Larval recruitment in response to manipulated field flows

Settlement responses to boundary-layer flow of several invertebrate taxa, including the hydroid Tubularia crocea, the bryozoans Bugula turrita and Schizoporella unicornis, and the tube-building polychaete Hydroides dianthus were studied in manipulated field flows in Great Harbor, Massachusetts, USA. During three experiments in 1989 and two in 1990, densities of newly-recruited larvae were measured on flat plates, whose flow regimes had been manipulated by altering the leading-edge configurations. Settlement responses to flow were strongly species-specific, with T. crocea preferring regions of both high turbulence and strong shear stress, and S. unicornis settling a most exclusively in regions of high shear stress. B. turrita settled most prominently in regions of reduced shear stress, exhibiting settlement patterns that closely approximated predictions from a model of passive particle contact. H. dianthus showed a moderate avoidance of regions with high shear stress. These results indicate that boundary-layer flows affect settlement of several common encrusting species, a probable consequence of larval behaviors such as substrate rejection or exploration in response to flow. These responses are likely to generate patchiness during initial colonization of natural habitats, and certainly affect colonization of settlement panels commonly used in marine ecological studies.