfall and flooding) and abandoned mine workings. Many of the caves that the species occurs in are connected to rivers that run above ground for the first 50 to 100 km and then disappear into the ground. Populations may be found close to the surface or as much as 300 m underground depending on the thickness of the Karstic formation. Olms hide in crevices or bottom sediment when disturbed, and are generally found in water temperatures from 6ºC to 12ºC, although the pigmented subspecies P. anguinus parkelj (the “black olm”) prefers in warmer surface waters. The black olm was first discovered in 1986 in the Dobli?ice karst spring near ?rnomelj, Slovenia, in an area smaller than 100 square kilometres. It is very different from the obligate cave-dwelling “white” olm, in that its skin is fully pigmented (black or brown), it has small but functional eyes, and its non sight-related sensory system is more insensitive.

Olms feed on detritus and endemic cave invertebrates, such as crabs, snails and insects, swallowing its prey whole. The olm is a master of starvation. It can consume large amounts of food at once, and store nutrients as large deposits of fats and sugar (as glycogen) in the liver. When food is scarce, olms reduce their activity and metabolic rate, and can reabsorb their own body tissues during severe food shortages. Captive experiements have shown that olms can survive for up to 10 years without food. This is a long-lived species, with a maximum longevity of up to 58 years or more, and sexually maturity if not achieved until the age of 7-12 years. Anecdotal evidence suggests that olms can in fact live for more than 100 years. The olm swims by serpentine (or snake-like) undulation of the body, rather like an eel. Most observations on the life history of this salamander were made in captivity and they have been bred in the Subterranean Laboratory of the CNRS, in the French Pyrenees, since 1955.

This species reproduces very slowly. Adults aggregate in suitable areas such as cracks and under rocks. The males establish a territory when breeding, which is furiously defended against competing males. When a female enters such a territory, the courtship commences. The male fans with his tail in the direction of the female's head (possibly even secreting a female-attracting pheromone) and touches the female's cloaca (reproductive urinogenital opening) with his snout. The female then touches the male's cloaca with her snout and then follows the male who walks 50-100 mm forward and deposits a spermatophore (a small packet of sperm). The pair then moves forward again until the female can take up the spermatophore with her cloaca. Courtship may be repeated several times within a few hours. After leaving the male's territory, the female establishes her own egg-laying territory. After 2-3 days the female starts to lay eggs and can continue doing so for up to 25 days, laying a total of up to 70 eggs under rocks. The eggs are then guarded by the female until they hatch. The rate of egg development is dependent upon water temperature: eggs develop in 182 days at 8ºC; in 123 days at 11ºC; and in 86 days at 15ºC. The development of larvae is also highly temperature-dependent. The larvae are 20 mm long when they hatch and live on yolk stored in the cells of their digestive tract for a month. In some cases the eggs are retained within the body and two fully-formed young are produced, and it has been hypothesised that female olms give birth to live young at lower temperatures and lay eggs at higher temperatures. However, but rigorous observations have not confirmed this idea.

There is no clear metamorphosis, as the olm is a neotenic salamander retaining larval characteristics, such as gills and a tail fin, throughout its life. In other amphibians, metamorphosis is regulated by a hormone called thyroxine, produced by the thyroid gland. The thyroid is normally developed and functioning in the olm, so the lack of metamorphosis is due to the unresponsiveness of key tissues to thyroxine. Olms are social, and usually aggregate either under stones or in rock fissures, although males become aggressive and antisocial during the breeding season. The scarcity of food in the acve environment makes fighting energetically costly, so encounters between males usually only involve display.

