Coastal animals have their own tidal timer -- separate from the 24-hour body clock

(Press-News.org) University of Leicester researchers have revealed that coastal animals have their own biological tidal timer, which is separate from their 24-hour body clock.

Experts from the University's Department of Genetics have published a paper in Current Biology which reveals the discovery of an independent clock driving coastal animals' tidal rhythms.

The paper, Dissociation of Circadian and Circatidal Timekeeping in the Marine Crustacean Eurydice pulchra, follows nearly ten years of research by Leicester geneticists, along with colleagues at the Universities of Bangor, Aberystwyth and Cambridge.

The researchers observed the behaviour of Eurydice pulchra, the speckled sea louse which lives along the sandy shores around the UK coast.

Eurydice have a regular 12.4 hour swimming cycle matching the incoming tides which sees them actively swimming around to find food as the tide comes in, and burying themselves in the sand as the tide goes out.

Eurydice also have a 24 hour 'circadian' rhythm in pigmented cells called melanophores, which grow during the day (acting as sunscreen) and become smaller at night when they are not required.

It had been thought for many years that these patterns of tidal behaviour in coastal organisms were driven by the 24-hour circadian clock, the ubiquitous and genetically encoded 'body clock' which controls the rhythmic behaviour and physiology of almost every terrestrial higher organism, including humans.

But the new paper shows that tidal behaviour is not governed by the circadian clock but is instead controlled by a dedicated 12.4-hour "circatidal pacemaker".

The researchers caught Eurydice from the coast near Bangor every season and monitored their behaviour and clock molecules in laboratories at Bangor and Leicester.

The team were able to "turn off" Eurydice's circadian clock by genetically knocking down two of the key circadian clock genes, which they had identified.

They were also able to block the circadian cycle by exposing Eurydice to a constant bright light for many days.

With both of these genetical and environmental manipulations, the circadian rhythm in the sunscreen cells became severely disrupted, but the tidal swimming rhythm just kept on ticking every 12.4 hours.

This showed conclusively that the animals' tidal patterns are independent of their 24 hour circadian clock.

The researchers believe these results – along with a parallel study on coastal worms published simultaneously in the journal Cell Reports on 26 September which came to a similar conclusion - suggest that many other coastal animals will have similar circatidal systems.

Principal investigator Professor Charalambos Kyriacou, Professor of Behavioural Genetics in the University's Department of Genetics, said: "Most coastal animals will have 24 hour rhythms in some aspects of their physiology, but their most important geophysical cycle is the tides coming in and out and so they have evolved circatidal rhythms to cope with this rapidly changing environment.

"Eurydice pulchra have both a 24 hour rhythm and a 12.4 hour rhythm, which relates to the gravitational pull of the moon on the earth's tides.

"People have speculated that the 24 hour clock could be driving the 12.4 hour clock, because all it would take is for two 24 hour clocks in the brain to work in antiphase with each other.

"This would give two approximate 12 hour rhythms. We have shown that if we knock down the genes that code for their 24 hour clock, the animal still has very robust tidal rhythms.

"This shows that the 12.4 hour clock is independent from the circadian clock. I expect tidal rhythms in many coastal organisms will follow this rule including insects, crabs, even plants."

### The project was carried out in collaboration with Bangor University and Professor Simon Webster and Dr David Wilcockson (the latter now a lecturer in Aberystwyth), and Dr Michael Hastings MRC Laboratory of Molecular Biology in Cambridge with the molecular laboratory work carried out by Dr Lin Zhang of Leicester's Department of Genetics.

The project was funded by grants from the Biotechnology and Biological Sciences Research Council (BBSRC) and the University of Leicester's College of Medicine, Biological Sciences and Psychology, as well as core funding from the Medical Research Council to Dr Michael Hastings.

The team have now secured a new four-year grant from BBSRC to sequence the Eurydice pulchra genome.