Circadian clocks in a blind fish
Do animals that have evolved for millions of years underground, completely isolated from the day-night cycle, still "know" what time it is? Does a normal circadian clock persist during evolution under constant darkness? A new study directly tackles these fundamental questions by investigating a species of cavefish, Phreatichthys andruzzii, which has lived isolated for 2 million years beneath the Somalian desert. Many fish species have evolved in the absence of sunlight in cave systems around the world, sharing a common set of striking adaptations including eye loss. The new study, published September 6 in the online, open access journal PLoS Biology, reports that this cavefish has an unusual circadian clock; it ticks with an extremely long period (up to 47 hours), and is completely blind.