Garra andruzzii showing the pale colour and lack of eyes typical of cavefish. The large red spot on the head is the blood-filled gills, visible through the semi-transparentgill cover
There are more than 200scientifically described species of obligate cavefish found on all continents, except Antarctica.[3][4] Although widespread as a group, many species have very small ranges and arethreatened.[5][6]
Many adaptions seen in cavefish are aimed at surviving in a habitat with little food.[1] Living in darkness,pigmentation and eyes are useless, or an actual disadvantage because of their energy requirements, and therefore typically reduced in cavefish.[14][15][16] Other examples of adaptations are larger fins for more energy-efficient swimming, and a loss ofscales andswim bladder.[17][18] 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.[19] 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 thepineal gland (pineal eye),[20] andCongo blind barbs arephotophobic, despite only havingretinas andoptical nerves that are rudimentary and located deep inside the head, and completely lacking alens.[21] In the most extreme cases, the lack of light has changed thecircadian rhythm (24-hour internal body clock) of the cavefish. In the Mexican tetra of the cave form and inGarra andruzzii the circadian rhythm lasts 30 hours and 47 hours, respectively.[22][23] This may help them to save energy.[22] Without sight, other senses are used and these may be enhanced. Examples include thelateral line for sensing vibrations,[24][25][26] mouth suction to sense nearby obstacles (comparable toecholocation),[27] andchemoreception (via smell andtaste buds).[28][29] Although there are cavefish in groups known to haveelectroreception (catfish andSouth American knifefish), there is no published evidence that this is enhanced in the cave-dwellers.[30] 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.[31]
Recent genomic studies on Chinese cavefish (Sinocyclocheilus) have highlighted the role ofpolyploidy in facilitating these adaptations. For example, in the allotetraploid speciesSinocyclocheilus 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.[32]
Some fish species that live buried in the bottom of aboveground waters, livedeep in the sea or live in deep rivers have adaptations similar to cavefish, including reduced eyes and pigmentation.[33][34][35]
Cavefish are quite small with most species being between 2 and 13 cm (0.8–5.1 in) instandard length and about a dozen species reaching 20–23 cm (8–9 in). Only three species grow larger; two slenderOphisternon swamp eels at up to 32–36 cm (13–14 in) in standard length and a much more robustundescribed species ofmahseer at 43 cm (17 in).[37][38] 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.[15][31] In their habitat, cavefish are often thetop predators, feeding on smaller cave-living invertebrates, or aredetritivores without enemies.[18] Cavefish typically have lowmetabolic rates and may be able to survive long periods of starvation. A captivePhreatobius cisternarum did not feed for a year, but remained in good condition.[39] 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.[40]
In the dark habitat, certain types ofdisplays are reduced in cavefish,[17] 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 thecourtship of the cave form of the Mexican tetra the pair produce turbulence through exaggeratedgill and mouth movements, allowing them to detect each other.[16] In general, cavefish are slow growers and slow breeders.[2] Breeding behaviors among cavefish vary extensively, and there are both species that areegg-layers andovoviviparous species that give birth to live young.[16] Uniquely among fish, the genusAmblyopsis brood their eggs in the gill chambers (somewhat likemouthbrooders).[41]
Although many cavefish species are restricted tounderground lakes, pools orrivers in actual caves, some are found inaquifers and may only be detected by humans when artificial wells are dug into this layer.[39][42] Most live in areas with low (essentially static) or moderate water current,[1][31] but there are also species in places with very strong current, such as thewaterfall climbing cavefish.[43] 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.[1][44]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) andguano frombats that roost in caves.[1][44][45] Cavefish are primarily restricted to freshwater.[1] A few species, notably the cave-dwellingviviparous brotulas,Luciogobius gobies,Milyeringa sleeper gobies and the blind cave eel, live inanchialine caves and several of these tolerate varioussalinities.[1][46][47][48][49]
The more than 200scientifically described obligate cavefish species are found in most continents, but there are strong geographic patterns and thespecies richness varies.[3] The vast majority of species are found in the tropics orsubtropics.[50] Cavefish are strongly linked to regions withkarst, which commonly result in undergroundsinkholes and subterranean rivers.[1][7]
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.[3][7] In contrast, only 9 species are known from Africa, 5 from Oceania,[7] and 1 from Europe.[4][51] 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.[1][3][52] No other country has more than 5 cavefish species.[7][53][54]
Being underground, many places where cavefish may live have not been thoroughlysurveyed. New cavefish species are described with some regularity andundescribed species are known.[5][7] 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,[56] and by 2015 this had surpassed 200 species.[3] It has been estimated that the final number might be around 250 obligate cavefish species.[57] For example, the first cavefish in Europe, aBarbatula stone loach, was only discovered in 2015 inSouthern Germany,[4][51] and the largest known cavefish,Neolissochilus pnar (originally thought to be a form of thegolden mahseer), was only definitely confirmed in 2019, despite being quite numerous in the cave where it occurs inMeghalaya, India.[37][38][58] Conversely, their unusual appearance means that some cavefish already attracted attention in ancient times. The oldest known description of an obligate cavefish, involvingSinocyclocheilus hyalinus, is almost 500 years old.[50]
As of 2019[update], the following underground-living fish species with various levels oftroglomorphism (ranging from complete loss of eyes and pigment, to only a partial reduction of one of these) are known.[1][3][52][64]Phreatobius sanguijuela andPrietella phreatophila, the only species with underground populations in more than one country,[65][66] 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) andundescribed species.
