Olm

The **olm** or **proteus** (*Proteus anguinus*) aka "The Human Fish" is an aquatic salamander in the family Proteidae, the only exclusively cave-dwelling chordate species found in Europe. In contrast to most amphibians, it is entirely aquatic; it eats, sleeps, and breeds underwater. Living in caves found in the Dinaric Alps, it is endemic to the waters that flow underground through the extensive limestone of the karst of Central and Southeastern Europe, specifically southern Slovenia, the basin of the Soča River (Italian: *Isonzo*) near Trieste, Italy, southwestern Croatia, and Bosnia and Herzegovina.

It is also called the "human fish" by locals because of its skin color, similar to that of Caucasian people (translated literally from Slovene: *človeška ribica*, Bosnian: *čovječja ribica* and Croatian: *čovječja ribica*), as well as "cave salamander" or "white salamander". In Slovenia, it is also known by the name *močeril*, which translates as "the one that burrows into wetness". 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 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. It has three toes on its forelimbs, but only two toes on its hind feet. It also exhibits neoteny, retaining larval characteristics like external gills into adulthood, like some American amphibians, the axolotl and the mudpuppies (*Necturus*). The olm is the only species in the genus *Proteus* and the only European species of the family Proteidae, whose other extant genus is *Necturus*.

## Anatomy

**External Appearance**
The olm's body is snakelike, 20–30 cm (8–12 in) long, with some specimens reaching up to 40 centimetres (16 in).[9] 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 internal organs can be seen shining through on the abdominal part of the body. The resemblance in color to that of white humans is the reason why the *Proteus* is called *human fish* in some languages. However, the olm's skin retains the ability to produce melanin. When exposed to light, it will gradually turn dark, and in some cases the larvae are also colored. Its 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. The sexes are very similar in appearance, with males having a somewhat thicker cloaca than females.

### **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**

The eyes are regressed, but retain sensitivity to light. 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. 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.

#### **Chemoreceptors**

The olm's body is snakelike, 20–30 cm (8–12 in) long, with some specimens reaching up to 40 centimetres (16 in). 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 internal organs can be seen shining through on the abdominal part of the body. The resemblance in color to that of white humans is the reason why the *Proteus* is called *human fish* in some languages. However, the olm's skin retains the ability to produce melanin. When exposed to light, it will gradually turn dark, and in some cases the larvae are also colored. Its 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. The sexes are very similar in appearance, with males having a somewhat thicker cloaca than females.

### **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**

The eyes are regressed, but retain sensitivity to light. 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. 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.

#### **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. The nasal epithelium, located on the inner surface of the nasal cavity and in the Jacobson's organ, is thicker than in other amphibians.[20] 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. 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. The ethological experiments indicate that the best hearing sensitivity of *Proteus* is between 10 Hz and up to 15,000 Hz. The lateral line supplements inner ear sensitivity by registering low-frequency nearby water displacements.

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. In 2002, *Proteus* *anguinus* was found to align itself with natural and artificially modified magnetic fields.