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 (approximately 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 (approximately 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 significant difference in growth rate of both species. However, assumptions are made on differences in food source and/or feeding behaviour of P. rhomboïdes and G. glycymeris