BEIJING, Jan. 29 (Xinhuanet) -- Cavefish whose eyes no longer function still can detect light with the help of a light-sensitive organ in their brains, recent research reveals.

The blind cave-dwelling form of the Mexican tetra (Astyanax mexicanus) evolved from surface-dwelling ancestors whose eyes degenerated after the fish shifted their habitat into complete darkness a million or more years ago. These albino cavefish dwell today in freshwater caves in northeastern Mexico, with skin growing over their now useless eyes.

Researcher Masato Yoshizawa, a neuroethologist at the University of Maryland, said the discovery was pure luck. As he was cleaning out bowls with young cavefish larvae in them, Yoshizawa saw that after a shadow passed slowly over their heads, the fish clearly responded by swimming to the surface.

Investigating what did not seem possible, Yoshizawa and University of Maryland colleague William Jeffery checked the fish's eyes. Although adult cavefish lack functioning eyes, cavefish embryos begin developing eye structures early in their development, which later degenerate.

The researchers searched young cavefish for light-sensitive pigments, but did not see the molecules in the fish's eyes. However, Yoshizawa and Jeffery did find the compounds in the animals' pineal gland, an organ in their brains.

The pineal gland is present in most creatures with a backbone, including humans. The organ helps control the body's day-night cycle — hence its light sensitivity in fish. The pineal gland is also sensitive to light in amphibians and reptiles, but not in mammals.

When the scientists experimentally removed eyes and pineal glands from the young cavefish, they found the fish only retained their shadow response if they had their pineal gland too. In other words, the pineal gland helped them detect light.

This light sensitivity fades away as the cavefish grow older, the researchers found. The light-sensitive molecules seem programmed to shut off, possibly after the eyes are supposed to kick in or when the skull gets too thick for much light to penetrate it.