Animals match their biological rhythms to the day–night cycle thanks to their circadian clocks, which are usually kept on time using cues from light. But do organisms that live in perpetual darkness, such as the blind cavefish (Phreatichthys andruzzii; pictured), have such clocks? Cristiano Bertolucci at the University of Ferrara in Italy and his colleagues show that they do. However, the clock's oscillations are set by feeding times, not light, and cycle every 47 hours, rather than every 24.
The team compared zebrafish (Danio rerio), which have light-entrained clocks, with the cavefish and found that two light-sensitive proteins, melanopsin and TMT-opsin, were present in both fish but mutated in the cavefish. The zebrafish versions of the proteins, when expressed in cavefish cells, restored the cells' sensitivity to light — proving that the proteins function as the photoreceptors that coordinate fish circadian clocks.
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Biological clocks work in the dark. Nature 477, 253 (2011). https://doi.org/10.1038/477253e
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DOI: https://doi.org/10.1038/477253e
