Twenty-four hour cycles known as circadian rhythms are responsible for a lot of animal behaviors, ranging from the dawn chorus of birds to jet lag in people. Some fish prepare for the day’s hunt by ramping up their metabolism. But increasing oxygen consumption uses up a lot of energy — so what happens with fish who never see the light of day? According to a new PLoS One study, a species of small, eyeless cavefish has no metabolic circadian rhythm, and it doesn’t matter if it’s swimming around in light and dark conditions or in constant darkness. This has led to energy savings of nearly 30 percent.
The Mexican tetra Astyanax mexicanus naturally comes in two variants: the fully-eyed form living close to the surface and multiple eyeless, cave-dwelling forms occupying deep waters. The two types experience dramatic differences in daily light exposure, food availability, and predation — and all of these differences can influence adaptation. So when seasonal flooding washes surface fish into caves, their conditions tend to worsen.
But all of the cave-adapted A. mexicanus morphs descended from surface ancestors. To explore evolutionary adaptation in fish residing in near or total darkness, a trio of Lund University researchers led by Damian Moran measured oxygen consumption (or metabolic rate) of both Pachón cave and surface fish from northeastern Mexico. They placed the fish in a contraption that allowed them to maintain a fixed swimming speed under alternating light and dark cycles, as well as constant dark photoperiods.
Their lab experiments show that the eyeless cavefish use 27 percent less energy than surface fish. During light-dark cycles, surface fish displayed a circadian rhythm in oxygen consumption — that is, they showed an increase in oxygen demand during the day hours — and they maintain this metabolic rhythm even in experimental settings with constant darkness. In dark conditions, they expended 38 percent more energy than cave forms under equivalent conditions.
Blind cavefish, on the other hand, display no metabolic rhythm in either light-dark or constant dark conditions. “Some organisms have stronger circadian rhythms, and some weaker,” Moran tells Time. “But these fish have none at all.”
This loss of a circadian rhythm likely enables cavefish to save energy. By eliminating circadian rhythm in metabolism, the fish can live in perpetually dark, food-limited environments. “While animals that live on the planet’s surface need autonomous circadian rhythms to tune their physiology to their daily activities,” Moran says in a news release, “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.”
Image: H. Zell via Wikimedia CC BY-SA 3.0
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