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EvolutionEvolution

This Fish Has No Circadian Rhythm, and It’s Better Off For It

ByBridget MorawskiNOVA NextNOVA Next

The blind Mexican tetra, a species favored by evolution researchers, has revealed yet another quirk of natural selection—the cave fish doesn’t have an circadian rhythm, a curious tick that isn’t normally found in the wild.

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Swedish researchers discovered the quirk when they compared the metabolic rates of the cave-dwelling form with its surface dwelling counterpart when the two were exposed to varying lighting conditions designed to mimic day/night cycles.

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blind-mexican-tetra
The blind Mexican tetra saves energy by doing away with its metabolic clock.

Biological clocks are linked to circadian rhythms, which take their cues from the rising and setting of the sun. It is what drives the humans to sleep at night and be productive during the day. Circadian rhythms also tie into metabolic functions; certain times of day cause our metabolism to either ramp up or slow down energy production.

To mimic day and night conditions for the tetra, researchers fiddled with the amount of light that each form of the fish was exposed to. While oxygen intake of the surface-dwelling tetra fluctuated based on its metabolic rate, researchers noticed something curious about the cave-dwelling tetra’s metabolic cycles. They simply weren’t there, and based on the data the researchers gathered, that absence appears to confer an evolutionary advantage. Here’s Elizabeth Palermo:

Regardless of whether it was light or dark, the fish consumed roughly the same amount of oxygen, the researchers found. By forgoing the circadian rhythms that control metabolism, the blind Mexican cavefish was able to expend nearly 30 percent less energy in a 24-hour period than its surface-dwelling counterparts.

Species living in extreme ecosystems are known to adapt in extreme ways. The cave-dwelling Mexican tetra doesn’t just lack a circadian rhythm, it also lacks eyes, a common adaptation to low-light conditions. A variety of salamanders, spiders, and small underground mammals, for example, have developed ways of feeling their way around their environment without the aid of eyesight.

The fish’s missing circadian rhythm has researchers stumped. Damian Moran, one of three scientists involved in designing and carrying out the study, says the study raises more questions than it answers. Here’s Palermo again:

“Not only do we not really know that much about circadian energy use in animals in general, we don’t even know how to consider animals that don’t have these circadian rhythms,” Moran said.

“We tend to assume that these rhythms are always adaptive, that they serve some really important purpose. But what happens in animals that don’t have these cycles? It’s a real conundrum.”

Circadian rhythms help prepare the body so it can perform certain tasks efficiently depending on the time of day, such as producing enzymes to digest food depending on meal times. But few studies have considered the metabolic cost of circadian rhythms in wild animals, in part because they’re so difficult to find. The Mexican tetra, with its surface- and cave-dwelling forms, gives scientists a near-perfect laboratory to study them. This discovery sheds more light on how animals adapt to survive in food-limited environments.

Photo credit: Joachim S. Müller/Flickr (CC BY-NC-ND)

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