Cavefish are losing sleep through evolution

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It seems that adapting to life in a cave will lead to insomnia, at least that's the findings of recent research conducted by biologists at New York University.

Blind cavefish (Astyanax mexicanus), also exist in eyed surface dwelling populations. Being a model specimen for this particular area of research, this popular aquarium tetra was able to add sleeplessness to those other known traits of cave life: blindness, loss of eyes, loss of pigmentation, changes in feeding behaviour and metabolism.

"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."

Borowsky goes on to explain that the fish do sleep, but only for relatively short periods, and that upon waking up they remain active for a relatively long time; a sleep-and-wake cycle that is repeated throughout the continued darkness of the cave.

To conduct the experiment the team used specimens from a surface population, and specimens from three separate populations of cave dwelling populations from Pachón, Tinaja, and Molino, in North Mexico.  

Sleep profiles showed the surface fish to be diurnal, sleeping at night and mostly awake through the day, with a short period of sleep towards the middle of the day. 

In contrast, the cave forms had a drastically reduced sleep phenotype – sleeping an average of 110-250 minutes per 24hr period, compared to an average of over 800 minutes for the surface fish. 

The experiments confirm that the separate populations converged, through evolution, on their shared phenotypes of reduced eyes, pigmentation and reduced sleep.

Researchers also crossed surface and cave specimens and studied the resultant hybrids to further their studies of the behavioural differences.  The studies demonstrated that cavefish differ from surface fish in sleep behaviour as the result of a few dominant gene mutations, that became fixed in the cave populations as they adapted to cave 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, and his team plans to investigate further to parse out the evolutionary forces driving the convergence.

Borowsky notes that it may not be a case that cavefish need less sleep.  Rather, a case of necessity, whereby, they live in an environment scarce of food and simply don’t have the luxury of missing out on the chance of a meal – although this theory is not tested.

For further information see the paper: Erik R. Duboué, Alex C. Keene, Richard L. Borowsky. Evolutionary Convergence on Sleep Loss in Cavefish Populations. Current Biology, 07 April 2011.