Researchers at Aberystwyth and Bangor, along with colleagues at Cambridge and Leicester, have found the speckled sea louse has two body clocks.
Writing in the journal Current Biology, they've confirmed the existence for the first time of a body clock which follows the 12.4 hour cycle of the tide.
That's as well as the one which most land-based animals have, including humans, which reacts to light and dark; the circadian clock.
They say the discovery of the circadian clock was a major breakthrough in biology so finding the tidal clock presents a new perspective on how organisms define biological time.
It could have implications for the welfare and productivity of commercially important marine animals.