Proteus anguinus Laurenti, 1768
Olm, Blind cave salamander, Grottenolm, hulepadde, koopaolm, Proteo, Protée anguillard, hulesalamander, White Olm, Black Olm, odmieniec jaskiniowy, olmi, covecija ribica, cloveska ribica
© 2017 Dr. Joachim Nerz (1 of 30)
The average total length lies between 23-25 cm. They may grow up to 30 cm and rarely more than 30 cm. Black Proteus can grow up to 40 cm or more. Males are somewhat smaller than females. Other sexually dimorphic characteristics include the shape and size of the cloaca during breeding activity, with the males having a larger and more elongated swollen cloaca than the females.
Distribution and Habitat
Country distribution from AmphibiaWeb's database: Bosnia and Herzegovina, Croatia, Italy, Slovenia. Introduced: France.
This species prefers underground water systems in karst formations, with calm, usually well-oxygenated water and a constant low water temperature between 8ºC (winter) and 11ºC (in one locality rarely up to 14ºC in summer) (Sket 1997; Honnegger 1981). It occurs in caves and may also be found in abandoned mines, from close to the surface to depths as much as 300 m underground, depending on how thick the karst formation is (Stuart et al. 2008). During periods of high rainfall and flooding it may be found in cave entrances (Stuart et al. 2008). The variant known as the black olm can be found in warmer surface waters (Stuart et al. 2008).
Life History, Abundance, Activity, and Special Behaviors
The Olms live in subterranean waters, and is therefore a difficult subject for field observations. It does occur in caves that are accessible to humans, but as these contain hardly any adults, these accessible parts of caves must be seen as marginal parts of the biotope. Most observations on the life history of this salamander have been made in captivity. They have been bred in the Subterranean Laboratory of the CNRS, in the French Pyrenees (Station D'Ecologie Expérimental du CNRS, at Moulis, France) for more than 50 years, since 1955. The following life history account is made using data from observations on captive salamanders.
Although adults aggregate in suitable spots as in cracks and under rocks, males establish a territory when breeding, which is furiously protected from competing males. When a female enters such a territory, the courtship begins. The male fans with his tail in the direction of the female's head. The male touches the female's cloaca with his snout. The female then touches the male's cloaca with her snout and then follows the male who walks 5-10 cm forward after which the male deposits a spermatophore. The pair then moves forward again until the female can take up the spermatophore with her cloaca. Courtship can be repeated several times within a few hours. After leaving the male's territory, the female establishes an 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. Eggs are guarded by the female. The diameter of the eggs directly after laying is 4-5 mm and can increase through water uptake to 8-9 mm. Unconfirmed historical observations of vivipary exist; it was long thought that female Proteus gave birth to only two well-developed young at lower temperatures and laid eggs at higher temperatures, but this has not been confirmed by rigorous observations. The eggs develop in 182 days at 8ºC, 140 days at 10ºC, 123 days at 11ºC, and in 86 days at 15ºC. Development of larvae is highly temperature-dependent. At 10ºC it takes another 14 years to reach sexual maturity. There is no clear metamorphosis; P. anguinus is a neotenic salamander, maintaining external gills, tail fin and other juvenile characteristics throughout its life.
Proteus anguinus is thought to be the longest-lived amphibian species. Using data spanning more than 50 years from a 400-animal captive breeding colony at the CNRS in Moulis, France, the predicted maximum lifespan is over a century, and the average adult olm lifespan is 68.5 years (Voituron et al. 2010). If the predicted maximum lifespan is accurate, it is more than double that of the next longest-lived species, the Japanese giant salamander (Andrias japonicus, at 55 years. Individual specimens have been kept under semi-natural conditions in concrete basins for up to 70 years (Prof. B. Bulog, personal communication). This species reaches sexual maturity at 15.6 years and lays 35 eggs every 12 years, on average (Voituron et al. 2010).
The diet consists of insect larvae, mostly Trichoptera, Ephemeroptera, Plecoptera and Diptera larvae, molluscs (Belgrandiella), and amphipods (Niphargus, Asellus, Synurella) (Bizjak-Mali 1995; Bizjak-Mali and Bulog 2004). In captivity worms are also readily eaten (Boehme et al. 1999).
Sensory adaptations to cave dwelling
Visual pigments in the regressed eye and the pineal of the depigmented and pigmented subspecies were studied by immunocytochemistry (Kos et al. 2001). The presence of visual pigments indicates retained light sensitivity in both subspecies. In the retina of the black Proteus are principal rods, red-sensitive cones and a third photoreceptor type, which might represent a blue- or UV-sensitive cone. The majority of these outer segments of the regressed eye of unpigmented Proteus showed immunolabelling for the red-sensitive cone.
The pineal organ influences skin pigmentation, metamorphosis and gonadal development, and controls circadian rhythms through secretion of pineal hormones. The pineal structure is very similar in all Proteus individuals analyzed. In Proteus the pineal organ is reduced in size; it has degenerated photosensitive cells and can be found only by serial sectioning of the brain. The pineal organ probably possesses some control over the physiological processes also in Proteus, taking into account the presence of visual pigments (Kos et al. 2001). Behavioral experiments revealed that the skin itself is also sensitive to light, and immunocytochemical analysis also supported the existence of photosensitive pigment in Proteus' integument. Photosensitivity of the integument is due to the pigment melanopsin inside pigment cells called melanophores. (Kos et al. 2001).
