A good deal of pioneering work on tidal clocks was done on sea-lice, relatives of the wood-lice. These marine isopod crustaceans show fabulous rhythms when taken from the shore and placed under controlled conditions where they experienced no daily or tidal cues such as light/dark or turbulence. Here are two species, one from NW Europe (Eurydice) and one form North America (Excirolana) studies by the highly influential scientists Naylor and Enright, respectively. Under each image a plot of activity data is shown. Arrows under Eurydice show times of expected high water at the home beach. For both, it is clear that activity (swimming) occurs on a circa 12.4h basis.