cavefish

File:Typhleotris madgascarensis.jpg is an opportunistic feeder on various invertebrates{{FishBase | genus = Typhleotris | species = madagascariensis | month = April| year = 2017}}{{cite journal | author=Rasoloariniaina | author2=Ganzhorn | author3=Riemann | author4=Raminosoa | name-list-style=amp | year=2016 | title=Water quality and biotic interaction of two cavefish species: Typhleotris madagascariensis Petit, 1933 and Typhleotris mararybe Sparks & Chakrabarty, 2012, in the Mahafaly Plateau groundwater system, Madagascar | journal=Subterranean Biology | volume=18 | pages=1–16 | doi=10.3897/subtbiol.18.8321| doi-access=free }}]]

Many aboveground fish may enter caves on occasion, but obligate cavefish (fish that require underground habitats) are extremophiles with a number of unusual adaptations known as troglomorphism. In some species, notably the Mexican tetra, shortfin molly, Oman garra, Indoreonectes evezardi, and a few catfish, both "normal" aboveground and cavefish forms exist.Plath, M.; and Tobler, M. (2007). Sex recognition in surface- and cave-dwelling Atlantic molly females (Poecilia mexicana, Poeciliidae, Teleostei): influence of visual and non-visual cues. acta ethol 10: 81–88Gross, J.B. (2012). The complex origin of Astyanax cavefish. BMC Evolutionary Biology 12: 105.{{Cite iucn | author = Harrison, I.J. | title = Garra barreimiae | volume = 2015 | page = e.T8916A3147989 | date = 2015 | doi = 10.2305/IUCN.UK.2015-2.RLTS.T8916A3147989.en | access-date = 23 December 2017}}{{cite journal|author=Ng, H.H.|author2=Kottelat, M. |name-list-style=and|year=1998|title=Pterocryptis buccata, a new species of catfish from western Thailand (Teleostei: Siluridae) with epigean and hypogean populations|journal=Ichthyological Research|volume=45 |issue=4|pages=393-399}}

