While most animals have a clear day/night cycle, the blind Mexican tetra fish- also known as Astyanax mexicanus– has relinquished its circadian rhythm in order to increase energy conservation.

According to researchers at Lund University in Sweden, blind cavefish are able to conserve energy by 27% through eliminating day/night differences. This allows them to maintain a metabolic state somewhere between sleep and wakefulness, and spring into action when the need arises.

According to senior research Dr. Damian Moran, “These cave fish are staying in an environment without light, without the circadian presence of food or predators, they have actually got absolutely nothing to get all set for, so it appears like they have actually just chopped away this increase in anticipation for the day. They live a demand-led life as opposed to an anticipation-led life.”

The blind tetra fish has long been the focus of evolutionary studies, including a major one conducted by Harvard Medical Center’s Department of Genetics in collaboration with Woods Hole. That study, published in 2013, examined why the fish had lost its sight- concluding that the change showed an unusual evolutionary pattern.

Researchers at Lund University had originally set out to study the energy cost of losing sight, but soon discovered that the fish had no circadian rhythm. The study was conducted by placing the fish in a plume respirometer, a closed-loop device, and measuring activity levels by changing water speeds.

While the blind tetra fish may have lost its sight, and reduced its eyes to two blank useless spheres, it surprisingly still contains a pineal gland. This is the gland in the brain responsible for managing circadian rhythms. Scientists are unsure why the fish have stopped using this gland, but suspect that it is for energy conservation reasons.

Similar studies on the cavefish’s lack of circadian rhythm have also been published by the University College of London and the Autonomous University of Mexico.