Recruitment variation in Macoma balthica:: a laboratory examination of the match/mismatch hypothesis

Long-term observation (1973 to 2001) of recruitment in the intertidal bivalve Macoma balthica (L.) in the western Wadden Sea has revealed that interannual variation in recruitment may be possibly governed by match/mismatch mechanisms. Mismatch occurs when spawning takes place before or at the beginning of the spring phytoplankton bloom, whereas matching occurs when spawning takes place during the spring bloom. To test the match/mismatch hypothesis, a laboratory experiment was conducted in which 6 batches of M balthica larvae were reared at different starting times during a phytoplankton spring bloom (April to May 2002). Larvae were reared in Wadden Sea seawater with the natural phytoplankton assemblage at 2 larval concentrations. As controls, larvae of the same batches were reared in filtered seawater with Isochrysis galbana. A clear effect of fertilisation date on larval growth and development rates was observed, which however could not be explained by phytoplankton concentration differences. In addition, no effects of fertilisation date or food level on mortality were detected. Hence, in this experiment, larval performance was affected by the timing of spawning, although the underlying mechanisms remain unclear.