FERRARA, Italy -- How 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. The circadian clock is a highly conserved physiological timing mechanism that allows organisms to anticipate and adapt to the day-night cycle. Since it ticks with a period that is not precisely 24 hours, it is vital that it is reset on a daily basis by signals such as light to ensure that it remains synchronized with the day-night cycle. The molecular mechanisms whereby light regulates the clock remain poorly understood. Fish have emerged as useful models to study how light regulates the clock since in most of their tissues, direct light exposure resets the clock. This differs from the situation in mammals, where light regulates the clock only indirectly through the eyes. However, the identity of the photoreceptors that must be widely expressed in fish tissues has remained a mystery.