The persistence and modulation of endogenous circatidal rhythmicity in Lipophrys pholis (Teleostei)
In constant conditions, freshly-collected Lipophrys pholis show an endogenous circatidal activity rhythm, the initial activity peaks of which are phased to the expected time of high tide. The rhythm usually damps out over a few days but it may re-appear spontaneously or as a result of disturbance caused by handling and transfer to the experimental apparatus. The free-running period is more variable in fish kept in non-tidal conditions for prolonged periods than in those recorded shortly after capture. The non-circatidal periodicity shown by some fish may be the result of stable coupling in antiphase of desynchronised oscillators. There is no semilunar variation of the circatidal rhythm and no influence of the slight diurnal inequality in tidal period upon the rhythm's periodicity, at least at the site studied. The activity rhythm of Lipophrys varies seasonally.