SETTLEMENT OF THE SERPULID POLYCHAETE HYDROIDS-ELEGANS (HASWELL) ON THE ARBORESCENT BRYOZOAN BUGULA-NERITINA (L.) - EVIDENCE OF A CHEMICALLY MEDIATED RELATIONSHIP

The tube building polychaete Hydroides elegans Haswell was found livin g attached to colonies of the arborescent bryozoan Bugula neritina (L. ) in Port Shelter, Hong Kong. Field data collected during the period o f January through May 1996, showed that H. elegans density reached 77. 6 individuals of H. elegans per g wet weight of B. neritina. Density o f H. elegans on B. neritina at depths from the surface to 0.5 m was lo wer than that at depths below 1 m. In January-March, when there were n o H. elegans settling on PVC plates or found on natural substrata, num bers 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. neri tina and on the bottom of dishes containing B. neritina leachate. Comp ounds extracted from the leachate of B. neritina induced 74% of H. ele gans 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 amb erlite XAD-2, indicating they are lipophilic in nature, induced over 7 0% metamorphosis in H. elegans larvae at 56 mu g/ml seawater. A biofil m from one of four strains of bacterial isolates associated with the s urface of B. neritina induced low levels of metamorphosis in H. elegan s larvae, while other bacterial isolates were detrimental to the survi val of juvenile H. elegans. Field experiments further demonstrated tha t H. elegans settled preferentially on Phytagel discs embedded with wh ole extracts of B. neritina over control Phytagel discs. Metabolites f rom B. neritina deterred feeding on alginate pellets by assemblages of local fishes in field assays. Metabolites originating from B. neritin a, bacteria colonizing B. neritina, and the complex structure of B. ne ritina contributed to the recruitment of H. elegans to B. neritina sur faces. Hydroides elegans may gain a refuge from predation by associati ng with B. neritina colonies both from its structural and chemical att ributes. (C) 1998 Elsevier Science B.V.