Chemical control of bacterial epibiosis and larval settlement of Hydroides elegans in the red sponge Mycale adherens

The sponge, Mycale adherens, usually occurs within the fouling community of Hong Kong waters, yet its body surface is rarely fouled by other macroorganisms. In this study, sponge-associated bacteria were isolated using enrichment culture techniques and compared with indigenous bacterial isolates from an inanimate reference site in the close vicinity. Bacterial isolates were identified phylogenetically by 16S rRNA gene sequence analysis. The comparison between culturable bacterial communities from the sponge and indigenous benthic bacteria revealed differences both in the total number of isolates and their phylogenetic affiliation. Laboratory bioassays utilizing monospecies bacterial films revealed that a significant portion of sponge-associated bacteria had either an inhibitory or neutral effect on larval settlement of the fouling polychaete, Hydroides elegans. In contrast to natural biofilms, which harbor ca 65% of bacteria with at least some sort of inductive effect on H. elegans, statistical analysis showed that only 25% of sponge-associated bacteria were classified as "inductive" strains while the remaining 75% were classified as "non-inductive" strains. Waterborne metabolites of sponges affected the larvae of H. elegans in a concentration-dependent manner by either exerting a toxic or an anti-settlement effect. Organic solvent extracts of sponge tissue weakly inhibited growth of bacterial strains isolated from marine biofilms. A potential antifouling mechanism in the sponge M. adherens is discussed.