The natural habitat for the olm is the fresh water reserves found in dark underground caves of the Dinaric Alps along the Adriatic Sea, from north-eastern Italy to Bosnia and Herzegovina and possibly Serbia and Montenegro. It was introduced in the Parolini Grotto, Vicenza, northern Italy and a cave in the French Pyrenees that is a subterranean laboratory of the CNRS (Centre National de la Recherche Scientifique), France. It prefers underground water systems in limestone and dolomite Karst formations, with calm, well-oxygenated water and a constant low water temperature between 6ºC (winter) and 9-12ºC (summer). Olms may be found in cave entrances (especially during episodes of high rainfall and flooding) and abandoned mine workings. Many of the caves within the olm’s range are connected to rivers that run above ground for the first 50 to 100km and then disappear into the ground. Populations may be found close to the ground surface or as much as 300 m underground depending on the thickness of the Karstic formation where they dwell.
The olm is restricted to subterranean aquatic habitats within the Dinaric Alps in the countries of Bosnia and Herzegovina, Italy, Slovenia, Croatia and possibly Serbia and Montenegro. The species has also been introduced to a cave of the subterranean laboratory of the CNRS, France in the Pyrenees, and at least one of the north-eastern Italian populations is introduced. The olm’s range extends from southern Slovenia and adjoining northeast Italy through coastal Croatia and western Bosnia in Bosnia and Herzegovina, although it has yet to be officially recorded in western parts of Serbia and Montenegro despite considerable anecdotal evidence of its presence. The pigmented subspecies Proteus anguinus parkelj is only found in Bela Krajina, south east Slovenia.
Population Estimate
There is currently is little information available on the abundance of this species. However the IUCN Red List of Threatened species states that the olm is apparently most common in Slovenia and Croatia, although a decline has been observed in the populations of Goriza (Italy) and Postojna (Slovenia). Specifically, the number of individuals of the subspecies P. anguinus parkelj is thought to be very low.
Population Trend
The olm is thought to be in decline by the IUCN Red List of Threatened Species.
The olm is listed as Vulnerable in the IUCN Red List of Threatened Species because its area of occupancy is less than 2,000 km sq., its distribution is severely fragmented, and there is continuing decline in the extent and quality of its habitat, and in the number of mature individuals.
The main threats to this species are changes to the forested and pastoral land above the subterranean cave systems inhabited by the olm, brought about by tourism, economic changes and increasing water pollution. Alterations to the vegetation system above the Dinaric karsts have a direct influence of the quality of the habitat available for olms underground. This species is highly dependent upon clean water, and is therefore very susceptible to pollution. Other localised threats to this species include water abstraction and hydroelectric damming schemes. There is some illegal collection of this species for the pet trade, but the extent of this is currently unknown. It has also been used as pig food by local farmers.

In addition, the olm has also been depleted in the wild by over-collection in some parts of its range, and the invasion of olm habitat by surface-water fish (mainly the artificially-introduced "Californian trout"). Specific issues affecting the olm include: chemical pollution by inorganic and organic compounds; alteration, destruction and ecological degradation of surface waterways on the surface of the karst for hydro-electric projects; the construction of urban and rural housing in formerly undeveloped areas; the introduction of European Aid programmes for agricultural development in karst polje areas, causing pollution of underground water regimes by agro-chemicals; the increase in local demands for fresh water resulting in excessive pumping of water from specific types of natural underground habitats (estavelles, vrela etc.); the casual or uncontrolled dumping of domestic and industrial rubbish into intermittently-active karst hydrological features, such as ponors (swallow holes); and sinking streams and rivers, springs, estavelles, which is a big problem across Slovenia, Croatia and Bosnia & Herzegovina).
Conservation Underway
The olm is listed on Appendix II of the Berne Convention, and on Annexes II and IV of the EU Natural Habitats Directive, which affords it substantial legislative conservation throughout Europe. It is recorded in the Slovenian National Red List and is protected by national legislation in both Slovenia and Italy. In Slovenia the species is present in caves that are protected by national legislation, and much of the distribution of this species is within proposed national or international protected areas, such as Kocevski and Kraski regional parks and NATURA 2000 sites. In Italy it is found in the Riserva Naturale Regionale dei Laghi di Doberdò e Pietrarossa. Hunting or keeping a limited number of olms is allowed, but only under strictly controlled circumstances determined by local authorities. Additionally, there is much research undertaken on oms in institutions such as the Subterranean Laboratory of the CNRS, in the French Pyrenees, and the Proteus Postojna Speleobiological station in Slovenia.

Specifically, in Slovenia the olm is considered to be a national treasure. The olm was first protected in Slovenia in 1922 along with all other cave fauna, but the protection was not effective and a substantial black market for this species developed. In 1982, the olm was placed on a list of rare and endangered species, which also prohibiting the trade of the species. After joining the European Union, Slovenia established mechanisms for protection of the species included in the EU Habitats Directive. The olm is included in a Slovenian IUCN Red List of Threatened Species. The Postojna cave and other cave networks inhabited by olm were also included in the Slovenian part of the Natura 2000 network.

In Croatia, the olm is protected by legislation designed to protect all amphibians. Collecting is possible only for research purposes by permission of the National Administration for Nature and Environment Protection.

In Bosnia and Herzegovina a 30 year project is being coordinated by the Devon Karst Research Society to study the olm in its natural environment and conserve it with the cooperation of local stakeholders.

This project supports in-country EDGE Fellows to help conserve relevant EDGE species

This series aims to raise awareness of lesser-known, yet fascinating species.

