Many species of fish have evolved to living under strange conditions, but few are more intriguing than those that have adapted to living in complete darkness. Some of these fish have developed a tendency to live at great depths in the ocean where no light penetrates, while others have found refuge in equally dim locations such as caves, wells, and subterranean streams. This specialization to living in complete darkness has arisen many times during the course of evolution, and the physical and behavioral attributes these species have developed are not confined to a single taxonomic group of fish. Some 32 species of fish have been observed to exhibit this cave-dwelling behavior. Most of these are small species, measuring some 3 in (7 cm) in length; the Kentucky blind fish (Amblyopsis spelaea), which lives in limestone caves, is one of the largest known cave-dwelling species, with adults reaching a length of almost 8 in (20 cm).
A few of these species have functional eyes but, for the vast majority, vision is of little use. Some of them are completely blind, with the vestiges of eyes still visible beneath a thin layer of skin. In many species, functional eyes are present in the young fry, but these either disappear or are covered up by a layer of skin as they grow and mature. Many cave fish have little or no skin pigment as these species have no need for body coloring, a feature widely used for communication purposes among other fish. Some cave fish do appear to be a pinkish color, but this is the result of the animal's blood vessels showing through the pale skin, rather than any pigmentation.
Apart from the specialized adaptations these fish have developed, what is interesting to scientists is the fact that so many species of different groups have evolved independently to living in perpetual darkness. In Africa, a group of Cyprinid fish (related to the carp) has adapted to living in underground streams and wells, while several species of catfish in Africa, the United States, and parts of South America have developed a similar habit. In Central America, the brotulid fish (Family Brotulidae) of Mexico and Cuba are typically cave-dwellers, lacking functional eyes and body pigment. Similar features have been detected among the Amblyopsidae, a group of five species of small freshwater fish that only occur in the United States. Some of these species actually live in slow-moving streams and swamps in the open air in southern Atlantic coastal plain, yet they are all blind and live amongst the rubble and vegetation in deeper water, well out of direct light.
To compensate for their lack of vision, all of these species have developed other means of locating food and finding a mate, two of the essential features for survival. Many species have developed sensory organs capable of detecting prey either through chemical means or touch. Most are thought to be sensitive to vibrations. Some of these species retain their body scales but, in others, these have been lost during the course of evolution, enabling them to develop additional sensory organs on the skin. In addition to this range of specialized adaptations, one feature that all of these fish have developed to a higher level than other ocean or freshwater species, is an improved lateral line system—a series of grooves or canals that run along each side of the body and extend over the head to the eyes, snout, and jaws. These lines are well equipped with sensory organs known as neuromasts, each of which consists of a group of sensory cells with fine hair-like projections that extend beyond the body wall. They function not only in orientation and balance, but also in helping the fish locate potential food sources.
While many of these adaptations assist with the detection and gathering of food, it is also likely that some nourishment is obtained on a chance basis. Very little is known about the feeding behavior of cave fish, but they are known to eat a wide range of insects, small crustaceans, smaller fish, and probably some detritus. Some species are even thought to feed on fecal droppings from overhead bat roosts.
The reasons why species should evolve to living in total darkness are not immediately obvious, and there is probably no single explanation for this phenomenon. Cave environments are known to provide a relatively stable habitat in terms of temperature fluctuations, but the species living in caves and wells are totally reliant on food being brought to them by underground streams. As such, they are highly vulnerable to external factors as subterranean aquifers are becoming increasingly tapped for irrigation purposes, and many sites may be at risk from drying out either temporarily or permanently. These species are also at risk from water-borne pollutants in agricultural runoff and other waste products entering their underground watercourses. For these reasons, many species of cave fish are endangered. On the other hand, these fish experience a reduced level of feeding competition as so few species have succeeded in colonizing these habitats. They are also relatively safe from predators, which is an obvious advantage provided they can obtain enough food in their immediate habitat.
Scientists now believe that the colonization of caves and wells by these fish was a deliberate, rather than accidental, move. Many species are known to live in and around cave openings and pools near underground springs. It is therefore possible that these dim habitats were gradually explored by a few different species that may have already favored living in poorly lit conditions, such as beneath rocks, or in the murky depths of swamps and lakes. Through a gradual progression and corresponding behavioral and anatomical changes, these fish could have eventually moved further into the cave system, exploiting the untapped food resources of these habitats.