Cavefish

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. There are more than 200 scientifically described species of obligate cavefish found on all continents, except Antarctica. Although widespread as a group, many cavefish species have very small ranges and are seriously threatened. Cavefish are members of a wide range of families and do not form a monophyletic group. Typical adaptations found in cavefish are reduced eyes and pigmentation. 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. 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), 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 Phreatichthys andruzzii the circadian rhythm lasts 30 hours and 47 hours, respectively. 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, 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. 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. Cavefish are quite small with most species being less than 10 cm (4 in) long and very few able to surpass 20 cm (8 in). At up to about 40 cm (16 in), the blind cave eel is the longest known cavefish. 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 ovoviparous species that give birth to live young. Uniquely among fish, the genus Amblyopsis brood their eggs in the gill chambers (somewhat like mouthbrooders). 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, 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.