This project focuses on the olm (Proteus anguinus), a cave dveling amphibian which is the only real cave vertebrate species in Europe. It is ranked 18th on EDGE list (and identified as a focal species) and 3rd on the Top 50 Evolutionarily Distinct Amphibians list.

In the area of the Dinaric karst, the olm is recognized as the most popular member of the subterranean fauna. Despite its popularity, general knowledge of the species’ ecology and biology is limited as it is a species that is actually rarely seen in nature and that is very vulnerable to human activity. Furthermore, there is a growing need for education of local communities in order to help the protection of this unique species.

The main reasons why the olm is endangered are the increasing and uncontrolled spread of urban areas and human made infrastructure, the uncontrolled excessive pollution of water habitats, and the use of crevices and cave entrances in olm habitats as landfill sites.

The olm is a species of European Community interest and a species in need of particularly strict protection, such as designating special areas of conservation and possibly reintroduction.

Conservation Proposed
The subspecies Proteus anuinus parkelj (the black olm) is in need of protection because its habitat is limited to only a few holes in connection with subterranean networks in a very small geographic area. This subspecies is very different from the “white” olm and may be considered as a separate species in the future. Further research is required to determine the taxonomic status of the black olm so that appropriate conservation actions can be developed.

The olm's survival is dependant on both large aquatic cave systems and the conservation of woodland and pastoral land above. A Conservation Action Plan should be developed for this species to recommend a dual management strategy for above ground and subterranean habitat conservation to maintain the cave system of the olm as an unpolluted and high-quality habitat for this extremely unusual species. This should include regular population monitoring and water quality assessments in olm habitat.

The olm has been bred at the Subterranean Laboratory of the CNRS, in the French Pyrenees since 1955. In the future it may be important establish a captive breed programme for conservation purposes for this species. Captive animals could then be a source of new individuals to repopulate the natural habitats within the olm’s known range whilst its habitat is protected and an environmental management plan is developed for this species and its habitat.
Associated EDGE Community members

Nick is a naturalist and broadcaster with a passion for unique species.

Marine and freshwater programme coordinator at ZSL with a passion for mangroves and corals

I am a biologist from Croatia working on amphibian and reptile ecology and conservation.

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Bulog, B., Mihajl, K., Jeran, Z. and Toman, M.J. 2002. Trace element concentrations in the tissues of Proteus anguinus (Amphibia, Caudata) and the surrounding environment. Water, Air, and Soil Pollution 136: 147-163.

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Hervant, F., Mathieu, J. and Durand, J.-P. 2000. Metabolism and circadian rhythm in the European blind salamander (Proteus anguinus) and a facultative cave dweller, the Pyrenean newt (Euproctus asper). Canadian Journal of Zoology 78: 1427-1432.

Hervant, F., Mathieu, J. and Durand, J.-P. 2001. Behavioural, physiological and metabolic responses to long-term starvation and refeeding in a blind cave-dwelling (Proteus anguinus) and a surface-dwelling (Euproctus asper) salamander. Journal of Experimental Biology 204: 269-281.

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Kletecki, E., Jalzic, B. and Rada, T. 1996. Distribution of the olm (Proteus anguinus, Laur.) in Croatia. Mémoires de Biospéologie 23: 227-231.

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Sket, B. and Arntzen, J.W. 1994. A black, non-troglomorphic amphibian from the karst of Slovenia: Proteus anguinus parkelj n. ssp. (Urodela: Proteidae). Bijdragen tot de Dierkunde 64(1): 33-53.

Stet, B. and Arntzen, J.W. 1994. A black, non-troglomorphic amphibian from the karst of Slovenia: Proteus anguinus parkelj n. ssp. (Urodela: Proteidae). Bijdragen tot de Dierkunde 64(1): 33-53.

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if you can provide new information to update this species account or to correct any errors, please email us at info@edgeofexistence.org

Forum comments
  1. Space

    Five other great photos of Olm (Proteus anguinus)

    Posted 4 years ago #
  2. wildam

    Olm (Proteus anguinus)




    Posted 5 years ago #
  3. Dag

    The subterranean ecosystems lack primary producers such as plants and algae. So all the organisms rely on the nutrients imported from the ground above, carried by the water to underground habitats. The olm can feed both on the invertebrates (primary mollusks, annelids, crustaceans, insects) imported from the above ground and those native to the subterranean water ecosystems. There is a variety of animal species that are native to subterranean water ecosystems, many of which are endemic. There are sponges, mollusks, leaches, crustaceans and other invertebrates that represent a diverse troglobiontic fauna.

    Posted 6 years ago #
  4. Anonymous

    what do they eat???????

    Posted 6 years ago #

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