In the dark, cave fish follows its own rhythm

An odd biological clock may help elucidate how daily cycles are set

A fish that swims in limestone caverns under the Somalian desert has something to tell scientists about keeping time. Despite living in permanent darkness, with no difference between day and night, this blind cave-dweller still has its own quirky sense of rhythm.

Fish that have lived in dark caves under the Somalian desert for millions of years have lost their eyes, scales and coloring, and may be losing clocks that set daily body rhythms to light and dark cycles. Saulo Bambi

The Somalian cave fish, Phreatichthys andruzzii, has an inner timekeeper that ticks out a roughly 47-hour cycle set by food rather than sunlight, scientists from Italy, Germany and Spain report online September 6 in PLoS Biology. This odd biological clock may teach scientists more about the molecular pathways that govern such clocks, why clocks are important to organisms and how living things adapt when their clocks are no longer tied to cycles set by the rising and setting of the sun.

Most animals, plants and some kinds of bacteria follow the sun’s cue in setting their own daily clocks. These biological, or circadian, clocks help govern sleeping, waking and feeding times, the rise and fall of blood pressure and other daily rhythms. Generally, circadian clocks follow an approximately 24-hour cycle and are reset largely by sunlight. When people’s circadian clocks aren’t set correctly, jet lag and even long-term health problems can result. Researchers study fish and other organisms to learn how circadian clocks’ gears mesh.

Somalian cave fish have been cut off from the sun for up to 2.6 million years. Adapting to life in the dark has not only caused the fish’s eyes (as well as its scales and skin coloring) to disappear, but also altered its clock, say study authors Nicholas S. Foulkes of the Karlsruhe Institute of Technology in Germany, Cristiano Bertolucci of the University of Ferrara in Italy and their colleagues.

In laboratory tests, the cave fish did not respond to the 12-hour light-and-dark cycles that set circadian clocks in closely related zebra fish. But after scientists replaced two faulty light-sensing proteins in cave fish cells with ones from zebra fish, the cave fish’s clock responded to the light-dark cycles, indicating that core components of the fish’s biological clock still work.

“We’re watching a clock in the process of being broken,” says Foulkes. In another million years or so the cave fish may lose other important clock parts, he says.

But other researchers see a different scenario. “I think one of the remarkable things is how intact the clock still is,” says Jennifer Liang, a developmental biologist at the University of Minnesota Duluth who studies circadian clocks. Instead of breaking, the clock may be adapting to other environmental cues, she says.

Indeed, the cave fish and zebrafish clocks could both be set by feeding times, although the cave fish packs about 47 hours into its biological day while the zebra fish follows a 24-hour cycle, the researchers show. The finding confirms that the cave fish clock is still ticking even though it can no longer be set by light. In contrast, zebra fish clocks respond to both light and food.

And although it is always the same temperature in the cave fish’s subterranean home, raising the temperature in the lab speeded up the cave fish clock and lower temperatures slowed it. That’s also unusual, as most organisms’ circadian clocks are insulated from temperature changes.

Over time, the cave fish may have gradually jettisoned the unneeded insulating capacity along with the light sensors, says Robert Lucas, a neuroscientist and circadian clock researcher at the University of Manchester in England. “The clock state described may be the end state of evolution for the cave fish.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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