Somalian cavefish were collected in the wild at the oasis of Bud-Bud in the centre of the Somalian desert. Ancestors of cavefish entered the large phreatic layers of the Somalian desert that developed in Eocene horizontal limestone formations and became isolated with the extinction of epigean sister species as the result of extreme climatic changes. Evolution during millions of years in perpetual darkness leads to mutations in non-visual opsin genes and an aberrant, blind circadian clock.
Credit: Saulo Bambi
EDINBURGH: Despite living for millions of years underground, completely isolated from the day-night cycle, blind cavefish are still able to 'know' what time it is, using an internal circadian clock.
In new research published in PLoS Biology today, scientists have found that cavefish have a working internal timing mechanism that operates on a 47 hour cycle, as opposed to the approximately 24 hour cycle in humans. Despite living in constant darkness, the fish have evolved a mechanism to set their clocks according to their feeding behaviour.
"This work can help us to answer questions such as if animals live in an environment where there is no day - night cycle, do they still maintain a clock that can be regulated by light?" said lead researcher Nicholas Foulkes from the Karlsruhe Institute of Technology in Germany.
"By considering these cavefish as 'natural mutants', they have much to teach us about how light regulates clocks and how clocks work in general, and provides the first evidence for how normal fish clocks directly sense light."
Setting by the Sun
The circadian clock is an animal's internal timing mechanism, allowing it to anticipate and adapt to changes in the day-night cycle. It controls not only whether we fall asleep or are awake, but also regulates most of our body functions according to the time of day. Since its particular rhythm runs for approximately 24 hours, it must be reset on a daily basis following cues such as low light to ensure that it remains synchronised.
Disruptions to the circadian rhythm can have adverse effects - in humans, a common example is jet lag, and bipolar disorder and some sleep disorders are also associated with irregular functioning of the circadian clock.
The way that light regulates the clock is still not completely 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, until now, the identity of the photoreceptors that must be widely expressed in fish tissues has remained a mystery.
Blind cave dwellers
Despite evolving in complete darkness beneath the Somalian desert, cavefish (Phreatichthys andruzzii), were found to still operate with an unusual circadian clock, running over a long period of 47 hours. By comparing the behaviour of a captive colony of cavefish with zebrafish, a distant relative, the researchers were able to identify how these blind cave dwellers were able to reset an accurate circadian clock in complete darkness.
"The research looked at locomotor activity and clock gene expression in both species when they were exposed to a light-dark cycle. While [we] obtained evidence for a robust circadian clock in the zebrafish that was synchronised with the light cycle, no rhythmicity was detected in the cavefish," said Foulkes. "However, in a comparable study where both fish were exposed to an alternative timing signal - a regular feeding time - both the zebrafish and cavefish displayed circadian clock rhythmicity. Thus, [we] concluded that the cavefish still have a clock that can be regulated by feeding behaviour, but which cannot be reset by light."
The team was also surprised to find that eye loss was not the reason that changes in light did not reset the clock. Instead it was due to genetic mutations in two light-sensitive 'opsin' photoreceptors found on the body of the fish.
Food and our clocks
According to the researchers, this discovery could help us to decipher how food may regulate our internal clocks, and provides further evidence for the role of the environment in influencing circadian rhythms.
"The discovery that cavefish have a circadian clock system that is insensitive to light but can be entrained by food is remarkable," said Christophe P. Ribelayga of the University of Texas Health Science Centre in the U.S., who was not involved in the study. "Endogenous circadian clocks must remain synchronised with the day and night cycle. Although light is widely considered as the most powerful environmental cue to entrain clocks, many details about the entrainment pathways activated by light and other cues, such as regular feeding schedule, remain elusive."
He added, "Future experiments in cavefish will dissect the food-entrainable clock and likely provide interesting insights into the mechanism of feeding entrainment in vertebrates."
