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Eyeless fish is timeless (literally): No internal clock, no circadian rhythm

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The eyeless Mexican cavefish is a species of blind fish that have now been shown to have another unusual adaptation: their metabolisms show no circadian rhythm. These are the biological processes that are timed to occur over a 24-hour cycle, linking bodies to the cycles of day and night, a new study reveals.

Astyanax mexicanus from inside the Pachon Cave were studied as they swam, and their speed was compared to the fish taken from the surface. Researchers discovered the lack of an internal clock saved the creatures 27 percent of the energy used by their surface-dwelling cousins. Additional oxygen was required by animals from the surface during daytime, leading to a loss of energy. When "night" conditions were reproduced in the laboratory, fish from the surface expended 38 percent more energy than cave dwellers. The lack of circadian rhythms could allow A. mexicanus to utilize additional energy in their dark light-deprived environment.

"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," Damian Moran, a biologist from Lund University in Sweden, said.

Circadian rhythms have been measured not just in humans but also in animals, plants, and fungi.

"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," Moran told the press.

Moran and his team suggest that a lack of circadian rhythms, such as that found in the Mexican cavefish, may also be found in other species that live in dark environments, where food is difficult to acquire. Many such species live in caves, as well as deep under the surface of the ocean, where light rarely passes.

The animals are also not targets of natural predators and live their lives in a semi-conscious state between wakefulness and sleep, according to researchers.

Mexican cavefish, also called Mexican tetra, are small, otherwise unremarkable fish that have lost any coloration, along with their eyesight. They have relatively powerful jaws, and males of the species possess tiny hooks on their anal fins. The animals also exhibit a dorsal fin and a forked tail on their plain silvery bodies, which can grow to be up to five inches in length.

Investigation of Mexican cavefish and the discovery of their lack of circadian rhythms were detailed in the online journal Plos One.

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