Blind cavefish know what time of day it is – and contrary to one suggestion that was offered recently, they don't dial the speaking clock!
Even in complete darkness blind cavefish can reset their internal, or circadian, clock – a process that normally requires a light/dark cycle to work.
A new study published in the open access journal PLoS Biology reveals that despite two million years living in the dark underneath the Somalian desert, a species of blind cavefish – Phreatichthys andruzzii – maintains a circadian clock in the absence of light, albeit a 47-hour clock.
The circadian clock is an internal mechanism that allows organisms to adapt to a day-night cycle, and needs resetting daily by signals, typically light – just how these mechanisms work is poorly understood. Fish make useful subjects for studying the circadian clock as unlike mammals, where light only enters through the eyes, fish regulate their clock through direct light exposure to most of their tissues, and it is the identity of the photoreceptors that have remained a mystery.
"Cavefish give us a unique opportunity to understand how profoundly sunlight has influenced our evolution," explains Cristiano Bertolucci, co-author of the study.
By comparing the circadian clock of the cavefish against that of a typical fish – the Zebra danio - through a series of experiments, the authors were able to conclude that not only could the cavefish retain a clock in the absence of light (with feeding behaviour acting as the trigger instead) but that it is mutations in two photoreceptors that left the clock unable to respond to light – and not the lack of eyes.
"This work holds great importance for two major fields of interest," explains Nicholas Foulkes, another co-author of the study. "First, it provides a fascinating new insight into how evolution in constant darkness affects animal physiology. While most detailed molecular studies of cavefish have focused on the mechanisms underlying eye loss, very little is known about other, broader adaptations to life without sunlight.
"Second, this work provides the first genetic evidence for the identity of the widely expressed photoreceptors in fish. This study sets the stage for a more complete understanding of how clocks respond to their environment."
For more information, please see the paper: Cavallari N, Frigato E, Vallone D, Fro¨ hlich N, Lopez-Olmeda JF, et al. (2011) A Blind Circadian Clock in Cavefish Reveals that Opsins Mediate Peripheral Clock Photoreception. PLoS Biol 9(9): e1001142. doi:10.1371/journal.pbio.1001142
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