Many adaptions seen in cavefish are aimed at surviving in a habitat with little food. Living in darkness, pigmentation and eyes are useless, or an actual disadvantage because of their energy requirements, and therefore typically reduced in cavefish.{{cite journal | author = Rantin B., Bichuette M.E. | year = 2013 | title = Phototactic behaviour of subterranean Copionodontinae Pinna, 1992 catfishes (Siluriformes, Trichomycteridae) from Chapada Diamantina, central Bahia, northeastern Brazil | journal = International Journal of Speleology | volume = 41 | issue = 1| pages = 57–63 | doi = 10.5038/1827-806X.42.1.7 | doi-access = free }}{{cite web| author=Owen, J. | title=How This Cave-Dwelling Fish Lost Its Eyes to Evolution | url=http://news.nationalgeographic.com/2015/09/150911-blind-cavefish-animals-science-vision-evolution/ | archive-url=https://web.archive.org/web/20150914002623/http://news.nationalgeographic.com/2015/09/150911-blind-cavefish-animals-science-vision-evolution/ | url-status=dead | archive-date=14 September 2015 | publisher=National Geographic | date=11 September 2015 | access-date=14 May 2017}}{{cite book | author=Burton, M. | display-authors=etal | title=International Wildlife Encyclopedia, volume 3, Bro–Che | year=2002 | edition=3rd | page=410 | publisher=International Society for Subterranean Biology | isbn=978-2-9527084-0-1 }} Other examples of adaptations are larger fins for more energy-efficient swimming, and a loss of scales and swim bladder.{{cite journal|author=Romero, S.|author2=Green, S.M. |name-list-style=and |year=2005|title=The end of regressive evolution: examining and interpreting the evidence from cave fishes|journal=Journal of Fish Biology|volume=67|issue=1|pages=3-32}}{{cite book|author=Parzefall, J. |author2=Trajano, E. |name-list-style=and |year=2010|chapter=Behavioral Patterns in Subterranean Fishes|editor= Trajano, E.|editor2= Bichuette, M.E.|editor3= Kapoor, B.G.|title=Biology of Subterranean Fishes|isbn=978-1578086702}} The loss can be complete or only partial, for example resulting in small or incomplete (but still existing) eyes, and eyes can be present in the earliest life stages but degenerated by the adult stage.{{cite journal | author=Secutti, S. | author2=E. Trajano | name-list-style=amp | year=2009 | title=Reproductive behavior, development and eye regression in the cave armored catfish, Ancistrus cryptophthalmus Reis, 1987 (Siluriformes: Loricariidae), breed in laboratory | journal=Neotropical Ichthyology | volume=7 | issue=3 | doi=10.1590/S1679-62252009000300016 | pages=479–490| doi-access=free }} In some cases, "blind" cavefish may still be able to see: Juvenile Mexican tetras of the cave form are able to sense light via certain cells in the pineal gland (pineal eye),Choi, C.Q. (28 January 2008). [http://www.livescience.com/9555-blind-fish.html Blind Fish Still Able to 'See'.] LiveScience. Retrieved 28 February 2016. and Congo blind barbs are photophobic, despite only having retinas and optical nerves that are rudimentary and located deep inside the head, and completely lacking a lens.{{cite book | author=Vreven, E. | author2=A. Kimbembi ma Ibaka | author3=S. Wamuini Lunkayilakio | name-list-style=amp | chapter=The Congo blind barb: Mbanza-Ngungu's albino cave fish | editor=Darwall | editor2=Smith | editor3=Allen | editor4=Holland | editor5=Harrison | editor6=Brooks | title=The diversity of life in African freshwaters: Underwater, under threat | publisher=IUCN | pages=74–75 | year=2011 | isbn=978-2-8317-1345-8}} In the most extreme cases, the lack of light has changed the circadian rhythm (24-hour internal body clock) of the cavefish. In the Mexican tetra of the cave form and in Garra andruzzii the circadian rhythm lasts 30 hours and 47 hours, respectively.Palermo, E. (24 September 2014). [http://www.livescience.com/47987-energy-saving-mexican-cavefish.html Blind Cavefish Froze Its Internal Clock to Save Energy.] LiveScience. Retrieved 28 February 2016.Battison, L. (10 September 2011). [https://www.bbc.com/news/science-environment-14844053 Fish living in dark caves still feel the rhythm of life.] BBC News. Retrieved 28 February 2016. This may help them to save energy. Without sight, other senses are used and these may be enhanced. Examples include the lateral line for sensing vibrations,Burt de Perera, T. (2004). "Spatial parameters encoded in the spatial map of the blind Mexican cave fish, Astyanax fasciatus". Anim.Behav 68: 291–295.Weber, A. (1995). The lateral line system of epigean and cave dwelling catfishes of the genus Rhamdia (Pimelodidae, Teleostei) in Mexico. Mem Biospeol 22: 215–225.{{Cite journal|last1=Yoshizawa|first1=Masato|last2=Gorički|first2=Špela|last3=Soares|first3=Daphne|last4=Jeffery|first4=William R.|date=September 2010|title=Evolution of a Behavioral Shift Mediated by Superficial Neuromasts Helps Cavefish Find Food in Darkness|url= |journal=Current Biology|language=en|volume=20|issue=18|pages=1631–1636|doi=10.1016/j.cub.2010.07.017|pmc=2946428|pmid=20705469|bibcode=2010CBio...20.1631Y }} mouth suction to sense nearby obstacles (comparable to echolocation),Poppick, L. (2 April 2014). [http://www.livescience.com/44571-blind-cavefish-use-mouth-suction.html Mouth Vision: Blind Fish Suctions Water to Navigate.] LiveScience. Retrieved 28 February 2016. and chemoreception (via smell and taste buds).Bibliowicz, J.; Alié, A.; Espinasa, L.; Yoshizawa, M.; Blin, M.; Hinaux, H.; Legendre, L.; Père, S.; and Rétaux, S. (2013). Differences in chemosensory response between eyed and eyeless Astyanax mexicanus of the Rio Subterráneo cave. EvoDevo 25.{{cite journal | author=Kasumyan, A.O. | author2=E.A. Marusov | name-list-style=amp | year=2015 | title=Chemoorientation in the feeding behavior of the blind Mexican cavefish Astyanax fasciatus (Characidae, Teleostei) | journal=Russian Journal of Ecology | volume=46 | issue=6 | pages=559–563 | doi=10.1134/s1067413615060053| bibcode=2015RuJEc..46..559K | s2cid=17283377 }} Although there are cavefish in groups known to have electroreception (catfish and South American knifefish), there is no published evidence that this is enhanced in the cave-dwellers.{{cite journal | author=Soares, D. | author2= M.L. Niemiller | year=2013 | title=Sensory Adaptations of Fishes to Subterranean Environments | journal=BioScience | volume=63 | issue=4 | pages=274–283 | doi=10.1525/bio.2013.63.4.7| doi-access=free }} The level of specialized adaptations in a cavefish is generally considered to be directly correlated to the amount of time it has been restricted to the underground habitat: Species that recently arrived show few adaptations and species with the largest number of adaptations are likely the ones that have been restricted to the habitat for the longest time.{{cite journal | author=Bockmann, F.A. | author2=R.M.C. Castro | name-list-style=amp | year=2010 | title=The blind catfish from the caves of Chapada Diamantina, Bahia, Brazil (Siluriformes: Heptapteridae): description, anatomy, phylogenetic relationships, natural history, and biogeography | journal=Neotropical Ichthyology | volume=8 | issue=4 | pages=673–706 | doi=10.1590/s1679-62252010000400001| doi-access=free }}

