Circadian clocks in a blind fish

September 6, 2011, Public Library of Science

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 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 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 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.

"Cavefish give us a unique opportunity to understand how profoundly sunlight has influenced our evolution," explains Cristiano Bertolucci, co-author of the study. The authors' starting point was to compare the circadian clock of the blind, Somalian cavefish with that of a "normal" fish – the zebrafish. They studied the locomotor activity and clock gene expression in both species when they were exposed to a light-dark cycle. While they obtained evidence for a robust circadian clock in the zebrafish that was synchronized with the light cycle, no rhythmicity was detected in the cavefish. However, in a comparable study where both fish were exposed to an alternative timing signal, a regular feeding time, both zebrafish and cavefish displayed circadian clock rhythmicity. Thus, they concluded that the cavefish still has a clock that can be regulated by feeding behaviour, but which cannot be reset by light. In a more detailed study, they were able to show that the cavefish retains a clock that ticks with an abnormally long period. Strikingly, they also found that the lack of its resetting by light is not due to eye loss in this fish; instead, mutations in two widely expressed opsin photoreceptors leave the clocks in most tissues unable to respond to light.

"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 to life without sunlight. Second, this work provides the first genetic evidence for the identity of the widely expressed in fish. This study sets the stage for a more complete understanding of how clocks respond to their environment."

Explore further: Keeping the daily clock ticking in a fluctuating environment: Hints from a green alga

More information: Cavallari N, Frigato E, Vallone D, Frohlich 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

Related Stories

Team creates math model for circadian rhythm

August 27, 2007

The internal clock in living beings that regulates sleeping and waking patterns -- usually called the circadian clock -- has often befuddled scientists due to its mysterious time delays. Molecular interactions that regulate ...

Mouse vision has a rhythm all its own

August 23, 2007

In the eyes of mammals, visual information is processed on a daily schedule set within the eyes themselves—not one dictated by the brain, according to a new report in the August 24 issue of the journal Cell, a publication ...

Recommended for you

Study links genes to social behaviors, including autism

October 18, 2018

Those pesky bees that come buzzing around on a muggy summer day are helping researchers reveal the genes responsible for social behaviors. A new study published this week found that the social lives of sweat bees—named ...

Bioceramics power the mantis shrimp's famous punch

October 18, 2018

Researchers in Singapore can now explain what gives the mantis shrimp, a marine crustacean that hunts by battering its prey with its club-like appendages, the most powerful punch in the animal kingdom. In a paper publishing ...

Expanding the optogenetics toolkit

October 18, 2018

Controlling individual brain cells using light-sensitive proteins has proven to be a powerful tool for probing the brain's complexities. As this branch of neuroscience has expanded, so has the demand for a diverse palette ...

Staying a step ahead of the game

October 18, 2018

Trypanosoma brucei, which causes sleeping sickness, evades the immune system by repeatedly altering the structure of its surface coat. Sequencing of its genome and studies of its 3-D genome architecture have now revealed ...

Elucidating cuttlefish camouflage

October 18, 2018

The unique ability of cuttlefish, squid and octopuses to hide by imitating the colors and texture of their environment has fascinated natural scientists since the time of Aristotle. Uniquely among all animals, these mollusks ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.