The lack of circadian timing in two intertidal invertebrates and its significance in the circatidal/circalunidian debate.

The control of the timing of gaping in the clam Austrovenus stutchburyi does not involve a circadian mechanism. The lack of a circadian timer is shown by the uniform pattern in gaping in constant conditions (CC) regardless of the relationship between the tidal cycle and the day/night cycle at the time of collection; it is also shown by the fact that, in nontidal conditions, a 24 h light-dark (LD) cycle fails to entrain persistent rhythmicity. The crab Macrophthalmus hirtipes also lacks a circadian oscillator. Both of these intertidal dwellers, however, exhibit endogenous tidal rhythmicity. The features of the tidal patterns they display in CC can be accounted for using the circalunidian (i.e., approximately 24.8 h) model of a tidal timing system but, as they lack circadian timers, cannot be explained by the circatidal (i.e., approximately 12.4 h)/circadian model. This demonstrates that, of the two hypothetical models of the clock system driving endogenous tidal rhythms, only the circalunidian one has universal applicability.