The olm (Proteus anguinas) is an amphibian most notable for its unusual appearance and adaptations to life in complete darkness. It's the only European vertebrate that lives solely underground.Olms are found in subterranean caves in central Europe, where they are rarely seen. They grow between 8 and 12 inches longRead on to see how these creatures, once thought to be baby dragons, are perfectly adapted to their dank, dark habitats. 

1. The olm goes by many names. The olm is also known as the proteus, cave salamander, and white salamander. In Slovenia, it is called "močeril," which translates to "the one that burrows into wetness." Another local name for the creature is "oveja Ribica," which means "human fish."

2. They only live in underground caves. Olms are entirely aquatic and found only in the deep underground lakes and pools of caves in the Dinaric Alps — parts of Slovenia, Italy, Croatia, and Herzegovina.

3. They have translucent skin. Since they spend their entire lives in complete darkness, olms have skin devoid of pigment, giving them a whitish-pink color. Their internal organs can be seen through the skin on their abdomens.

4. Olms are blind. The eyes of the olm are underdeveloped, covered by a layer of skin. It's an adaptation to its dark surroundings. Though the eyes are regressed, they retain a sensitivity to light. Olm larvae have normal eyes for the first four months of life before they start to regress. By the time they are adults, olms are blind.

5. Olms rely on keen senses other than sight. The front part of the olm's head is equipped with sensitive chemo-, mechano-, and electroreceptors, which it uses to navigate and find prey. They have powerful senses of smell and hearing, as well as the ability to detect electric fields with ampulary organs. Recent behavioral studies suggest they are also able to detect and orient to magnetic fields.

6. They can go a long time between meals. Olms have small mouths with tiny teeth that form a sieve to keep larger food particles in their mouths. They feed on insect larvae, small crabs, and snails, which they swallow whole. Olms can consume large amounts of food at once and store nutrients in the liver. When food is scarce, they reduce their metabolic activity and even reabsorb their own tissues. Experiments show they can live up to 10 years without food.

7. Olms might be the longest-lived amphibian species. Olms can live for at least 50 years, and one study suggests a maximum lifespan of over 100 years.

8. Olms never grow up. Unlike most amphibians, olm larvae do not go through a  distinct period of metamorphosis. Instead, the larvae develop directly into adults at about four months of age, but they retain some of their juvenile characteristics, like frilly gills and tail fins.

9. Olms are gregarious, except males during breeding season. Adult olms aggregate in cracks and under rocks, but sexually active males establish territories that they defend from other males. Once he chooses an area and chases other males away, the male will secrete a pheromone to attract females. When a female approaches, he fans his tail toward her head. Then he touches her cloaca with his snout, and she does the same to him. He walks in front of the female and deposits a spermatophore, which she picks up with her cloaca and stores in a special structure called a spermathecae. Then the female leaves to establish an egg-laying territory, where she will watch over the developing eggs.

10. Olms were once thought to be baby dragons. Their natural habitat is mostly inaccessible to people, but after heavy rains, olms occasionally wash up from their underground lairs. Medieval legend considered them baby dragons — their snake-like bodies and frilly gills corresponded to descriptions of dragons, and their soft pale skin looked like that of a human newborn. 

References and Other Resources:

Bulog, B. (1989). Differentiation of the inner ear sensory epithelia of Proteus anguinus (Urodela, Amphibia). Journal of Morphology 202: 325–338. doi:10.1002/jmor.1052020303.

Dumas, P. and Chris, B. (1998). The olfaction in Proteus anguinus. Behavioural Processes 43: 107–113. doi: 10.1016/S0376-6357(98)00002-3.

Guillaume, O. (2000). Role of chemical communication and behavioural interactions among conspecifics in the choice of shelters by the cave-dwelling salamander Proteus anguinus (Caudata, Proteidae). Can. J. Zool. 78(2): 167–173. doi:10.1139/z99-198.Istenič, L. and Bulog, B. (1984). Some evidence for the ampullary organs in the European cave salamander Proteus anguinus (Urodela, Amphibia). Cell Tissue Res 235: 393–402. doi:10.1007/bf00217865.

Meaton, J. (2011). "Proteus anguinus," Animal Diversity Web. Accessed June 15, 2015 at http://animaldiversity.org/accounts/Proteus_anguinus/.

Proteus anguinas, Arkive. Accessed June 15, 2015 at http://www.arkive.org/cave-salamander/proteus-anguinus/.

Schegel, P.and Bulog, B. (1997). Population-specific behavioral electrosensitivity of the European blind cave salamander, Proteus anguinus. Journal of Physiology (Paris) 91: 75–79. doi:10.1016/S0928-4257(97)88941-3.

Voituron, Y., De Fraipont, M., Issartel, J., Guillaume, O., and Clobert, J. (2010). Extreme lifespan of the human fish (Proteus anguinus): a challenge for ageing mechanisms. Biology Letters 7(1): 105–107. doi:10.1098/rsbl.2010.0539.