Olm

The Olm or Proteus (Proteus anguinus) is an amphibian that lives in the subterranean waters of the Dinaric karst from the Soča river basin near Trieste in Italy through southern Slovenia and southwestern Croatia to Herzegovina. It is the only species in the genus Proteus, the only European species of the family Proteidae, and the only European cave-dwelling caudate.it is a relative to the axolotl (mexican walking fish)it unlike the axolotl has internal gills with only a small amount of gill showing. It is also called the "human fish" or "humanfish", because of its human-like skin (translated literally from Slovenian: Človeška ribica and Croatian: Čovječja ribica), Cave Salamander, or White Salamander.[2]
This animal is most notable for its adaptations to life in the complete darkness of its underground habitat. The Olm's eyes have atrophied, leaving it blind, while its other senses, particularly those of smell and hearing, are acute. It also has no skin pigmentation. In contrast to other amphibians, the Olm is wholly aquatic, not only breeding underwater but living its entire life there.[3] This is possible due to their retention of larval characteristics, such as external gills, into adulthood.

Anatomy

External appearance

The Olm's body is snakelike, 20–30 cm (8–12 in) long, with some specimens reaching up to 40 cm (16 in).[4] 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,[5] making it yellowish-white or pink in color.[3] The internal organs can be seen shining through on the abdominal part of the body. The resemblance in color to human skin 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 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.[3] They are red in color because the oxygen-rich blood shows through the unpigmented 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.[6] 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.[7] The pineal body has also regressed photoreceptive cells, retaining visual pigment like photoreceptive cells of the regressed eye. The Pineal organ in Proteus probably possesses some control over the physiological processes.[8] Behavioral experiments revealed that the skin itself is also sensitive to light.[9] Photosensitivity of the integument is due to the pigment melanopsin inside specialized cells called melanophores. Preliminary immunocytochemical analysis support the existence of photosensitive pigment also in Proteus' integument[10][11]

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.[12] The nasal epithelium, located on the inner surface of the nasal cavity and in the Jacobson's organ, is thicker than in other amphibians.[13] 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.

Mechano- and electroreceptors

The sensory epithelia of the inner ear are very specific differentiated and enables 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[14][15] Little is known about the hearing of Proteus, but occasionally observed reactions to sounds have indicated its possibility (Bulog, personal observ.). This suggested a hearing capability even and especially under water. 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.[16] The lateral line supplements inner ear sensitivity by registering low-frequency near by 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.[17] Like some other lower vertebrates, the Olm has the ability to register weak electric fields.[18] 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.[19]