Species includes both aboveground and belowground forms (aboveground also in Central America). Sometimes considered a part ofAstyanax mexicanus[67][68][69]
Species includes both aboveground and belowground forms (aboveground also in United States). Cave form sometimes considered a separate species,A. jordani[69]
Formerly placed inBarbus orPuntius instead. Aboveground populations have also been assigned to this species,[71] but its taxonomy is unresolved and a review has suggested that at least some of the underground populations might belong toPuntius binotatus or anundescribed species instead.[72][73]
Species includes both aboveground and belowground forms (aboveground also in the United Arab Emirates). A population in the United Arab Emirates has been reported to be underground,[52] but this is incorrect[3]
First described as a species of cavefish based on a single specimen, but a later review suggested that it was found in an area without underground waters and only is analbinistic individual of the abovegroundCobitis fahireae[79]
Aboveground populations widespread in Europe. Belowground population only discovered in 2015 and tentatively included in this species based ongenetic evidence. Only known cavefish in Europe[4]
Traditionally in genusSchistura orTriplophysa.[80][81] Species includes both aboveground and belowground populations; the latter sometimes recognized as a separatesubspeciesmicrophthalmus.[64]
Originally described asMonopterus indicus by K. C. Eapen, but as this name was already taken by theBombay swamp eel, it was redescribed asMonopterus eapeni in 1991. When the species was moved to the genusRakthamichthys, theindicusspecific epithet was revived.
Has mistakenly been reported to occur in Papua New Guinea,[3] but it is fromWestern New Guinea, the Indonesian part of the island.[93] The family Butidae was formerly considered a subfamily of Eleotridae[3]
One of two species in a unique fish family closely related to truesnakeheads. Displays relatively fewtroglomorphisms despite living in underground aquifers, and thus could either be a recent arrival to the subterranean ecosystem or possibly asubtroglophile that periodically moves between the underground and surface.[60]
One of two species in a unique fish family closely related to truesnakeheads. Displays relatively fewtroglomorphisms despite living in underground aquifers, and thus could either be a recent arrival to the subterranean ecosystem or possibly asubtroglophile that periodically moves between the underground and surface.[61]
The cave form of theMexican tetra 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 seriouslythreatened. In 1996, more than 50 species were recognized as threatened by theIUCN and many, including several that are rare, have not been assessed at all.[2] For example, thecritically endangeredAlabama cavefish is only found in theKey Cave and the entire population has been estimated at less than 100 individuals,[96] while the critically endangeredgolden cave catfish only is found in the Aigamas cave in Namibia and has an estimated population of less than 400 individuals.[97] TheHaditha cavefish from Iraq and theOaxaca cave sleeper from Mexico may already beextinct, as recent surveys have failed to find them.[98][99] In some other cases, such as theBrazilian blind characid which went unrecorded byichthyologists 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).[42] Living in very stable environments, cavefish are likely more vulnerable to changes in the water (for example, temperature oroxygen) than fish of aboveground habitats which naturally experience greater variations.[44] The main threats to cavefish are typically changes in the water level (mainly throughwater extraction ordrought),habitat degradation and pollution, but in some casesintroduced species and collection for theaquarium trade also present a threat.[5][6] Cavefish often show little fear of humans and can sometimes be caught with the bare hands.[18] Most cavefish lack natural predators, although larger cavefish may feed on smaller individuals,[18] and cave-livingcrayfish,crabs,giant water bugs andspiders have been recorded feeding on a few species of cavefish.[100][101][102][103]
Caves in some parts of the world have been protected, which can safeguard the cavefish.[55] In a few cases such as theOmani blind cavefish (Oman garra), zoos have initiatedbreeding programs as a safeguard.[12] In contrast to the rarer species, the cave form of the Mexican tetra is easily bred in captivity and widely available toaquarists.[69][104] This is the most studied cavefish species and likely also the most studied cave organism overall.[105] As of 2006, only six other cavefish species have been bred in captivity, typically by scientists.[57]
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