Settlement of the serpulid polychaete Hydroides elegans (Haswell) on the arborescent bryozoan Bugula neritina (L.): evidence of a chemically mediated relationship

The tube building polychaete Hydroides elegans Haswell was found living attached to colonies of the arborescent bryozoan Bugula neritina (L.) in Port Shelter, Hong Kong. Field data collected during the period of January through May 1996, showed that H. elegans density reached 77.6 individuals of H. elegans per g wet weight of B. neritina. Density of H. elegans on B. neritina at depths from the surface to 0.5 m was lower than that at depths below 1 m. In January-March, when there were no H. elegans settling on PVC plates or found on natural substrata, numbers on B. neritina were ca. 5 per g wet weight. H. elegans settled on B. neritina and grew rapidly as mean diameter of tubes increased from 605 mu m in February to 936 mu m in March. In laboratory experiments, larvae of H. elegans settled and metamorphosed on branches of B. neritina and on the bottom of dishes containing B. neritina leachate. Compounds extracted from the leachate of B. neritina induced 74% of H. elegans larvae to metamorphose at a concentration of 16 mu g/ml seawater, compared to 5% in dishes containing only filtered seawater (controls). Metabolites from the leachate of B. neritina which were bound to amberlite XAD-2, indicating they are lipophilic in nature, induced over 70% metamorphosis in H. elegans larvae at 56 mu g/ml seawater. A biofilm from one of four strains of bacterial isolates associated with the surface of B. neritina induced low levels of metamorphosis in H. elegans larvae, while other bacterial isolates were detrimental to the survival of juvenile H. elegans. Field experiments further demonstrated that H. elegans settled preferentially on Phytagel discs embedded with whole extracts of B. neritina over control Phytagel discs. Metabolites from B. neritina deterred feeding on alginate pellets by assemblages of local fishes in field assays. Metabolites originating from B. neritina, bacteria colonizing B. neritina, and the complex structure of B. neritina contributed to the recruitment of H. elegans to B. neritina surfaces. Hydroides elegans may gain a refuge from predation by associating with B. neritina colonies both from its structural and chemical attributes. (C) 1998 Elsevier Science B.V.