SIMILARITY IN FOOD SOURCE AND TIMING OF FEEDING IN DEPOSIT-FEEDING AND SUSPENSION-FEEDING BIVALVES

Although adapted to different methods of feeding, deposit- and suspension-feeding bivalves in the Wadden Sea (The Netherlands) appear to rely partly on the same limiting food sources. Two conditions for food competition, viz. use of the same food source and simultaneous feeding (in relation to season and tidal cycle), were studied in the field. The algal species composition in the stomachs of 3 suspension feeders (Cerastoderma edule, Mya arenaria and Mytilus edulis) and 2 deposit feeders (Macoma balthica and Scrobicularia plana) showed high correlations with the species composition of the water column, but lower correlations with the species composition of the sediment surface. During the phytoplankton spring-bloom periods in April and May, the stomachs of the suspension feeders C. edule, M. arenaria and M. edulis, and the deposit feeder M. balthica contained maximum amounts of algae. During the tidal cycle, the stomachs of the suspension feeders generally contained minimum amounts of algae when the tidal flat was drained and maximum amounts around times of high water. The stomachs of the deposit feeder M. balthica, however, contained relatively high amounts of algae during the entire tidal cycle. These results indicate that, although some difference in feeding behaviour was observed between deposit and suspension feeders, both groups rely on the same food source at the same time, irrespective of type of feeding.