Science News

Sea louse with tidal clock

Thu, 26th Sep 2013

Chris Smith

A 12-hour tide-predicting clock, running inside the brains of small marine crustaceans, has been discovered by UK scientists.

Eurydice pulchra is a small speckled sea louse resembling a miniature shrimp. It lives along Euridyce pulchra - sea louseAtlantic coastlines and emerges from its hiding place in the sand twice per day to swim and feed, but always around the time of the high tide, every twelve hours.

Scientists had speculated that, in animals showing this sort of activity pattern, a chronometer similar to - but running twice as fast as - the twenty-four hour body clock in our own brains was responsible for driving this behaviour.

But now researchers from Aberystwyth and Cambridge have discovered that, in the sea louse at least, there's a very different tide-specific clock, which runs independently of the animal's main body clock.

The finding is important because it might inform our understanding of the workings of other sorts of biological clocks, including those that help animals keep track of the seasons, or even the circaluna clock shown recently to operate in humans and affect our sleep patterns at different phases of the moon cycle.

In the new study, which is published in Current Biology, author David Wilcockson placed sea louse specimens, collected fresh from the local beach, into conditions of constant darkess in the laboratory.

Despite the absence of light or a rising and falling tide, the animals showed regular patterns of activity every 12 hours. They also altered their skin colouration over the same time period, at night time displaying prominent dark speckles which disappeared again during what would be daytime for them.

Interestingly, they also were active for longer during what would be their night time swimming forays than during the equivalent daytime swimming episodes.

This showed they were keeping time using some kind of internal clock.

But then the researchers disrupted the animals' clocks both by exposing them constantly to light and by using a genetic technique called RNA interference to shut off the animals' body clocks.

This played havoc with their skin speckle pattern, and they also stopped swimming for longer during what would have been their night.

Yet, despite this disruption to their normal body clocks, they nevertheless continued to display 12 hourly episodes of swimming behaviour interspersed with periods of rest.

This shows that the tidal and circadian (daily) clocks in these animals are two totally separate entities that run side by side but use largely different and independent molecular clockwork mechanisms. What the mechanism is though, as yet remains a mystery...

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I read elsewhere that it was every 12.4 hours. This leads me to wonder if there's any connection between that and us having a 25 hour clock which has to be corrected every day by the light of dawn instead of a 24 hour clock which doesn't need correcting to such a huge degree. David Cooper, Fri, 27th Sep 2013

Hi David

you're right - it is 12.4 hours - I simplified it to 12, because perhaps unfairly, I assumed that most people wouldn't be interested in this particular detail. But the reason it's 12.4h, I believe, is because tides move on about 1 hour each day; so adopting this rhythm correctly anticipates the later time of the next tide. chris, Fri, 27th Sep 2013

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