Blind cavefish got no (circadian) rhythm

Small, silver fish called Mexican tetra (Astyanax mexicanus) live in some Texas and Mexican rivers. Some members of the species — eyeless and blind — can be found in nearby freshwater caves. Sometimes the sighted fish wash into a cave, but they don’t do nearly as well as their blind brethren. Any surface dweller unlucky enough to end up in the dark would have some disadvantages: It would have to adapt to the loss of light and forage for unfamiliar foods, which may be not as abundant as those found in their home waters. But the fish’s biggest disadvantage may be its metabolism.

Blind cavefish have lost their circadian rhythms and have developed more efficient metabolisms than the fish that live in the light, researchers report September 24 in PLOS ONE.

To measure tetras’ metabolism, Damian Moran and colleagues at Lund University in Sweden placed fish in a contraption that let the fish swim in place while the researchers tracked their oxygen consumption, a measure of their metabolism. Surface and cave fish were placed in the tank under constant darkness  or 12-hour light-and-dark cycles for 7 or 8 days. Then the researchers compared how the fish did under the different light regimes.

All the fish took a few days to acclimate to the laboratory conditions. In the light-and-dark conditions, surface fish showed a clear circadian pattern to their oxygen consumption. These fish ramped up their metabolism by about 20 percent during the day. That increase in metabolism would let them have more energy for their hunts and feeding, which take place in the light.

When put into all-dark conditions, the fish still displayed some of that rhythm, even though there was no sunshine to trigger it. And their metabolism increased; the fish consumed about 16 percent more energy over a 24-hour period.

The cave-dwelling fish, in contrast, didn’t display any rhythm in their metabolism in either all-dark or dark-and-light conditions. And that metabolism was far more efficient than that of the fish that normally lived in surface waters. Compared with surface fish in their natural light-and-dark condition, cave fish in their natural darkness expended 27 percent less energy in a day. And compared with a surface fish that washed into the dark, cave fish used 38 percent less energy.

So if there’s some advantage for a blind fish that loses its circadian rhythm, why haven’t they replaced the sighted ones that live in the light? The cave fish are well adapted to their dark homes. They don’t have to spend any energy on eyes or pigmentation — though the amount the surface dwellers use for that isn’t all that much. They’ve got great senses of touch and taste to help them forage on the cave floor for food. And their efficient metabolism may be the most important of their cave-living adaptions, the researchers note. But if one of these fish ends up outside its cave in surface waters, it’s incredibly vulnerable to predators (both fish and birds). And a metabolism advantage doesn’t mean much if you quickly get eaten.