Exposure of biofilms to meiofaunal copepods affects the larval settlement of Hydroides elegans (Polychaeta)

This study reports significant differences in larval settlement of the polychaete Hydroides elegans (Haswell, 1883) on biofilms treated either by different densities of live harpacticoid copepods or conditioned by particulate organic material (POM) retrieved from harpacticoid copepod cultures. The copepods used in this study were the facultatively predatory Tisbe japonica and the detritivorous Nitocra spinipes. The resulting biofilms covering experimental surfaces exhibited differences in bacterial abundances, bacterial community profiles and bioactivity, as evidenced by differential attractivity provided as percentages of larval settlement. The density of settled larvae decreased on biofilms modified by increasing numbers of copepods. The highest bacterial densities (16085 +/- 915 cells mm super(-2)) were found on natural biofilms conditioned with POM from cultures of T. japonica. The lowest bacterial densities (4020 +/- 765 cells mm super(-2)) were found on biofilms grown in the presence of the highest numbers of live T. japonica. Spearman-rank correlation results revealed a positive trend between increasing bacterial densities and larval settlement. Increasing copepod abundances were correlated with reduced bacterial densities and caused changes in the bacterial community profiles, which were identified by terminal restriction fragment analyses. Changes in microbial biofilms due to the conditioning effects of copepods or their remains/exudates are assumed to be responsible for differential larval settlement. Hence, the present laboratory study demonstrates that effects of predators can be mediated by biofilms and affect larval settlement indirectly.