A Mystery of Evolution: the Blind Cave Fish

by Robert W. Bly, founder, AquariumDetective.com

I go to the fish store every week to buy live food for my stingray and see what's new.

This week, I added a pair of blind cave fish. These small, peaceful, hardy community fish originated in deep caves in Mexico, where the fish live in total darkness. Biologists believe the blind cave fish have been living in these pitch black cave waters for a million years.

Two physical features relating to the absence of light in their native habitat make the blind cave fish a fascinating aquarium novelty. First, they are albinos. Their color is white tinged with pink from the blood vessels beneath the skin.

Second, they are sightless and do not have eyes. That seems logical, because living in total darkness makes eyesight unnecessary.

Albinism is also related to living in the dark: in a world without light, there is no color, so pigmentation is unnecessary.

However, having eyes or skin pigment in the dark seems like it would not be a hindrance. So why did the species evolve in such a way that they lost their eyes and color over time?

One possible reason is that the body requires protein to produce the pigment melanin. Food sources, including those containing protein, may be scarcer in a dark isolated cave than lighted waters.

Through random mutation, a small number of albinos appear in animal populations. The albino fish could use their limited supply of protein solely for growth and reproduction, increasing their population over time.

What about sight? One explanation is that blindness is an economical adaptive mutation. The energy used by the embryo to grow eyes could instead be applied to growing other, more useful organs and features.

Another possibility is that having eyes in total darkness increased fish mortality. The fish, unable to see, bump into rocks and damage the eyes. The eyes become infected, and the infected fish die.

Those fish born without eyes are better suited to the dark. They too cannot see, but the eye sockets are covered, and there is no eye susceptible to easy infection.

A paper published in the Journal of Experimental Biology (1/18/08) reports that blind cave fish, while lacking a pair of eyes, have a structurally intact "pineal" eye. A pineal eye is a rudimentary eye found in some lizards and other species.

The pineal gland is an organ in the brain that helps control the body's day-night cycle. So the pineal eye is not an eye in the traditional sense; it is a concentration of light-sensitive pigments in the fish's pineal gland.

The paper reports that blind cave fish are not, as it turns out, totally blind. Their pineal eye can detect light, and cave fish larvae swim toward light.

However, the blind cave fish do not rely on the limited vision of the pineal eye for guidance. Instead, they have developed rows of gel-covered hairs, called capula, extending from their bodies.

Like the cilia in a human ear, the blind cave fish's capula provide a mechanical sensor for detecting moving fluid. For the human ear, that fluid is air; for the fish, it's the water.

When a cave fish approaches or is approached by an object, the fish senses the motion or presence of the object with its capula, in essence enabling it to detect the moving or stationery object like a radar-sense.

With this underwater sensor array covering its skin, the blind cave fish, though sightless, can avoid predators and obstacles, navigate safely through the aquarium, and locate food.

Though the radar mechanism seems more than sufficient to meet the cave fish's navigational needs, eyesight can be restored in several ways.

In one experiment, lenses from sighted fish were transplanted to the blind cave fish. After the operation, the blind cave fish began to develop eyes.

In another study, blind cave fish from one population were mated with blind cave fish from other caves from which they had been isolated. In some cases, the offspring of these unions could see.

The cave fish is not the only animal born blind, though other blind animals seem to also have evolved that way from living in dark caves.

The "olm" is a blind newt that lives in caves. "Troglobites" are tiny spider-like animals that are blind and have no eyes or pigmentation. They live in total darkness in caves in Australia.