Flow-mediated feeding in the field: consequences for the performance and abundance of a sessile marine invertebrate

Abstract

The ecology of sessile marine invertebrates that feed on waterborne particles is closely related to their surrounding fluid environment. However, few studies have quantified how water flow affects the feeding success of such organisms under field conditions and whether the results of shortterm feeding assays can predict longer-term patterns of growth and reproduction. I examined the influence of natural variation in current speed on the feeding success, colony growth rate and population abundance of an encrusting bryozoan Membranipora serrilamella using transplant experiments at nearshore reefs along the coast of Santa Barbara, California, USA. Feeding success was highest at intermediate ambient flow speeds (10 to 12 cm s–1) and lowest at low (0 to 5 cm s–1) and fast (> 20 cm s–1) speeds. Growth rates varied such that the greatest increases in bryozoan colony size occurred at reefs where average temperatures were the coolest and current speeds were intermediate. A test of the hypothesis that high feeding success translates into faster growth rates revealed that colonies with the highest predicted consumption grew the most. Further, spatial variation in M. serrilamella abundance mirrored that of colony growth, as percent cover of the bryozoan was highest at reefs where ambient current speeds were intermediate. These nonlinear functional relationships likely result from reductions in food delivery at low flows and deformation of feeding structures at high flows. The consequences of dynamic flow conditions for both short-term and integrated measures of performance suggest that flow-mediated food acquisition plays an important role in determining the distribution, abundance and morphology of encrusting bryozoans and other sessile, suspension-feeding invertebrates.