Manila clams,
Ruditapes philippinarum, removed from their natural environment and maintained for 9 weeks in continuously immersed conditions exhibited a clear
endogenous circatidal rhythm in oxygen consumption. The clams exhibited a semidiurnal rhythmicity in oxygen consumption after
showing a diurnal pattern in the first few days (5 to 7 d) of the experiment. The results of the present study indicate that
activity rhythms of clams are controlled not only by exogenous factors, but also by an endogenous circatidal periodicity.
Received: 8 April 1998 / Accepted: 1 February 1999