Trends and Threats
The Olm is extremely vulnerable to changes in its environment due to its adaptation to the specific conditions in caves. Water resources in the karst are extremely sensitive to all kinds of pollution (Bulog et al. 2002) . The contamination of the karst underground waters is due to the large number of waste disposal sites leached by rainwater, as well as to the accidental overflow of various liquids. The reflection of such pollution in the karst underground waters depends on the type and quantity of pollutants, and on the rock structure through which the waters penetrate. Self-purification processes in the underground waters are not completely understood, but they are quite different from those in surface waters. Among the most serious chemical pollutants are chlorinated hydrocarbon pesticides, fertilizers, polychlorinated biphenyls (PCBs), which are (or were) used in a variety of industrial processes and in the manufacture of many kinds of materials; and metals such as mercury, lead, cadmium, and arsenic. All of these substances persist in the environment, being slowly, if at all, degraded by natural processes. In addition, all are toxic to life if they accumulate in any appreciable quantity.
Slovenian caves became famous for the animals they contained and which could not be found elsewhere. Due to its rarity the Olm is also popular among collectors, leading to possible overcollection. Honnegger (1981) also lists overcollecting, for scientific use or as pig-food by farmers, as a threat to this species.
The Olm is included in Appendices II and IV of the EU Habitats Directive (92/43/EEC). Appendix II seeks to preserve favorable conservation status in animal and plant species along with their habitats by protecting the species or defining special areas of conservation. These areas of conservation form the Natura 2000 network. Appendix IV further defines "animal and plant species of community interest in need of strict protection." Hunting or keeping a limited number of Olms is allowed only under strictly controlled circumstances, determined by local authorities.
The Olm was first protected in Slovenia in 1922 along with all cave fauna, but the protection was not effective and a substantial black market came into existence. In 1982 it was placed on a list of rare and endangered species in Slovenia. This list also had the effect of prohibiting trade of the species. After joining the European Union, Slovenia had to establish mechanisms for protection of the species included in the EU Habitats Directive. The Olm is included in the Slovenian Red List of endangered species. The Postojna cave and other caves inhabited by the Olm were also included in the Slovenian part of the Natura 2000 network.
On the IUCN Red List, the Olm is listed as Vulnerable because of its fragmented and limited distribution and ever-decreasing population.
In Slovenia much of the range lies within proposed national or international protected areas (Kocevski Regional Park; Kraski Regional Park; NATURA 2000 sites). In Italy it occurs within the Riserva Naturale Regionale dei Laghi di Doberdò e Pietrarossa (Stuart et al. 2008).
Relation to Humans
Nearly 300 years after the first written mention of the unpigmented Proteus in Janez Vajkard Valvasor's “The Glory of the Duchy of Carniola (1689)”, a pigmented form (described as a subspecies, Proteus anguinus parkelj, the black olm) was discovered at Jelsevniscica in Jelsevnik near Crnomelj in Slovenia. This limited area is the unique locality of the black olm. A presentation on the unique black population has been prepared by the research group for functional-morphological studies of vertebrates, under the guidance of Prof. dr. Bulog. This presentation is located in the house of the family Zupancic in Jelsevnik and is prepared on ten color panels with texts and photo documentation. The presentation is intended for tourists and also for teaching and research purposes, for better recognition of the unique part of Slovenian natural heritage.
A popular scientific film dealing with our studies of biological peculiarities of endemic cave salamander supports this presentation and was recently created in collaboration with National TV of Slovenia (Bulog et al. 2003).
Possible reasons for amphibian decline
General habitat alteration and loss
Proteus is the only cave-adapted vertebrate in Europe. Current genetic research under the direction of Dr. Boris Sket of the University of Ljubljana suggests that Proteus anguinus is actually a complex of several species, with phylogenetic analysis revealing six cryptic lineages (see Trontelj et al. 2007).
Functional-morphological and environmental studies of Proteus have been performed at the Department of Biology, Biotechnical Faculty (BF), University of Ljubljana, Slovenia for more than thirty years, with the most recent twenty years under the guidance of Prof. Dr. Boris Bulog.
This species was featured in News of the Week on 16 March 2020:
Blind, aquatic, and with a great sense of smell, olms (Proteus anguinus) are enigmatic amphibians that reproduce once every 12 years and live for over 100. In a recent study, Balázs et al (2019) surveyed a population of olms in Eastern Herzegovia for eight years to discover that over this timeframe, individuals rarely move more than a few meters. Amazingly, one individual did not move for over seven years. In the absence of any major predators, the high site fidelity of Proteus anguinus may instead be related to extreme pressures to conserve energy in low-nutrient cave habitats. Studies of olms in the wild are rare, and this study contributes to our understanding of how long-lived amphibians survive in such challenging environments. (RTarvin)
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Originally submitted by: Boris Bulog, Arie van der Meijden (first posted 1999-12-26)
Edited by: Kellie Whittaker, Michelle S. Koo (2021-03-17)
Species Account Citation: AmphibiaWeb 2021 Proteus anguinus: Olm <https://amphibiaweb.org/species/4229> University of California, Berkeley, CA, USA. Accessed Dec 5, 2023.
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Citation: AmphibiaWeb. 2023. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 5 Dec 2023.
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