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

Intake pipes are unique habitats that provide an experimental environment f or studying the role of hydrodynamics and larval settlement in community de velopment. In this study, we used 5 and 10 mm (inner diameter) tubes as exp erimental models to mimic intake pipe environments to study the settlement patterns at different flow rates of the bryozoan Bugula neritina, the polyc haete Hydroides elegans, and barnacles of the genus Balanus. Clean, unfilme d PVC tubes were used to examine settlement of B. neritina. PVC tubes, on w hich biofilm had been allowed to develop for 48 h, were used for studying a ttachment of H. elegans, while clean tubes were used for investigating sett lement of Balanus spp. In all but very low flows, the flow velocity and Rey nolds number were poorly correlated with patterns of larval settlement. A h ydrodynamic measure, which is theoretically independent of the size of the tube, the so called 'velocity gradient', was well correlated with the highe st settlement for all 3 species. Comparisons of results from field and labo ratory experiments reveal slight differences. Settlement of the small elong ate 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 t o 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 b ryozoans ranged from 8 to 25 s(-1), while those for barnacles ranged from 5 0 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. B arnacles did not settle at <30 s(-1) velocity gradients. Although the optim al velocity gradient (approx. 20 s(-1)) for settlement of B. neritina was m uch lower than that for barnacles, some B. neritina settled at velocity gra dients of >400 s(-1). H. elegans had the narrowest range of settlement in r elation 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 specifi city is related to larval morphology, swimming ability, and behavior.