Through evolution, cavefish have lost sleep

Cave life is known to favor the evolution of a variety of traits, including blindness and loss of eyes, loss of pigmentation, and changes in metabolism and feeding behaviour. Now researchers reporting online on 7 April in Current Biology have added sleeplessness to that list.

'Cave-adapted fish sleep less - much less - than closely related surface fish,' said Richard Borowsky of New York University. 'In some ways, their sleep phenotypes are similar to those of humans with sleep disorders.'

The fish do sleep, but only for relatively short periods, Borowsky explained. Once they wake up, they remain active for a relatively long time. That sleep-and-wake cycle is repeated throughout the continued darkness of the cave.

The discovery was made by studying three cave populations of Astyanax mexicanus, a species ideal for such studies because it includes eyed surface and numerous blind cave populations. Although most of those cave populations have been founded independently by individuals living on the surface, they have nonetheless converged over the course of evolution on similar traits suitable to their new environments.

Borowsky and Erik Duboue, lead author of the study, got the idea that the transition from surface to cave might be associated with less sleep based on anecdotal, nighttime observations of fish in their laboratory. Their surface fish clearly sleep at night. They could be found inactive at the bottom of their tanks with fins drooping. The cavefish, on the other hand, actively patrol their laboratory homes through the wee hours.

To further study the basis of those behavioural differences, the researchers allowed surface and cave individuals to mate in the lab and studied their hybrid offspring. Those studies yielded evidence showing that cavefish differ from surface fish in sleep behaviour because of a few dominant gene mutations that became fixed in the cave populations as they adapted to their new life.

'We have documented a cave-related phenotype unsuspected until now that might turn out to be the most basic adaptation of aquatic vertebrates to cave life,' Borowsky said. His team plans to investigate further to parse out the evolutionary forces driving the convergence.

It remains to be tested, but Borowsky doesn't think the cavefish need less sleep, exactly. Rather, they may need to be awake more. 'These fish live in an environment where food is generally scarce and episodically and unpredictably present,' he said. 'If you are asleep when a bit of food floats by, you are out of a meal and out of luck.'

The discovery might offer clues to understanding variation in sleep among other species and even sleep disorders in humans. That's because the genes controlling sleep patterns in the fish are very likely to play the same roles in other animals.

'This study is the first step to identifying the responsible genes,' he said.