Some fish species that live buried in the bottom of aboveground waters, live deep in the sea or live in deep rivers have adaptations similar to cavefish, including reduced eyes and pigmentation.{{cite journal | author=Schaefer | author2=Provenzano | author3=De Pinna | author4=Baskin | name-list-style=amp | title=New and Noteworthy Venezuelan Glanapterygine Catfishes (Siluriformes, Trichomycteridae), with Discussion of Their Biogeography and Psammophily | journal=American Museum Novitates | year=2005 | issue=3496 | pages=1–27 | doi=10.1206/0003-0082(2005)496[0001:nanvgc]2.0.co;2| hdl=2246/5665 | s2cid=19506818 | url=https://www.biodiversitylibrary.org/item/291729 }}Uiblein, F.; Ott, J.A.; and Stachowitsch, M. (1996). Deep-sea and extreme shallow-water habitats: Affinities and Adaptations. Biosystematics and Ecology-Series, Band 11. {{ISBN|978-3-7001-2574-7}}.Lucanus, Oliver (2013). First Notes on the Husbandry of the Blind Cichlid Lamprologus lethops from the Congo River. Cichlid News vol. 22(1): 6–11.

File:Cryptotora thamicola.jpg has several adaptions that allow it to climb and "walk" in a tetrapod-like fashion{{cite journal | author=Flammang, B.E. | author2=A. Suvarnaraksha | author3=J. Markiewicz | author4=D. Soares | name-list-style=amp | title=Tetrapod-like pelvic girdle in a walking cavefish | year=2016 | journal=Scientific Reports | volume=6 | pages=23711 | doi=10.1038/srep23711 | pmid=27010864 | pmc=4806330 | bibcode=2016NatSR...623711F }}]]

Cavefish are quite small with most species being between {{cvt|2|and(-)|13|cm|in|1}} in standard length and about a dozen species reaching {{cvt|20-23|cm|in|0}}. Only three species grow larger; two slender Ophisternon swamp eels at up to {{cvt|32-36|cm|in}} in standard length and a much more robust undescribed species of mahseer at {{cvt|43|cm|in}}.{{cite journal | author1=Harries, D. | author2=T. Arbenz | author3=N. Dahanukar | author4=R. Raghavan | author5=M. Tringham | author6=D. Rangad | author7=G. Proudlove | year=2019 | title=The world's largest known subterranean fish: a discovery in Meghalaya (NE India) of a cave-adapted fish related to the Golden Mahseer, Tor putitora (Hamilton 1822) | journal=Cave and Karst Science | volume=46 | issue=3 | pages=121–126 }}{{cite news| author=Main, D. | date=12 February 2020 | title=World's largest cave fish discovered in India | url=https://www.nationalgeographic.com/animals/2020/02/worlds-largest-cave-fish-found-in-india/ | archive-url=https://web.archive.org/web/20200213170406/https://www.nationalgeographic.com/animals/2020/02/worlds-largest-cave-fish-found-in-india/ | url-status=dead | archive-date=13 February 2020 | publisher=National Geographic | access-date=27 June 2020 }} The very limited food resources in the habitat likely prevents larger cavefish species from existing and also means that cavefish in general are opportunistic feeders, taking whatever is available. In their habitat, cavefish are often the top predators, feeding on smaller cave-living invertebrates, or are detritivores without enemies. Cavefish typically have low metabolic rates and may be able to survive long periods of starvation. A captive Phreatobius cisternarum did not feed for a year, but remained in good condition.{{cite journal|url=http://www.scielo.br/pdf/paz/v45n26/a01v4526.pdf|title=New data on Cistern Catfish, Phreatobius cisternarum, from subterranean waters at the mouth of the Amazon River (Siluriformes, Incertae Sedis)|last=Muriel-Cunha|first=Janice|author2=de Pinna, Mário|journal=Papéis Avulsos de Zoologia|pages=327–339|volume=35|year=2005}} The cave form of the Mexican tetra can build up unusually large fat reserves by "binge eating" in periods where food is available, which then (together with its low metabolic rate) allows it to survive without food for months, much longer than the aboveground form of the species.{{cite web | author=Dutchen, S. | url=https://www.sciencedaily.com/releases/2015/07/150713161426.htm | title=Fat fish illuminate human obesity | date=13 July 2015 | publisher=ScienceDaily, Harvard Medical School | access-date=26 April 2017}}

