The speckled sea louse, a marine cousin of the wood louse, has two body clocks - one for time and other for tides, say researchers at Aberystwyth University.
Some lice had their circadian cycle, the one used by land dwellers including humans to tell the time by changes in light and dark, switched off.
Despite being out of their habitat, the lice continued to swim every 12.4 hours as they would with changes of the tide.
The specimens were collected from a beach near Bangor in north Wales.
The speckled sea louse Eurydice pulchra grows up to 5mm in length and burrows deep into the sand at low tide and returns to the surface to swim and feed when the tide comes in.
It turns dark during the day as a protection against UV light while it turns white at night and becomes a vigorous swimmer.
Dr David Wilcockson, aquatic biologist at the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University, and senior author on the research, said the lice's circadian cycle was switched off by using genetic, pharmacological and in vitro cell biology techniques.
Yet the lice carried on swimming in time to anticipated tide changes.
He said: "The discovery of the circadian clock mechanisms in various terrestrial species from fungi to humans was a major breakthrough for biology.
"The identification of the tidal clock as a separate mechanism now presents us with an exciting new perspective on how organisms define biological time. It is a completely unexplored field."
Researchers from Aberystwyth, Bangor, Cambridge and Leicester Universities, were involved in the study, which is published in the journal Current Biology.
The teams believe similar solutions to measuring tidal change may have evolved in unrelated species in the same environment.
Dr Wilcockson said: "There is tremendous diversity in the oceans and biology is so inventive that it may have come up with many different solutions exploiting various mechanisms to solve the same challenges of life.
"Our work has revealed that evolution created a greater diversity of clock types than we ever thought previously, so we really are on the threshold of some exciting science."