Cave Fish Sleep When They Please to Save Energy

Mexican cave fish don't have a designated bedtime around which most of them sleep. That sets them apart from most animals and even some plants, for whom a specified time for sleep helps them to efficiently recharge. However, a new study suggests living in their lightless environment, sleeping whenever they please may help these fish save energy.

There are two types of tetra fish that can be found in the waters of Mexico. One, tiny and wide-eyed, can be seen in flashes of silvery scales as it darts around in shallow fresh water. The other you likely will never see, and if you do, it's because you brought a snorkel and a flashlight.

The cave-dwelling Mexican tetra has the same body shape of its sun-loving cousin, but it's slightly more transparent, and is completely blind - probably due to the fact that it doesn't have eyes.

Evolution long ago dictated that in an utterly lightless environment, the cave tetra's eyes were simply another means to risk injury and infection. A few lucky mutations and selective pressures later, the silvery fish lost its ability to grow eyes completely.

But that's not all it lost. According to a study recently published in the journal PLOS ONE, the Mexican cave tetra also has no circadian rhythm either.

Most animals, humans included, are slaves to their circadian rhythm, growing tired in predictable patterns that cannot change from day-to-day. This ensures that we catch some shut-eye when we are least active - usually at night - so that we will be refreshed by the time our bodies will need to expend the most energy.

Interestingly, when comparing cave tetra to their surface-dwelling counterparts, researchers found that the cave fish use about 30 percent less energy overall than surface fish.

"While animals that live on the planet's surface need autonomous circadian rhythms to tune their physiology to their daily activities, the results of our study show that animals that live in environments without 24 hour cycles can save energy by not ramping up their metabolism needlessly for a day that will never arrive," researcher Damian Moran of Lund University explained in a statement.

Moran and his colleagues admit that very little is actually understood about circadian rhythms, but if cave-dwelling fish can simply discard their internal clocks all together for the sake of saving energy, then it stands to reason that many other animals living in lightless, food-limited environments may do the same.