Speckled Sea Louse

• It keeps its activity rhythm every 12.4 hours even when isolated in a laboratory without tides: it is one of the classic models for studying the circatidal biological clock.
• As well as the tidal clock, it changes colour according to the time of day thanks to an independent circadian rhythm.
• It has a tidal clock of its own: an endogenous circatidal rhythm of about 12.4 hours that brings it out of the sand on the flooding tide, keeps it swimming for five or six hours and sends it back into the sand.
• That clock settled an old debate. For years it was argued that tidal rhythms were merely a submultiple of the circadian clock; experiments on this species showed that there is a circatidal pacemaker independent of the circadian system.
• It swims mainly at night, and if turbulence lifts it out of the sand by day it buries itself at once.
• It reaches densities of more than 1,500 individuals per square metre, and on one beach in south Wales more than 4,000 were recorded.

Habitat and environment

Sand of exposed beaches of the north-eastern Atlantic

Sources consulted: Zhang et al. (2013), Current Biology; Wilcockson et al.; Jones y Naylor (1970); MarLIN