In the dark habitat, certain types of displays are reduced in cavefish, but in other cases they have become stronger, shifting from displays that are aimed at being seen to displays aimed at being felt via water movement. For example, during the courtship of the cave form of the Mexican tetra the pair produce turbulence through exaggerated gill and mouth movements, allowing them to detect each other. In general, cavefish are slow growers and slow breeders. Breeding behaviors among cavefish vary extensively, and there are both species that are egg-layers and ovoviviparous species that give birth to live young. Uniquely among fish, the genus Amblyopsis brood their eggs in the gill chambers (somewhat like mouthbrooders).{{cite journal | author=Armbruster, J.W. | author2=M.L. Niemiller | author3=P.B. Hart | name-list-style=amp | title=Morphological Evolution of the Cave-, Spring-, and Swampfishes of the Amblyopsidae (Percopsiformes) | journal=Copeia | year=2016 | volume=104 | issue=3 | pages=763–777 | doi=10.1643/ci-15-339| s2cid=53608365 }}

File:Dama ciega blanca.jpg and other cave-dwelling brotulas are among the few species that live in anchialine habitats]]

Although many cavefish species are restricted to underground lakes, pools or rivers in actual caves, some are found in aquifers and may only be detected by humans when artificial wells are dug into this layer.Moreira, C.R.; Bichuette, M.E.; Oyakawa, O.T; de Pinna, M.C.C.; and Trajano, E. (2010). Rediscovery and redescription of the unusual subterranean characiform Stygichthys typhlops, with notes on its life history. Journal of Fish Biology (London: Wiley InterScience) 76 (7): 1815–1824. Most live in areas with low (essentially static) or moderate water current, but there are also species in places with very strong current, such as the waterfall climbing cavefish.{{Cite iucn | author = Vidthayanon, C. | title = Cryptotora thamicola | volume = 2011 | page = e.T41407A10459372 | date = 2011 | doi = 10.2305/IUCN.UK.2011-1.RLTS.T41407A10459372.en | access-date = 23 December 2017}} Underground waters are often very stable environments with limited variations in temperature (typically near the annual average of the surrounding region), nutrient levels and other factors.{{cite journal | author=Poulson, T.L. | author2=W.B. White | name-list-style=amp | year=1969 | title=The cave environment | journal=Science | volume=165 | issue=3897 | pages=971–981 | doi=10.1126/science.165.3897.971| pmid=17791021 | bibcode=1969Sci...165..971P }} Organic compounds generally only occur in low levels and rely on outside sources, such as contained in water that enters the underground habitat from outside, aboveground animals that find their way into caves (deliberately or by mistake) and guano from bats that roost in caves.{{cite web | author=McDowell, I. | date=10 November 2016 | title=Alabama Cavefish | url=http://www.encyclopediaofalabama.org/article/h-3833 | publisher=Encyclopedia of Alabama | access-date=16 May 2017}} Cavefish are primarily restricted to freshwater. A few species, notably the cave-dwelling viviparous brotulas, Luciogobius gobies, Milyeringa sleeper gobies and the blind cave eel, live in anchialine caves and several of these tolerate various salinities.{{cite journal | author=Nielsen | author2=Schwarzhans | author3=Hadiaty | name-list-style=amp | year=2009 | title=A blind, new species of Diancistrus (Teleostei, Bythitidae) from three caves on Muna Island, southeast of Sulawesi, Indonesia | journal=Cybium | volume=33 | issue=3 | pages=241–245}}{{cite journal | author=Møller | author2=Schwarzhans | author3=Iliffe | author4=Nielsen | name-list-style=amp | year=2006 | title=Revision of the Bahamian cave-fishes of the genus Lucifuga (Ophidiiformes, Bythitidae), with description of a new species from islands on the Little Bahama Bank | journal=Zootaxa | volume=33 | issue=1223 | pages=23–46| doi=10.11646/zootaxa.1223.1.3 }}{{FishBase |genus= Milyeringa|species= veritas| month = April | year = 2017}}{{cite web | url=http://www.environment.gov.au/cgi-bin/sprat/public/publicspecies.pl?taxon_id=66678 | title=Ophisternon candidum — Blind Cave Eel | publisher=Department of the Environment (Australia) | access-date=28 April 2017}}

