BioEd Online

Blind cavefish (Astyanax mexicanus) can sense light when young, even though their eyes lost their function over a million years of evolution. Scientists have found that the fish larvae can detect an overhead shadow and seek shelter by swimming towards it.

“We were surprised,” says Masato Yoshizawa from the University of Maryland in College Park, who led the research. “This animal has lived in perpetual darkness for about a million years.”

Young cavefish do have some rudimentary eye components, which degenerate even further as the animals age. But they do not have any photosensitive pigments. And when Yoshizawa cut out these components, the fish still responded to shadows.

The researchers found that the behaviour is driven by the pineal gland, a light-sensitive organ sometimes known as the ‘third eye’. This gland contains a light-sensitive pigment called rhodopsin, and when it was removed the fish’s shadow response disappeared. The study is published in the Journal of Experimental Biology¹.

Dark seekers

Yoshikawa discovered the cavefish’s shadow response while cleaning their tanks. “As a pipette was moved above them, we noticed that the larvae reacted by swimming toward the shadow it cast.” He realized that the larvae were just as attracted to the passing shadow as were sighted surface-dwelling individuals from the same species. As in their sighted cousins, this response to seek out shade is strongest in young animals.

The fish’s pineal gland may have been retained because it also supplies the body with melatonin, a key reproductive and seasonal growth hormone. The gland’s twin roles as a hormone factory and light detector are tightly linked. “We suspect that selective pressure to retain the body’s melatonin supply was greater than the passive accumulation of mutations that could have led to loss of the pineal gland’s light-detecting capacity,” says William Jeffery, a co-author on the study.

But the pineal gland’s ability to sense light might yet serve some limited use, in avoiding the odd ray of light that can make its way into a cave. “Caves are not always completely dark,” Jeffery explains. “Sometimes, there are ‘windows’ in the roof where light floods in.” It would be of benefit for larvae to detect and avoid these areas, where they would be more vulnerable to sighted predators.

Kingdom of the blind

Jim Bowmaker from University College London, UK, who studies the evolution of visual pigments, says “There’s a lot of evidence that pineal [glands have a] function in many animals with poorly developed visual systems. But it’s nice to see it demonstrated so neatly.”

Most previous work on pineal glands has been anatomical rather than behavioural. “Many deep-sea fish live without daylight and have reduced eyes, but some of them have large functional pineal glands with large photoreceptors and a lot of photosensitive pigment,” says Bowmaker. This seems to be the first test of the light-detecting abilities of the pineal in blind cave-dwelling animals, he says. ”Some cave amphibians might well show the same function.”

Jeffery speculates that the pineal gland’s light-detecting ability might eventually be lost as the cavefish spend more time in the dark. “This could be investigated by looking for light-detecting function in the pineal glands of evolutionarily older cave animals,” he says.