Larval settlement in turbulent pipe flows

In two experiments, larval settlement was studied within a series of pipes, each containing steady, turbulent, flowing seawater. The study was designed to determine effects of flow (speed, turbulence intensity and wall shear stress) and surface orientation on rates of settlement of larvae of various taxa, and to relate variability in patterns of settlement among orientations and flows to the concomitant variability expected in rates of larval supply or delivery to the pipe walls. Patterns of settlement of all five taxa (the barnacle Balanus spp., the mussel Mytilus trossulus, the serpulid polychaete Pseudochitinopoma occidentalis, the cyclostome bryozoan Tubulipora sp., and the terebellid polychaete Eupolymnia heterobranchia) generally conformed to predicted patterns of larval supply to pipe walls (the larval transport flux). Within each taxon results were consistent in the two experiments. For these taxa, the transport fluxes that most closely matched observed settlement fluxes were predicted by assuming that larvae exhibited average speeds of advection (by swimming or sinking) of comparatively low magnitude. These best-fit larval speeds are consistent with results from previous studies of larval swimming. However, some results indicate that settlement patterns also were affected significantly by processes operating after larval contact with pipe walls. In several instances settlement patterns exhibited a strong and significant effect of wall orientation that varied in intensity with flow speed in a manner not predictable from expected patterns of larval supply. The probable causes for this effect and statistical interaction vary among taxa, but in all cases involve consideration of the habitat, behavior or success of the post-settlement, benthic individual.