SUMMARY
Since its invasion of Europe in the early 1980s, the Asian clam Corbicula fluminea has become very abundant in nearly all western river systems. Today this species is one of the most important biomass producers in the River Rhine. Monitoring the valve movements of C. fluminea over a period of 2 years revealed a circadian rhythm in summer,with extended periods (10-12 h) of valve closure, predominantly in the morning hours. Altogether valve movements were very scarce, frequently fewer than four movements per individual per day.
Simultaneous measurements of heat dissipation and oxygen consumption(calorespirometry) revealed an intermittent metabolism in the clam. With the onset of valve closure, C. fluminea reduced its metabolic rate to 10%of the standard metabolic rate (SMR) measured when the valves were open. Nevertheless, this depressed metabolism remained aerobic for several hours,enabling the clam to save energy and substrates compared to the requirements of the tenfold higher SMR. Only during long-lasting periods of valve closure(more than 5-10 h) did the clams become anaerobic and accumulate succinate within their tissues (2 μmol g-1 fresh mass). Succinate is transported into the mantle cavity fluid, where it reaches concentrations of 4-6 mmol l-1. Because this succinate-enriched fluid must pass the gills when the valves open again, we suggest that this anaerobic end product is at least partly reabsorbed, thus reducing the loss of valuable substrates during anaerobiosis. Propionate was also produced, but only during experimental N2-incubation, under near-anoxic conditions.
The intermittent metabolism of C. fluminea is discussed as an adaption to efficiently exploit the rare food supply, saving substrates by the pronounced metabolic depression during valve closure.