The more than 200 scientifically described obligate cavefish species are found in most continents, but there are strong geographic patterns and the species richness varies. The vast majority of species are found in the tropics or subtropics.Ma, L.; and Y.-H. Zhao (2012). Cavefish of China. Pp. 107–125 in: White, W.B.; and D.C. Cuvier, editors. Encyclopedia of Caves. Elsevier. {{ISBN|9780123838322}} Cavefish are strongly linked to regions with karst, which commonly result in underground sinkholes and subterranean rivers.

With more than 120 described species, by far the greatest diversity is in Asia, followed by more than 30 species in South America and about 30 species in North America.Riesch, R.; Tobler, M.; and Plath, M. (2015). Extremophile Fishes: Ecology, Evolution, and Physiology of Teleosts in Extreme Environments. {{ISBN|978-3319133614}} In contrast, only 9 species are known from Africa, 5 from Oceania, and 1 from Europe.{{cite web |url=https://www.newscientist.com/article/2126523-first-ever-cavefish-discovered-in-europe-evolved-super-fast/ |title=First ever cavefish discovered in Europe evolved super-fast |author=Andy Coghlan |date=3 April 2017|publisher=New Scientist |access-date= 4 April 2017}} On a country level, China has the greatest diversity with more than 80 species, followed by Brazil with more than 20 species. India, Mexico, Thailand and the United States of America each have 9–14 species.Proudlove, G.S. (2010). Biodiversity and distribution of the subterranean fishes of the world. Pp. 41–63 in: Trajano, E.; Bichuette, M.E.; Kapoor, B.G., eds. The Biology of Subterranean Fishes. Science. {{ISBN|978-1578086702}} No other country has more than 5 cavefish species.{{cite journal | author1=Lina M. Mesa S. | author2=Carlos A. Lasso | author3=Luz E. Ochoa | author4=Carlos DoNascimiento | year=2018 | title=Trichomycterus rosablanca (Siluriformes, Trichomycteridae) a new species of hipogean catfish from the Colombian Andes | journal=Biota Colombiana | volume=19 | issue=1 | pages=95–116 | doi=10.21068/c2018.v19s1a09 | doi-access=free | hdl=20.500.11761/35246 | hdl-access=free }}{{cite journal | author1=Nguyen Dinh Tao | author2=Liang Cao | author3=Shuqing Deng | author4=E. Zhang | year=2018 | title=Speolabeo hokhanhi, A New Cavefish from Central Vietnam (Teleostei: Cyprinidae) | journal=Zootaxa | volume=4476 | issue=1 | pages=109–117 | doi=10.11646/zootaxa.4476.1.10 | pmid=30313345 | doi-access=free }}

File:Amblyopsis hoosieri 29330.jpg from Indiana in the United States was only described in 2014{{cite journal |last1=Chakrabarty |first1=Prosanta |last2=Prejean |first2=Jacques A. |last3=Niemiller |first3=Matthew L. |date=May 29, 2014 |title=The Hoosier cavefish, a new and endangered species (Amblyopsidae, Amblyopsis) from the caves of southern Indiana |url=https://zookeys.pensoft.net/articles.php?id=3824 |journal=ZooKeys |issue=412 |pages=41–57 |doi=10.3897/zookeys.412.7245 |pmid=24899861 |pmc=4042695 |doi-access=free |bibcode=2014ZooK..412...41C }}]]

