Cavefish

'' is an opportunistic feeder on various invertebrates 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. Many adaptions seen in cavefish are aimed at surviving in a habitat with little food. 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), 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. This may help them to save energy. mouth suction to sense nearby obstacles (comparable to echolocation), and chemoreception (via smell and taste buds). 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. has several adaptions that allow it to climb and "walk" in a tetrapod-like fashion 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. 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, ==Habitat==

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. Most live in areas with low (essentially static) or moderate water current, 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. ==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. Cavefish are strongly linked to regions with karst, which commonly result in underground sinkholes and subterranean rivers. In contrast, only 9 species are known from Africa, 5 from Oceania, 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. No other country has more than 5 cavefish species. from Indiana in the United States was only described in 2014 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. and by 2015 this had surpassed 200 species. For example, the first cavefish in Europe, a Barbatula stone loach, was only discovered in 2015 in Southern Germany, 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. 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. It has been argued that such species should be recognized as a part of the group of troglobitic fish. Phreatobius sanguijuela and Prietella phreatophila, the only species with underground populations in more than one country, 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. ==Conservation==

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. 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. The Haditha cavefish from Iraq and the Oaxaca cave sleeper from Mexico may already be extinct, as recent surveys have failed to find them. 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). Caves in some parts of the world have been protected, which can safeguard the cavefish. This is the most studied cavefish species and likely also the most studied cave organism overall. As of 2006, only six other cavefish species have been bred in captivity, typically by scientists. ==See also==