A new study conducted on Mexican tetra or blind cave fish reveals that these species show no metabolic circadian rhythm (A daily rhythmic activity cycle, based on 24-hour intervals, that is exhibited by many organisms). The study was conducted by Dr. Damian Moran, lead author and fish biologist and his team. It recently got published in the journal PLOS ONE.

Dr. Damian mentioned that most animals have a clear day/night circadian rhythm to their metabolism. This means that usually the organisms are least active at night and have a designed bedtime.

Dr. Damian added, "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".

However, these cave-dwelling organisms have eliminated that cycle, which has enabled them to use significantly less energy over a 24-hour period compared to its surface-dwelling counterparts. A 27% saving in energy use was reported.

There are two types of tetra fish that can be found in the waters of Mexico. One is tiny and wide-eyed surface dwelling tetra, which can be seen in flashes of silvery scales, and the other is blind cave dwelling, which can't be seen without using a flashlight or snorkel.

The cave-dwelling Mexican tetra has a slightly more transparent body and is completely blind as it doesn't have eyes. The species lost their ability to grow eyes completely due to the dark environment and mutations.

To compare the energy expenditure of surface-dwelling tetra with its blind cave-dwelling form, the researches placed the fish in a device known as a flume respirometer. The device is 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, thus allowing the scientists to examine how energy is used.