FOOD-PARTICLE SIZE AND SELECTION BY BIVALVE LARVAE IN A TEMPERATE EMBAYMENT

Epifluorescence microscopy was used to analyze the stomach contents of bivalve larvae collected in the Bale des Chaleurs (western Gulf of St . Lawrence, Canada) in order to document food-particle sizes, compare feeding among taxa, and compare the diet with the in situ phytoplankto n community. Stomach contents were mainly composed of small autotrophi c flagellates (<5 mu m) and cyanobacteria (<2 mu m), reflecting the mi crobial food web which characterizes these waters. More than half (55% ) of all veligers examined contained algal cells of 5 to 15 mu m, wher eas only 3% had cells of 15 to 25 mu m. Differences in the size ranges of ingested algal cells among similar-sized larvae of different speci es suggests that veligers actively selected food particles. Among the smallest veligers (185 to 260 mu m), scallops (Placopecten magellicanu s) and mussels (Mytilus edulis) ingested more <5 mu m and 5 to 15 mu m algae than clams (Mya arenaria). Among larger veligers (261 to 405 mu m), clams contained significantly more <5 mu m cells than mussels, wh ereas mussels contained significantly more 5 to 15 mu m algae than cla ms. Algal cells of 15 to 25 mu m were preferentially ingested by musse l veligers. Feeding also differed between different-sized veligers wit hin taxa, i.e. the smallest clam veligers ingested fewer of 5 to 15 mu m algae than the larger size classes. Mussel veligers ingested signif icantly more 15 to 25 mu m and fewer <5 mu m cells as their size incre ased. The dominance of ultraplankton in the nearshore waters of Bale d es Chaleurs and in the stomach contents suggests that veliger larvae m ay be an important export path for carbon produced by small phytoplank ton.