Eyeless Mexican cavefish show no metabolic1 circadian rhythm in either light and dark or constant dark conditions, according to a study published September 24, 2014 in the open-access journal PLOS ONE by Damian Moran from Lund University, Sweden, and colleagues. The Mexican tetra fish has two variants2, a fully-eyed fish living close to the surface and a blind, deep water, cave-dwelling fish. Scientists in this study used these two fish to study evolutionary3 adaptation in fish residing in near or total darkness. The two fish types experience differences in daily light exposure, food availability, and predation, which all may influence adaptation. The authors explored these evolutionary differences in laboratory-based experiments by measuring metabolic rate of both Pachn cave and surface fish at a fixed4 swimming speed under light and dark and constant dark phoTOPeriods.Researchers found that the eyeless cavefish use ~30% less energy than surface fish, depending on lighting5 conditions. Surface fish displayed a circadian rhythm in oxygen consumption on a light-dark cycle, and they found that this metabolic rhythm persists even in constant darkness. Cavefish, however, show no circadian rhythm in metabolism6, in either light-dark or constant dark conditions. The authors propose that this loss of a circadian rhythm enables cavefish to save energy. The authors suggest that elimination7 of the circadian rhythm in metabolism could be a general feature of animals that live in perpetually dark, food-limited environments, such as caves or the deep sea.Damian Moran added: While animals that live on the planets surface need autonomous8 circadian rhythms to tune9 their physiology10 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 ramping11 up their metabolism needlessly for a day that will never arrive.
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1 metabolic adj.新陈代谢的 参考例句： Impressive metabolic alternations have been undergone during embryogenesis.在胚胎发生期间历程了深刻的代谢变化。 A number of intoxicants are associated with metabolic acidosis.很多毒性物质可引起代谢性酸中毒。
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