Editing Olm (section)

== Description ==
===External appearance===
[[File:Protée aveugle.jpg|thumb|The olm as depicted by the French biologist [[Gaston Bonnier]] in 1907]]
The olm's body is [[snake]]like, {{convert|20|–|30|cm|in|round=0.5|abbr=on}} long, with some specimens reaching up to {{convert|40|cm|in|0}}, which makes them some of the largest cave-dwelling animals in the world.<ref name="WEB01">Weber A. (2000). ''Fish and amphibia''. In: Culver D.C. ''et al.'' (ed.): ''Ecosystems of the world: Subterranean Ecosystems'', pp. 109–132. Amsterdam: [[Elsevier]]</ref><ref>{{Cite web|url=https://www.theguardian.com/environment/2017/aug/27/slovenia-baby-dragons-postojna-salamaders-olms-proteus-anguinus|title=How Slovenia is helping its 'baby dragons'|first=Robin|last=McKie|date=August 27, 2017|via=The Guardian}}</ref> The average length is between 23 and 25&nbsp;cm.<ref name="blackburn">{{Cite journal |last=Blackburn |first=Daniel G. |date=2019-07-01 |title=The oviparous olm: Analysis and refutation of claims for viviparity in the cave salamander ''Proteus anguinus'' (Amphibia: Proteidae) |url=https://www.sciencedirect.com/science/article/pii/S0044523119300555 |journal=Zoologischer Anzeiger |language=en |volume=281 |pages=16–23 |doi=10.1016/j.jcz.2019.05.004 |bibcode=2019ZooAn.281...16B |s2cid=190885637 |issn=0044-5231|url-access=subscription }}</ref> Females grow larger than males, but otherwise the primary external difference between the sexes is in the [[cloaca]] region (shape and size) when breeding.<ref name="AW" /> 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 [[biological pigment|pigment]] [[riboflavin]],<ref>{{Cite journal |last1=Istenic L. |last2=Ziegler I. |name-list-style=amp |date=1974 |title=Riboflavin as "pigment" in the skin of ''Proteus anguinus'' L. |journal=Naturwissenschaften |volume=61 |issue=12 |pages=686–687 |bibcode=1974NW.....61..686I |doi=10.1007/bf00606524 |pmid=4449576|s2cid=28710659 }}</ref> making it yellowish-white or pink in color.<ref name="animal" />

The white skin color of the olm retains the ability to produce [[melanin]], and will gradually turn dark when exposed to light; in some cases the [[larva]]e are also colored. One population, the [[black olm]], is always pigmented and dark brownish to blackish when adult.<ref name="SA01"/> The olm's pear-shaped head ends with a short, dorsoventrally flattened snout. The mouth opening is small, with tiny [[tooth|teeth]] forming a [[sieve]] to keep larger particles inside the mouth. The nostrils are so small as to be imperceptible, but are placed somewhat [[Anatomical terms of location#Left and right (lateral), and medial|laterally]] near the end of the snout. The regressed [[eye]]s are covered by a layer of skin. The olm breathes with external [[gill]]s that form two branched tufts at the back of the head.<ref name="animal" /> They are red in color because the oxygen-rich [[blood]] shows through the non-pigmented skin.{{r|ley196802}} The olm also has rudimentary [[lung]]s, but their role in respiration is only accessory, except during [[Hypoxia (environmental)|hypoxic]] conditions.<ref name="AW" />

===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.<ref name="Schlegal 2006">{{cite journal | last1 = Schlegel | first1 = P.A. | last2 = Briegleb | first2 = W. | last3 = Bulog | first3 = B. | last4 = Steinfartz | first4 = S. | date = 2006 | language =  French | title = Revue et nouvelles données sur la sensitivité a la lumiere et orientation non-visuelle chez Proteus anguinus, Calotriton asper et Desmognathus ochrophaeus (Amphibiens urodeles hypogés) | journal = Bulletin de la Société herpétologique de France | volume = 118 | pages = 1–31 | url = https://pub.uni-bielefeld.de/record/1870980}}</ref> 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 receptor]]s.<ref name="SB01" />

====Photoreceptors====
Although blind, the olm swims away from light.{{r|ley196802}} The eyes are regressed, but retain sensitivity. They lie deep below the [[Dermis|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.<ref name="DUR01">{{cite journal | last = Durand | first = J.P. | date = 1976 | title = Ocular Development and Involution in the European Cave Salamander, ''Proteus Anguinus'' Laurenti | journal = The Biological Bulletin | volume = 151 | number = 3 | pages = 450–466 | doi = 10.2307/1540499| jstor = 1540499 | pmid = 1016662 | url = https://www.biodiversitylibrary.org/part/13965 }}</ref> 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.<ref name="LANG01">Langecker T.G. (2000). ''The effects of continuous darkness on cave ecology and cavernicolous evolution''. In: Culver D.C. ''et al.'' (eds.): ''Ecosystems of the world: Subterranean Ecosystems'', pp. 135–157. Amsterdam: Elsevier</ref> [[Ethology|Behavioral]] experiments revealed that the skin itself is also sensitive to light.<ref name="HAW01">{{Cite journal |last=Hawes |first=R.S. |date=1945 |title=On the eyes and reactions to light of Proteus anguinus |journal=Q. J. Microsc. Sci. |series=New Series |volume=86 |pages=1–53|pmid=21004249 }}</ref> Photosensitivity of the integument is due to the pigment [[melanopsin]] inside specialized cells called [[melanophore]]s. Preliminary immunocytochemical analyses support the existence of photosensitive pigment also in the animal's integument.<ref name="KOS01">Kos M. (2000). {{lang|sl|Imunocitokemijska analiza vidnih pigmentov v čutilnih celicah očesa in pinealnega organa močerila (Proteus anguinus, Amphibia, Urodela)}} (''Immunocitochemical analysis of the visual pigments in the sensory cells of the eye and the pineal organ of the olm (Proteus anguinus, Amphibia, Urodela).'') PhD thesis. Ljubljana: [[University of Ljubljana]]. {{in lang|sl}}</ref><ref>{{Cite journal |last1=Kos |first1=M. |last2=Bulog |first2=B. |display-authors=etal |date=2001 |title=Immunocytochemical demonstration of visual pigments in the degenerate retinal and pineal photoreceptors of the blind cave salamander (''Proteus anguinus'') |journal=Cell Tissue Res |volume=303 |issue=1 |pages=15–25 |doi=10.1007/s004410000298 |pmid=11236001|s2cid=30951738 }}</ref>