Being underground, many places where cavefish may live have not been thoroughly surveyed. New cavefish species are described with some regularity and undescribed species are known. As a consequence, the number of known cavefish species has risen rapidly in recent decades. In the early 1990s only about 50 species were known, in 2010 about 170 species were known,{{cite journal | author = Walsh S.J., Chakrabarty P. | year = 2016 | title = A new genus and species of blind sleeper (Teleostei: Eleotridae) from Oaxaca, Mexico: First obligate cave gobiiform in the Western Hemisphere | journal = Copeia | volume = 104 | issue = 2| pages = 506–517 | doi=10.1643/ci-15-275| s2cid = 89252631 | url = https://digitalcommons.lsu.edu/biosci_pubs/778 | url-access = subscription }} and by 2015 this had surpassed 200 species. It has been estimated that the final number might be around 250 obligate cavefish species.{{cite book | author=Proudlove, G. | title=Subterranean fishes of the world | year=2006 | publisher=International Society for Subterranean Biology | isbn=978-2-9527084-0-1 }} For example, the first cavefish in Europe, a Barbatula stone loach, was only discovered in 2015 in Southern Germany, and the largest known cavefish, Neolissochilus pnar (originally thought to be a form of the golden mahseer), was only definitely confirmed in 2019, despite being quite numerous in the cave where it occurs in Meghalaya, India.{{Cite journal |last1=Dahanukar |first1=Neelesh |last2=Sundar |first2=Remya L. |last3=Rangad |first3=Duwaki |last4=Proudlove |first4=Graham |last5=Raghavan |first5=Rajeev |date=2023-06-02 |title=The world's largest cave fish from Meghalaya, Northeast India, is a new species, Neolissochilus pnar (Cyprinidae, Torinae) |url=https://vertebrate-zoology.arphahub.com/article/101011/ |journal=Vertebrate Zoology |language=en |volume=73 |pages=141–152 |doi=10.3897/vz.73.e101011 |issn=2625-8498|doi-access=free }} Conversely, their unusual appearance means that some cavefish already attracted attention in ancient times. The oldest known description of an obligate cavefish, involving Sinocyclocheilus hyalinus, is almost 500 years old.

Obligate cavefish are known from a wide range of families: Characidae (characids), Balitoridae (hillstream loaches), Cobitidae (true loaches), Cyprinidae (carps and allies), Nemacheilidae (stone loaches), Amblycipitidae (torrent catfishes), Astroblepidae (naked sucker-mouth catfishes), Callichthyidae (armored catfishes), Clariidae (airbreathing catfishes), Heptapteridae (heptapterid catfishes), Ictaluridae (ictalurid catfishes), Kryptoglanidae (kryptoglanid catfish), Loricariidae (loricariid catfishes), Phreatobiidae (phreatobiid catfishes), Trichomycteridae (pencil catfishes), Sternopygidae (glass knifefishes), Amblyopsidae (U.S. cavefishes), Bythitidae (brotulas), Poeciliidae (live-bearers), Synbranchidae (swamp eels), Cottidae (true sculpins), Butidae (butid gobies), Eleotridae (sleeper gobies), Milyeringidae (blind cave gobies), Gobiidae (gobies) and Channidae (snakeheads).{{cite journal | last1 = Britz | first1 = Ralf | last2 = Kakkassery | first2 = Francy | last3 = Raghavan | first3 = Rajeev | year = 2014 | title = Osteology of Kryptoglanis shajii, a stygobitic catfish (Teleostei: Siluriformes) from Peninsular India with a diagnosis of the new family Kryptoglanidae | journal = Ichthyological Exploration of Freshwaters | volume = 24 | issue = 3 | pages = 193–207 }}{{Cite journal | last1=Raghavan | first1=Rajeev | last2=Dahanukar | first2=Neelesh | last3=Anoop | first3=V. K. | last4=Britz | first4=Ralf | year=2019 | title=The subterranean Aenigmachanna gollum, a new genus and species of snakehead (Teleostei: Channidae) from Kerala, South India | journal=Zootaxa | volume=4603 | issue=2 | pages=377–388 | doi=10.11646/zootaxa.4603.2.10 | pmid=31717234 | s2cid=164781147 }}{{Cite journal|last1=Ravi|first1=Charan|last2=Basheer|first2=V. S.|last3=Kumar|first3=Rahul G.|date=2019-07-17|title=Aenigmachanna mahabali, a new species of troglophilic snakehead (Pisces: Channidae) from Kerala, India|journal=Zootaxa|language=en|volume=4638|issue=3|pages=410–418|doi=10.11646/zootaxa.4638.3.6|pmid=31712470|s2cid=203899040 |issn=1175-5334}} Many of these families are only very distantly related and do not form a monophyletic group, showing that adaptations to a life in caves has happened numerous times among fish. As such, their similar adaptions are examples of convergent evolution and the descriptive term "cavefish" is an example of folk taxonomy rather than scientific taxonomy. Strictly speaking some Cyprinodontidae (pupfish) are also known from sinkhole caves, famously including the Devils Hole pupfish, but these lack the adaptations (e.g., reduced eyes and pigmentation) typically associated with cavefish. Additionally, species from a few families such as Chaudhuriidae (earthworm eels), Glanapteryginae and Sarcoglanidinae live buried in the bottom of aboveground waters, and can show adaptions similar to traditional underground-living (troglobitic) fish.{{cite journal | author=Britz, R. | title=Pillaiabrachia siniae, a new species of earthworm eel from northern Myanmar (Teleostei: Synbranchiformes: Chaudhuriidae) | journal=Ichthyol. Explor. Freshwaters | year=2016 | volume=27 | issue=1 | pages=41–47 }}{{cite journal | author=Villa-Verde | author2=Lima | author3=Carvalho | author4=Lima | name-list-style=amp | title=Rediscovery, taxonomic and conservation status of the threatened catfish Listrura camposi (Miranda-Ribeiro) (Siluriformes: Trichomycteridae) | journal=Neotrop. Ichthyol. | year=2013 | volume=11 | issue=1 | doi=10.1590/S1679-62252013000100006 | pages=55–64| doi-access=free }} It has been argued that such species should be recognized as a part of the group of troglobitic fish.

