Macrofouling in unidirectional flow: miniature pipes as experimental models for studying the interaction of flow and surface characteristics on the attachment of barnacle, bryozoan and polychaete larvae

Tubes constructed of different materials were used to examine the role of hydrodynamics and surface characteristics, as measured by contact angles, on larval attachment of marine polychaetes, bryozoans and barnacles. Tubes (10 mm inner diameter) of 7 different materials, ranging in contact angle from 25° to 113°, were used. Laboratory and field experiments were conducted using a range of flow rates from 1.7 to 84.9 cm s–1. For all 3 taxa of organisms, the results of field experiments were similar to those of the laboratory experiments. Larval settlement of the polychaete Hydroides elegans and the bryozoan Bugula neritina was the highest at a flow rate of ≤2.1 cm s–1, and decreased when the flow rate was increased. H. elegans larvae settled within the narrowest range of flow rates (1.7 to 9.1 cm s–1), B. neritina larvae settled over the widest range of flow rates (1.7 to 53.0 cm s–1), and barnacle larvae (Balanus spp.) settled in an intermediate range of flow rates (4.2 to 21.2 cm s–1). Barnacle larvae did not settle when the flow rate was <2.1 or >21.2 cm s–1, and the highest settlement was recorded at the flow rate of 10.6 cm s–1. Barnacle larvae showed the highest settlement in glass tubes (smallest contact angle) and the lowest in Teflon tubes (largest contact angle), while B. neritina showed the highest larval settlement in Teflon tubes and the lowest settlement in glass tubes. H. elegans also showed the highest larval settlement in Teflon tubes, but the lowest settlement in polyurethane and polyethylene tubes. Biofilm formation changed the contact angles of all the tubes, the degree of change varied among the 7 types of tubes. Bacterial densities of biofilm on the 7 types of tubes were significantly different and were not correlated with the initial contact angles of tubes. The bacterial densities on the Teflon tubes were the same as on glass tubes. Our results suggest a very complex interaction among substratum characteristics, flow rates, and larval settlement behavior and indicate the necessity for considering the interaction when one interprets laboratory bioassay data on larval settlement of marine invertebrates.