A cavefish is a variety of fish that has adapted to living in near to complete darkness. Owing to this environment, the fish lacks color and is often blind. Some varieties have no eyes at all. About 32 species of cavefish exist around the world.
Cavefish are most commonly found in caves, but can live in any dark aquatic environment. None have been found in caves without inlets. If there is no water running into the cave, no fish are able to reach the lake inside. The fish can live outside of caves as well. Some live in above-ground swamps, earning them the nickname swampfish.
Owing to two distinctive mutations, cavefish are generally easy to identify. First, because they live in a dark environment where vision is restricted or impossible, most adult cavefish do not have eyes. Some have tiny bumps or marks where the eyes once were; however, most adult cavefish are blind.
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Many varieties do have eyes when they are first hatched. As the fish matures, either the eyes atrophy or a layer of skin grows to cover them. Some studies have shown that when different varieties of cavefish are mated, they can produce sighted offspring. Scientists believe this works because the genetic mutation that causes blindness in the fish is different in each variety. When the two mutations mix, they tend to cancel each other out.
Without eyes, the fish have developed another way to sense the world around them. A series of tiny bumps, much like the taste buds on the human tongue, cover the skin of the fish. These bumps are able to sense temperature, water currents, and other things that help the fish to navigate.
The second distinctive mutation is lack of pigment. This is also owing to the darkness of their environment. Most fish use color as either camouflage or to communicate with other fish. For example, males of a given species may be more colorful than females of the same species. When sight is impossible, these colors are meaningless, so cavefish have no skin pigmentation at all. Some do appear pinkish because it is possible to see blood vessels through the colorless skin.
Some people keep these fish as pets. Aquarium owners like them because they are active and get along well with other fish. They are also easy to care for as they eat almost any food provided for them.