The olm, or proteus (Proteus anguinus), is a blind amphibian endemic to the subterranean waters of caves of the Dinaric karst of southern Europe. It lives in the waters that flow underground through this extensive limestone region including waters of the Soča river basin near Trieste in Italy, through to southern Slovenia, southwestern Croatia, and Herzegovina. The olm is the only species in its genus Proteus, the only European species of the family Proteidae, and the only European exclusively cave-dwelling chordate. It is also occasionally called the "human fish" by locals because of its human-like skin (translated literally from človeška ribica and čovječja ribica), as well as "cave salamander" or "white salamander."
This animal is most notable for its adaptations to a life of complete darkness in its underground habitat. The olm's eyes are undeveloped, leaving it blind, while its other senses, particularly those of smell and hearing, are acutely developed. It also lacks any pigmentation in its skin. In contrast to most amphibians, the olm is entirely aquatic, and it eats, sleeps, and breeds underwater. It also exhibits neoteny, retaining larval characteristics like external gills into adulthood.
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. 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 the chemical composition of water.
A new type of 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. Recently, Proteus anguinus has been found to align itself with natural and artificially modified magnetic fields.
The olm's embryonic development takes 140 days, after which it takes another 14 years to reach sexual maturity. The larvae gain adult appearance after nearly four months, with the duration of development strongly correlating with water temperature. Unconfirmed historical observations of vivipary exist, but it has been shown that the females possess a gland that produces the egg casing, similar to those of fish and egg-laying amphibians. It was long thought that female olm gave birth to live young at lower temperatures and laid eggs at higher, but rigorous observations have not confirmed that. The olm appears to be oviparous.
The female lays up to 70 eggs, each about in diameter, and places them between rocks, where they remain under her protection. The tadpoles are long when they hatch and live on yolk stored in the cells of the digestive tract for a month.
Development of the olm and other troglobite amphibians is characterized by heterochrony – the animal does not undergo metamorphosis and instead retains larval features. The form of heterochrony in the olm is neoteny – delayed somatic maturity with precocious reproductive maturity, i.e. reproductive maturity is reached while retaining the larval external morphology. In other amphibians, the metamorphosis is regulated by the hormone thyroxine, excreted 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.
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 crabs, snails and occasionally insects. 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.
Reproduction has only been observed in captivity so far. Sexually mature males have swollen cloacas, brighter skin color, two lines at the side of the tail, and slightly curled fins. No such changes have been observed in the females. The male can start courtship even without the presence of a female. He chases other males away from the chosen area, and may then secrete a female-attracting pheromone. When the female approaches, he starts to circle around her and fan her with his tail. Then he starts to touch the female's body with his snout, and the female touches his cloaca with her snout. At that point, he starts to move forward with a twitching motion, and the female follows. He then deposits the spermatophore, and the animals keep moving forward until the female hits it with her cloaca, after which she stops and stands still. The spermatophore sticks to her and the sperm cells swim inside her cloaca where they attempt to fertilize her eggs. The courtship ritual can be repeated several times over a couple of hours.
Longevity is estimated at up to 58 years. Individual specimens have been kept under semi-natural conditions in concrete basins for up to 70 years
The Black Proteus (Proteus anguinus parkelj Sket & Arntzen, 1994) is the only recognized subspecies of the olm, endemic to the underground waters near Črnomelj, Slovenia, an area smaller than . It was first found in 1986 by members of the Slovenian Karst Research Institute who were exploring the water from Dobličice karst spring in the Bela krajina region.
It has several features separating it from the type subspecies:
|Feature||Proteus anguinus anguinus||Proteus anguinus parkelj||Notes|
|Skin||Not pigmented.||Normally pigmented, dark brown or black in color.||The most obvious difference.|
|Head shape||Long, slender.||Shorter, equally thick. Stronger jaw muscles visible as two bulbs on the top of the head.|
|Body length||Shorter, 29–32 vertebrae.||Longer, 34–35 vertebrae.||Amphibians do not have a fixed number of vertebrae.|
|Tail||Longer in proportion to the rest of the body.||Shorter in proportion.|
|Eyes||Regressed.||Almost normally developed, although still small compared to other amphibians. Covered by a thin layer of transparent skin, no eyelids.||Regressed eye of White Proteus shows first of all immunolabelling for the red-sensitive cone opsin. The eye of Black Proteus has principal rods, red-sensitive cones and blue- or UV- sensitive cones.|
|Other senses||Specific and highly sensitive.||Some sensory organs, particularly electroreceptors, less sensitive.|
These features suggest that the Black Proteus has probably colonized underground habitats more recently and still retains some nontroglomorphic characteristics.
The first written mention of the olm is in Janez Vajkard Valvasor's The Glory of the Duchy of Carniola (1689) as a baby dragon. This was a reference to a folk story he did not really believe in. The first researcher to retrieve a live olm was a physician and researcher from Idrija, G.A. Scopoli; he sent dead specimens and drawings to colleagues and collectors.
Joseph Nicolai Laurenti, though, was the first to briefly describe the olm in 1768 and give it the scientific name Proteus anguinus. It was not until the end of the century that Carl Franz Anton Ritter von Schreibers from the Naturhistorisches Museum of Vienna started to look into this animal's anatomy. The animals were sent to him by Žiga Zois. Schreibers presented his findings in 1801 to The Royal Society in London, and later also in Paris. Soon the olm started to gain wide recognition and attract significant attention, resulting in thousands of animals being sent to researchers and collectors worldwide. The basis of functional morphological investigations in Slovenia was set up by Prof. dr. Lili Istenič in the 1980s. More than twenty years the Research Group for functional morphological Studies of the Vertebrates in the Department of Biology (Biotechnical Faculty, University of Ljubljana), is one of the leading groups studying the olm under the guidance of Prof.dr. Boris Bulog. There are also several cave laboratories in Europe, where olms have been introduced and are being studied. These are Moulis (France), Kent's Cavern (England), Han-sur-Lesse (Belgium) and Aggtelek (Hungary). They were also introduced into the Hermannshöhle (Germany) and Oliero (Italy) caves, where they still live today.
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 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. 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, which can threaten the species by taking too many animals out of the habitat.
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 olm 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. 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 a Slovenian red list of endangered species. The Postojna cave and other caves inhabited by olm were also included in the Slovenian part of the Natura 2000 network.
In Croatia, the olm is protected by the legislation designed to protect amphibians – collecting is possible only for research purposes by permission of the National Administration for Nature and Environment Protection. Conservation status in Bosnia and Herzegovina and Montenegro has not yet been defined.
On the IUCN Red List, the olm is listed as vulnerable because of its fragmented and limited distribution and ever-decreasing population.
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. The Postojna cave is one of the birthplaces of speleobiology due to the olm and other rare cave inhabitants. The image of the olm contributes significantly to the fame of the Postojna cave, which Slovenia successfully utilizes for the promotion of eco-tourism in Postojna and other parts of Slovenian karst. Tours of the Postojna cave also include a tour around the speleobiological station – the Proteus vivarium, showing different aspects of the cave environment.
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.