Cavefish

In some species, notably the Mexican tetra, shortfin molly, Oman garra, Indoreonectes evezardi, and a few catfish, both "normal" aboveground and cavefish forms exist.

[1] Living in darkness, pigmentation and eyes are useless, or an actual disadvantage because of their energy requirements, and therefore typically reduced in cavefish.

[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 the pineal gland (pineal eye),[20] 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.

[28][29] 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.

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.

[40] 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.

[42] 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.

[49] Cavefish are strongly linked to regions with karst, which commonly result in underground sinkholes and subterranean rivers.

[1][7][58][59][60] 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.

[1] 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.

[3] As of 2019[update], 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.

[96] The Haditha cavefish from Iraq and the Oaxaca cave sleeper from Mexico may already be extinct, as recent surveys have failed to find them.

[97][98] 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).

[41] 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.

[12] In contrast to the rarer species, the cave form of the Mexican tetra is easily bred in captivity and widely available to aquarists.