Cavefish or cave fish is a generic term for fresh and brackish water fish adapted to life in caves and other underground habitats. Related terms are subterranean fish, troglomorphic fish, troglobitic fish, stygobitic fish, phreatic fish, and hypogean fish.Romero, Aldemaro, editor (2001). The Biology of Hypogean Fishes. Developments in Environmental Biology of Fishes. Helfman, G.S. (2007). Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources, pp. 41–42. Island Press.
There are more than 200 scientifically described species of obligate cavefish found on all continents, except Antarctica. Although widespread as a group, many species have very small ranges and are threatened.Fenolio, D.B.; Zhao, Y.; Niemiller, M.L.; and Stout, J. (2013). In-situ observations of seven enigmatic cave loaches and one cave barbel from Guangxi, China, with notes on conservation status. Speleobiology Notes 5: 19–33.Proudlove, G.S. (2001). The conservation of hypogean fishes. Environmental Biology of Fishes 62: 201–213.
Cavefish are members of a wide range of families and do not form a monophyletic group. Typical adaptations include reduced eyes and depigmentation. __TOC__
Adaptations
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.
Many adaptations 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. Other examples of adaptations are larger fins for more energy-efficient swimming, and a loss of scales and swim bladder. 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. 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). 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. 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). Blind Cavefish Froze Its Internal Clock to Save Energy. LiveScience. Retrieved 28 February 2016.Battison, L. (10 September 2011). 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. mouth suction to sense nearby obstacles (comparable to echolocation),Poppick, L. (2 April 2014). 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. 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. 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.
Recent genomic studies on Chinese cavefish (Sinocyclocheilus) have highlighted the role of polyploidy in facilitating these adaptations. For example, in the allotetraploid species Sinocyclocheilus microphthalmus, duplicated gene sets allow for different evolutionary trajectories; one set of genes may retain essential functions while the other accumulates mutations that drive adaptation to darkness, such as relaxed selection on visual pathways.
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.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. .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.
Cavefish are quite small, with most species being between in standard length and about a dozen species reaching . Only three species grow larger: two slender Ophisternon swamp eels at up to in standard length and a much more robust undescribed species of mahseer at . 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. 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.
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).
Habitat
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. 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. 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. 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.
Range and diversity 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. 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. In contrast, only 9 species are known from Africa, 5 from Oceania, and 1 from Europe. 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. No other country has more than 5 cavefish species.
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, and by 2015 this had surpassed 200 species. It has been estimated that the final number might be around 250 obligate cavefish species. 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. 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.