The entrainment of persistent tidal rhythmicity in a filter-feeding bivalve using cycles of food availability.

Austrovenus stutchburyi is an intertidal, burrowing, filter-feeding bivalve. It exhibits a strong, approximately tidal pattern in the time of gaping of its valves when in constant laboratory conditions. Individuals that have been held in laboratory conditions long enough to lose their overt gaping pattern will respond to pulses of algae made available at a tidal frequency by opening to coincide with the times when food is available and remaining closed at other times. The opening of the valves is not solely a response to the arrival of the food; usually, their opening anticipates the arrival of the regular food pulses. The tidal pattern seen during the pulsed food treatment will continue for two cycles when food is withheld, indicating the involvement of an internal timing mechanism that is responsive to pulses of food. The short persistence of the food-entrained rhythm in constant conditions suggests that the food-entrained oscillator is only a minor component of the timing system, and this conclusion is supported by the finding that for the first 4-5 days when freshly collected clams are held in the laboratory and supplied with food pulses around the times of expected high water, the "circa" period of their gaping rhythm is not adjusted to a strictly tidal one. The characteristics of the food-entrained oscillator are discussed in relation to existing knowledge of tidal oscillators and food-entrained circadian oscillators.