COMPARATIVE-STUDIES OF TIDAL RHYTHMS .13. IS A CLAM CLOCK SIMILAR TO THOSE OF OTHER INTERTIDAL ANIMALS

Under constant laboratory conditions the gaping and closure of the she ll valves of the intertidal clam Chione stutchburyi show a clear patte rn related initially to the tides in its natural habitat. From a total of 63 clams immersed in water, 97% displayed a clear and persistent p attern consisting basically of two gaping intervals per circalunar day with the mean period length of the bimodal rhythm being 25.67 hours. An unusual feature in the rhythm is the irregularity with which the 'g aping' phase is expressed. Details of the shell gaping pattern in some individuals support the hypothesis of dual circalunidian systems cont rolling the circatidal pattern. The gaping rhythm is also expressed in air. It is not rephased by a single air to water stimulus but is reph ased by a 18.5 hr-long low temperature pulse. This is the first detail ed study of a bivalve clock, and the features of the timing system in Chione are compared to those of other intertidal animals.