Effects of halogenated metabolites from infaunal polychaetes on larval settlement of the spionid polychaete Streblospio benedicti

Considering that a variety of infaunal polychaetes release halogenated metabolites with potent bactericidal effects into the surrounding sediment, we tested the hypothesis that polychaete- derived halometabolites may either pose a direct negative effect on larvae or alter the bacterial com- munity in surface sediments, and thus indirectly influence larval settlement. Commercially available halogenated compounds identical or similar to the dominant halometabolites found in Streblospio benedicti and Capitella sp. I served as experimental proxies in single- and multiple-choice settlement assays with larvae of S. benedicti. In multiple-choice assays, the hypothesis of a direct influence of halogenated proxies on larval settlement was verified with 1-chlorononane at concentrations of 3.5 µg g -1 sediment. A similar effect was not observed with 2,6-dibromophenol. The no-choice assays did not show a direct influence of halometabolites on polychaete larvae. On the other hand, the hypothesis of an indirect influence of halogenated proxies on larval settlement via modification of the sediment microbial assemblage was clearly rejected. However, an analysis of similarity of the PCR-amplified bacterial genes coding for 16S rRNA corresponding to distinct phylotypes revealed a significant change in bacterial community richness between untreated and natural sediment spiked and incubated with halogenated proxies for 7 d. The change was characterized by the absence of up to 13 phylotypes and the appearance of up to 11 previously undetected phylotypes. The shift in bacterial community richness of sediments exposed to the halogenated proxies was not recognized by larvae, indicating that the halogenated proxies elicited no differential response in larvae of S. benedicti.