Eyeless fish that have evolved underground, completely isolated from the day-night cycle, may offer clues to how our body clocks work up here on dry land.
Authors of a report published today in the online journal PLoS Biology investigated a species of cavefish, Phreatichthys andruzzii, which has lived isolated for 2 million years beneath the Somalian desert.
The report shows the cavefish has an unusual circadian clock: it ticks over an extremely long period (nearly two full days). This is despite the fish not having exposure to daylight, which is thought to be a key driver in terrestrial body clocks.
Feel the rhythm
Earth’s environment is profoundly rhythmic: our world is a very different place at 3am than it is 3pm.
There’s an adaptive advantage in accurately predicting the daily change from dark to light, and an internal clock has evolved in all terrestrial species.
The human circadian system is known to be important in health and illness (especially mental health).
Guidelines for Bipolar Disorder, for example, prescribe the maintenance of regular 24-hour activity cycles to address a core vulnerability in the circadian system.
The circadian system is strongly represented across evolution, enabling us to use simpler systems in other species to help understand our own circadian system.
One of the key questions is how the circadian clock stays synchronised with the 24-hour light/dark cycle, given that in most species the body clock’s innate cycle is not 24 hours and needs to be reset (entrained) daily by environmental cues.
The most common entraining signal across all species is, not surprisingly, light itself.
Eyes wide shut
The PLoS study compared the circadian clock of Somalian cavefish with that of a “normal” fish – the zebrafish – to explore entrainment mechanisms.
There were three key findings:
1) The cavefish – for which complete eye degeneration occurred as a result of living in perpetual darkness – has a functional circadian system (albeit one that has an unusually long period of 47 hours). This underlines the biological significance of a body clock.
2) The circadian system of the cavefish does not respond to light, but is entrainable by cycles of food availability.
This finding opens important questions for future research because so-called food-entrainable oscillators are of interest in other species, including humans.
3) The lack of sensitivity to light was not due to the cavefish having no eyes. Light entrainment in the sighted zebra fish does not occur through the eyes but through non-visual opsin (a protein of the retina) photoreceptors in most of their tissues.
In the cavefish, mutations in these opsins led it to abandon light entrainability.
This is signficant because in the human case there’s great interest in the recent finding that light’s effects on the circadian clock are mediated by non-visual pathways in the brain, and these pathways involve opsins.
Will the cavefish findings cause ripples or deep stirrings in our knowlede of body clocks going forward? Perhaps we’d better sleep on it.