Effects of salinity on endogenous rhythm of the Manila clam, Ruditapes philippinarum (Bivalvia : Veneridae)

The Manila clam Ruditapes,cs philippinarum an intertidal bivalve, was expos ed to different salinity regimes (from 31.0-31.7 parts per thousand down to 20 parts per thousand, 15 parts per thousand, 10 parts per thousand, 5 par ts per thousand), and the endogenous rhythm in its oxygen consumption was s tudied using an automatic intermittent-flow respirometer. When exposed to s alinities reduced from 31.5 parts per thousand to 20 parts per thousand and 15 parts per thousand under otherwise constant conditions, the clams recov ered a clear endogenous circatidal rhythm in their oxygen-consumption rate after having dampened periods of 12 h and 48 h, respectively. At salinities less than 10 parts per thousand,, however, the oxygen-consumption rate was depressed greatly at the beginning of the experiment for about 36 h and th en increased to a level higher than normal, but the rhythm of oxygen consum ption was not recovered. The results of this study indicate that the Manila clam, a euryhaline organism, cannot maintain a normal metabolic activity a t a salinity lower th;tn 15 parts per thousand. All clams were dead after e xposure at a salinity of 5 parts per thousand for 7 days.