Prey Capture by the Sea Anemone Metridium senile (L.): Effects of Body Size, Flow Regime, and Upstream Neighbors.

The sea anemone Metridium senile is a quantitatively important passive suspension feeder in hard-bottom communities on the west coast of Sweden and occurs in aggregations with different size distributions. This study tests the hypothesis that different polyp sizes have different optimal flow regimes maximizing prey capture. Results showed that prey capture by M. senile is a function of both flow regime and polyp size, and different optimal flow regimes exist for different size classes. Large anemones had a maximum feeding efficiency at the slowest flow, medium-sized anemones at moderate flow, and small anemones at moderate- to high-flow regimes. Small anemones showed consistently higher feeding rates (per unit of biomass and area of tentacle crown) at all velocities above 10 cm s-1 and exhibited less flow-induced deformation of the tentacle crown, suggesting that small anemones are better at feeding in moderate- to high-flow habitats. Different vertical projections of large and small anemones in the boundary layer could only partly account for differences in feeding success among size classes. Feeding rate was also a function of upstream conspecifics, declining asymptotically to 30% of the maximum rate. The negative effects of neighbors on feeding in aggregations with clonal rather than polyp growth appear to be compensated for by the generally higher feeding efficiency of small polyps.