Predation of intertidal infauna on juveniles of the bivalve Macoma balthica

Juveniles of the bivalve Macoma balthica live on tidal flats in the Wadden Sea. This study examined the interaction of Macoma with the infaunal polychaetes Arenicola marina and Nereis diversicolor and the gastropod Retusa obtusa. The distribution of M. balthica spat on the flats, shortly after settlement in April, showed a positive correlation with the Arenicola distribution and a negative correlation with Nereis distribution. There were no locations where Macoma spat and Retusa occurred together. In August, Macoma spat had grown too large for predation by intertidal infauna.Small individuals of Macoma spat were found in stomachs of Arenicola (0.14 worm−1) and Nereis (0.05 worm−1). Laboratory experiments showed that Nereis and Retusa could reduce Macoma spat abundance, both in the absence and presence of sediment and alternative prey. Arenicola reduced the abundance of small Macoma (<1 mm) in sediment without, but not with, alternative prey.In field experiments, we manipulated the density of Arenicola in 0.25–1 m2 plots and of Nereis in 0.03 m2 cages and examined the effect on Macoma density several weeks later. We found a significant negative relation between densities of polychaetes and Macoma spat for both polychaete species in these experimental plots. Peculiarly, we found a significant positive relation between manipulated Nereis density and adult Macoma density in the cages; we cannot explain this.Consumption rates, calculated both from stomach contents and from field experiments, were 45 to 102 Macoma m−2 d−1 for Arenicola and 5 to 116 Macoma m−2 d−1 for Nereis. These values are higher than recorded consumption rates by epibenthic predators in the same area. Nevertheless, between-year differences in year-class strength could not be explained by differential abundance of these polychaetes.In conclusion, Arenicola and Nereis had a negative effect on the abundance of Macoma <1.5 mm, which was at least partly caused by direct consumption. Retusa obtusa can eat juvenile Macoma, but probably did not so in the study area, because there were no locations where Retusa and Macoma spat occurred together in the period that Macoma was <2 mm. [Copyright &y& Elsevier]