{{As of|2019}}, the following underground-living fish species with various levels of troglomorphism (ranging from complete loss of eyes and pigment, to only a partial reduction of one of these) are known.{{cite journal | author=Romero | author2=Zhao | author3=Chen | name-list-style=amp | title=The Hypogean fishes of China | journal=Environ Biol Fish | year=2009 | volume=86 | issue=1 | pages=211–278 | doi=10.1007/s10641-009-9441-3 | bibcode=2009EnvBF..86..211R | s2cid=41778476 }} Phreatobius sanguijuela and Prietella phreatophila, the only species with underground populations in more than one country,{{cite web | author= University of Texas at Austin | title=Rare, blind catfish never before found in US discovered in national park cave in Texas | url=https://www.sciencedaily.com/releases/2016/06/160617140602.htm | publisher=ScienceDaily | date=17 June 2016 | access-date=13 May 2017 }}{{cite journal | author1=Ohara, W.M. | author2=I.D. Da Costa | author3=M.L. Fonseca | year=2016 | title=Behavioiur, feeding habits and ecology of the blind catfish Phreatobius sanguijuela (Ostariophysi: Siluriformes) | journal=Journal of Fish Biology | volume=89 | issue=2 | pages=1285–1301 | doi=10.1111/jfb.13037 | pmid=27329067 | bibcode=2016JFBio..89.1285O }} are listed twice. Excluded from the table are species that live buried in the bottom of aboveground waters (even if they have troglomorphic-like features) and undescribed species.

Image:Astyanax mexicanus 01.jpg is easily bred in captivity and the only cavefish widely available to aquarists]]

