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 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.
"These cave fish are living in an environment without light, without the circadian presence of food or predators, they've got nothing to get ready for, so it looks like they've just chopped away this increase in anticipation for the day," Moran says.
Instead, the cave-dwellers exist in a more steady metabolic state that is somewhere between asleep and fully active.
"They live a demand-led life rather than an anticipation-led life."
The researchers stumbled across the discovery while investigating the energy savings associated with the absence of eyes in the fish.
To compare the energy expenditure of surface-dwelling tetra with its eyeless cave-dwelling form, Moran and colleagues placed the fish in a device known as a flume respirometer: a closed loop of water in which the fish swim and which can be regularly sampled to measure oxygen levels.
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 Mexican 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.
"What's particularly cool about these fish is this happened not just once, but many times, so you've got different populations of these cave forms which have independently evolved so it's a really great tool for studying evolution."
While the cave-dwelling tetra may have lost its eyes-and instead has two tiny, functionless balls- it has retained the pineal gland, a gland in the brain that is responsible for regulating sleep in the circadian rhythm.
"Maybe it has kept the pineal gland because that helps keep the metabolism down," says Moran.
"It's all about saving energy in these food-limited environments."