====Chemoreceptors====
[[Image:P anguinus-head1.jpg|thumb|right|The front part of the olm's head carries sensitive chemo-, mechano-, and electroreceptors.]]

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.<ref name="HUP01">Hüpop K. (2000). ''How do cave animals cope with the food scarcity in caves?''. In: Culver D.C. ''et al.'' (ed.): ''Ecosystems of the world: Subterranean Ecosystems'', pp. 159–188. Amsterdam: Elsevier</ref> The nasal [[epithelium]], located on the inner surface of the nasal cavity and in the [[Jacobson's organ]], is thicker than in other amphibians.<ref name="DC01">{{cite journal | last1 = Dumas | first1 = P | year = 1998 | title = The olfaction in Proteus anguinus | journal = Behavioural Processes | volume = 43 | issue = 2| pages = 107–113 | doi = 10.1016/s0376-6357(98)00002-3 | pmid = 24895999 | s2cid = 20825148 }}</ref> The [[taste buds]] are in the [[mucus|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.<ref>{{Cite journal |last1=Istenič |first1=L. |last2=Bulog |first2=B. |year=1979 |title=The structural differentiations of the buccal and pharyngeal mucous membrane of the Proteus anguinus Laur |journal=Biološki Vestnik |volume=27 |pages=1–12}}</ref>

====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 [[vibration]]s from the ground. The complex functional-morphological orientation of the sensory cells enables the animal to register the sound sources.<ref>{{Cite journal |last=Bulog |first=B |date=1989 |title=Differentiation of the inner ear sensory epithelia of Proteus anguinus (Urodela, Amphibia) |journal=Journal of Morphology |volume=202 |issue=3 |pages=325–338 |doi=10.1002/jmor.1052020303 |pmid=29865674|s2cid=46928861 }}</ref><ref name="BUL01">Bulog B. (1990). {{lang|sl|Čutilni organi oktavolateralnega sistema pri proteju Proteus anguinus (Urodela, Amphibia). I. Otični labirint}} (''Sense organs of the octavolateral system in proteus Proteus anguinus (Urodela, Amphibia). I. Otic labyrinth''). Biološki vestnik '''38''': 1–16 {{in lang|sl}}</ref> 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.
Behavioural (ethological) tests have shown that its sensitivity for detecting underwater sound waves reaches into the area of frequencies of sound waves between 10 and more than 12,000 Hz, while the greatest sensitivity is reached between  1,500 and 2,000 Hz. The ethological experiments indicate that the best hearing sensitivity of ''Proteus'' is between 10&nbsp;Hz and up to 12,000&nbsp;Hz.<ref name="Bulog and Schlegel 2000">Bulog B. & Schlegel P. (2000). ''Functional morphology of the inner ear and underwater audiograms of Proteus anguinus (Amphibia, Urodela)''. Pflügers Arch '''439'''(3), suppl., pp. R165–R167.</ref> The [[lateral line]] supplements inner ear sensitivity by registering low-frequency nearby water displacements.<ref name="Schlegal 2006" /><ref name="Bulog and Schlegel 2000" />

A new type of [[electroreception]] sensory organ has been analyzed on the head of ''Proteus'', utilizing light and [[Electron microscope|electron microscopy]]. These new organs have been described as [[ampullary organ]]s.<ref>{{Cite journal |last1=Istenič |first1=L. |last2=Bulog |first2=B. |date=1984 |title=''Some evidence for the ampullary organs in the European cave salamander Proteus anguinus'' (Urodela, Amphibia) |journal=Cell Tissue Res |volume=235 |issue=2 |pages=393–402 |doi=10.1007/bf00217865 |pmid=6705040|s2cid=25511603 }}</ref>

Like some other lower [[vertebrate]]s, the olm has the ability to register weak [[electric field]]s.<ref name="SB01">{{Cite journal |last1=Schlegel |first1=P. |last2=Bulog |first2=B. |date=1997 |title=Population-specific behavioral electrosensitivity of the European blind cave salamander, Proteus anguinus |journal=Journal of Physiology (Paris) |volume=91 |issue=2 |pages=75–79 |doi=10.1016/S0928-4257(97)88941-3 |pmid=9326735|s2cid=25550328 }}</ref> 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.<ref>Bulog B., Schlegel P. ''et al.'' (2002). ''Non-visual orientation and light-sensitivity in the blind cave salamander, ''Proteus'' anguinus'' (Amphibia, Caudata). In: Latella L., Mezzanotte E., Tarocco M. (eds.). 16th international symposium of biospeleology; 2002 Sep 8–15; Verona: Societé Internationale de Biospéologie, pp. 31–32.</ref>