Blind cave tetra has no day/night cycle

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The Blind cave tetra (Astyanax mexicanus) ditches circadian rhythm to save energy

The eyeless, cave-dwelling form of the Mexican tetra fish (Astyanax mexicanus) has surrendered its circadian rhythm for the sake of saving energy in its pitch-black habitat.

The absence of a day/night cycle in the cave-dweller's metabolism has resulted in a 27 per cent saving in energy use, the scientists report today in PLoS ONE.

Most animals have a clear day/night circadian rhythm according to their metabolism says lead author and fish biologist Dr Damian Moran, now a senior scientist at Plant and Food Research New Zealand.

"The reason why the metabolism is ramped up for day-active animals is that they are preparing for foraging, digestion and are anticipating all these physiological processes that they need to be ready for," he says.

However the cave-dwelling, eyeless form of the tetra appears to have eliminated that cycle, and as a result, uses significantly less energy over a 24-hour period compared to its surface-dwelling counterpart.

The fish's activity level was changed by varying the speed of the water movement, allowing the scientists to examine how energy use changed with activity. They also looked at how energy use changed over a 24-hour period.

The Blind cave tetra is a popular species among evolutionary biologists, says Moran, because the surface-dwelling and cave-dwelling forms are physically very different, but still similar enough that they can interbreed.

"Somewhere between 100,000 to one million years ago, you got surface fish somehow getting into these caves by accident or moving into them, and turning into these cave forms," says Moran.

"It's all about saving energy in these food-limited environments."