Although cavefish as a group are found throughout large parts of the world, many cavefish species have tiny ranges (often restricted to a single cave or cave system) and are seriously threatened. In 1996, more than 50 species were recognized as threatened by the IUCN and many, including several that are rare, have not been assessed at all. For example, the critically endangered Alabama cavefish is only found in the Key Cave and the entire population has been estimated at less than 100 individuals,{{Cite iucn | author = NatureServe | title = Speoplatyrhinus poulsoni | volume = 2013 | page = e.T20467A19033986 | date = 2013 | doi = 10.2305/IUCN.UK.2013-1.RLTS.T20467A19033986.en | access-date = 23 December 2017}} while the critically endangered golden cave catfish only is found in the Aigamas cave in Namibia and has an estimated population of less than 400 individuals.{{Cite journal | author = Bruton, M.N. | title = Threatened fishes of the world: Clarias cavernicola Trewavas, 1936 (Clariidae) | year = 1995 | journal = Environmental Biology of Fishes | volume = 43 | issue = 2 | page = 162 | doi = 10.1007/BF00002486 | bibcode = 1995EnvBF..43..162B | s2cid = 44350023 }} The Haditha cavefish from Iraq and the Oaxaca cave sleeper from Mexico may already be extinct, as recent surveys have failed to find them.{{Cite iucn | author = Freyhof, J. | title = Caecocypris basimi | volume = 2014 | page = e.T3450A19006223 | date = 2014 | doi = 10.2305/IUCN.UK.2014-1.RLTS.T3450A19006223.en | access-date = 23 December 2017}}{{cite web| author=Montanari, M. | title=This Rare Eyeless Cavefish Was Discovered Deep Underground In Mexico | url=https://www.forbes.com/sites/shaenamontanari/2016/06/30/this-rare-eyeless-cavefish-was-discovered-deep-underground-in-mexico/#6c08a48a4996 | work=Forbes | date=30 June 2016 |access-date=30 April 2017 }} In some other cases, such as the Brazilian blind characid which went unrecorded by ichthyologists from 1962 to 2004, the apparent "rarity" was likely because of a lack of surveys in its range and habitat, as locals considered it relatively common until the early 1990s (more recently, this species appears to truly have declined significantly). Living in very stable environments, cavefish are likely more vulnerable to changes in the water (for example, temperature or oxygen) than fish of aboveground habitats which naturally experience greater variations. The main threats to cavefish are typically changes in the water level (mainly through water extraction or drought), habitat degradation and pollution, but in some cases introduced species and collection for the aquarium trade also present a threat. Cavefish often show little fear of humans and can sometimes be caught with the bare hands. Most cavefish lack natural predators, although larger cavefish may feed on smaller individuals, and cave-living crayfish, crabs, giant water bugs and spiders have been recorded feeding on a few species of cavefish.{{cite web | title=Alabama Cavefish | publisher=U.S. Fish and Wildlife Service | url=http://www.fws.gov/ecos/ajax/docs/life_histories/E011.html | access-date=12 October 2011 | url-status=dead | archive-url=https://web.archive.org/web/20141007234105/http://www.fws.gov/ecos/ajax/docs/life_histories/E011.html | archive-date=7 October 2014 }}{{cite journal | author=Klaus, S. | author2=M. Plath | name-list-style=amp | year=2011 | title=Predation on cave fish by freshwater crab Avotrichodactylus bidens (Bott, 1969) (Brachyura, Trichodactylidae) in Mexican sulfur cave | journal=Crustaceana | volume=84 | issue=4 | pages=411–418 | doi=10.1163/001121611X560853}}{{cite journal | author=Tobler, M. | year=2009 | title=Does a predatory insect contribute to the divergence between cave- and surface-adapted fish populations? | journal=Biol. Lett. | volume=5 | issue=4 | pages=506–509 | doi=10.1098/rsbl.2009.0272| pmid=19443506 | pmc=2781934 }}{{cite journal | author=Horstkotte | author2=Riesch | author3=Plath | author4=Jäger | name-list-style=amp | year=2010 | title=Predation by three species of spiders on a cave fish in a Mexican sulphur cave | journal=Bull. Br. Arachnol. Soc. | volume=15 | issue=2 | pages=55–58 | doi=10.13156/arac.2010.15.2.55| s2cid=41990323 }}

Caves in some parts of the world have been protected, which can safeguard the cavefish. In a few cases such as the Omani blind cavefish (Oman garra), zoos have initiated breeding programs as a safeguard. In contrast to the rarer species, the cave form of the Mexican tetra is easily bred in captivity and widely available to aquarists.SeriouslyFish: [http://www.seriouslyfish.com/species/astyanax-mexicanus/ Astyanax mexicanus.] Retrieved 28 February 2016. This is the most studied cavefish species and likely also the most studied cave organism overall.{{cite book | author=Romero, A. | title=Cave Biology: Life in Darkness | year=2009 | publisher=Cambridge University Press |pages=147–148 | isbn=978-0-521-82846-8}} As of 2006, only six other cavefish species have been bred in captivity, typically by scientists.

• Cave salamander

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