Phase Shift of a Tidal Rhythm by Light-Dark Cycles in the Semi-Terrestrial Crab Sesarma pictum

The larval release activity of the semi-terrestrial crab Sesarma pictum was monitored for three-week periods under laboratory conditions of constant and cyclic light. Under conditions of constant dim light, the rhythm for the first ten days was unimodal (larval release just after the nocturnal high tide) and then became bimodal (no apparent synchrony with the tides or with other members of the population) for the remainder of the experimental period. On the other hand, in photoperiods similar to those in the field, the rhythm was maintained; the phase was bimodal and the timing of larval release was delayed 1-2 h from the predicted times of high water in the habitat. When the photoperiod was advanced or delayed, the tidal rhythm was phase-shifted accordingly. The photoperiod does entrain the release rhythm to bimodal tidal cycle. So the phase-shift of a tidal rhythm by 24-h LD cycles is a very difficult phenomenon to explain.