Composition and density of bacterial biofilms determine larval settlement of the polychaete Hydroides elegans

Larvae of the polychaete worm Hydroides elegans settle and metamorphose in response to marine biofilms. Phylogenetic relationships among 4 marine-biofilm bacterial species identified by 16S rDNA sequences were not predictive of their inductive capacity. Two bacterial species separated by a genetic distance of 30 % shared the greatest capacity for inducing metamorphosis in H. elegans. Two bacterial strains with only a 3 % divergence in 16S rRNA gene sequences were very different; one strain induced metamorphosis strongly while the other one did not. The percent of larvae that metamorphosed correlated positively with bacterial density in either natural biofilms or biofilms composed of a single bacterial species. When results of tests across 4 bacterial species were compared, the most inductive bacterial species was effective in much lower biofilm densities than weakly or non-inductive bacteria. Evidence presented here indicates that the cue that triggers settlement and metamorphosis in larvae of H. elegans is not unique to a single bacterial taxon and is likely to be an insoluble, surface-bound material.