A comparative ecophysiological study was carried out in September and November 2002 for two
bivalves, Paphia rhomboïdes and Glycymeris glycymeris, which are significantly harvested in the
Western Channel (French coast). Individual measurements of clearance and respiration rates were
performed for both species, in several experimental conditions of temperature and phytoplankton
concentrations. Those conditions were chosen to be in the range of those commonly observed in the subtidal area of the Western Channel. The mean value of clearance rate and oxygen consumption obtained from continuous data recording did not reveal any significant influence of temperature in the investigated range, but a significant difference between P. rhomboïdes and G. glycymeris. P. rhomboïdes showed a continuous feeding activity and faeces production and was characterised by high clearance rate (mean : 1.7 l.h-1 .g -1) regulated by the phytoplankton concentration and high assimilation efficiency (approx. 90%). On the opposite, G. glycymeris showed a periodic feeding activity and faeces production with low clearance rates independent of phytoplankton concentration (mean: 0.4 l.h-1 .g -1) and a lower assimilation efficiency (approx. 78%). Individual SFG values were calculated for both species and it appears that P. rhomboïdes exhibited higher SFG values than G.glycymeris, except for low concentration of algae. These results may help to explain the significantdifference in growth rate of both species. However, assumptions are made on differences in foodsource and/or feeding behaviour of Paphia and Glycymeris.