Macrofouling in unidirectional flow: Miniature pipes as experimental models for studying the effects of hydrodynamics on invertebrate larval settlement

Abstract

Intake pipes are unique habitats that provide an experimental environment for studying the role of hydrodynamics and larval settlement in community development. In this study, we used 5 and 10 mm (inner diameter) tubes as experimental models to mimic intake pipe environments to study the settlement patterns at different flow rates of the bryozoan Bugula neritina, the polychaete Hydroides elegans, and barnacles of the genus Balanus. Clean, unfilmed PVC tubes were used to examine settlement of B. neritina. PVC tubes, on which bigfilm had been allowed to develop for 48 h, were used for studying attachment of H. elegans, while clean tubes were used for investigating settlement of Balanus spp. In all but very low flows, the flow velocity and Reynolds number were poorly correlated with patterns of larval settlement. A hydrodynamic measure, which is theoretically independent of the size of the tube, the so called 'velocity gradient', was well correlated with the highest settlement for all 3 species. Comparisons of results from field and laboratory experiments reveal slight differences. Settlement of the small elongate larvae of H. elegans was offset to higher shear values in the tubes of smaller diameters. Flow velocities for the highest settlement were from 1 to 3 cm s-1 for B. neritina and H. elegans, and from 3 to 15 cm s-1 for barnacles. Velocity gradients for the highest settlement of tubeworms and bryozoans ranged from 8 to 25 s-1, while those for barnacles ranged from 50 to 120 s-1 Barnacles, as reported previously by other authors, did not settle in high numbers when velocity gradients were too low or too high. Barnacles did not settle at <30 s-1 velocity gradients. Although the optimal velocity gradient (approx. 20 s-1) for settlement of B. neritina was much lower than that for barnacles, some B. neritina settled at velocity gradients of >400 s-1 H. elegans had the narrowest range of settlement in relation to flow as settlement of this species was the highest from 8 to 20 s-1 and hardly occurred above 200 s-1. In general, larval settlement in response to flow is species specific. We suggest that this species specificity is related to larval morphology, swimming ability, and behavior.