Cavefish are ancient, scaleless, live in total darkness and lack any pigment. Now researchers have discovered that they have another unusual feature: a circadian rhythm, or body clock, of 47 hours, about twice the length of most animals, including humans. 

Normally, an important factor that helps regulate circadian clocks is sunlight. But the researchers, who reported their findings in the journal PLoS Biology, write that the cavefish’s clock is totally blind to light. 

“They have lived for millions of years completely isolated from the day-night cycle,” said the lead researcher, Nicholas Foulkes, a chronobiologist at Karlsruhe Institute of Technology in Germany. 

The researchers collected cavefish living beneath the Somali desert. They exposed the fish, along with zebra fish, to a day-night cycle. Only the zebra fish responded. The photoreceptor genes in the cavefish did not respond to light at all. In other words, Dr. Foulkes said, “their clocks are blind.” 

The clocks are nonetheless still functional. The researchers were able to regulate cavefish circadian clocks by creating a regular feeding schedule. 

“They do have clocks,” Dr. Foulkes said. “It’s just that they are not light-sensing.” 

He added that the research is providing new information about the evolution of circadian clocks and the effect of constant darkness. “We’re really looking at the evolution of this at the gene level,” he said.