Larval settlement in the serpulid polychaete Hydroides elegans in responseto bacterial films: an investigation of the nature of putative larval settlement cue

Larval settlement in the marine polychaete Hydroides elegans (Haswell) is i nduced by certain bacteria in marine biofilms. The exact nature of the sett lement cue that larvae of H. elegans receive from these bacteria remains un known. In this study, we revealed some properties of the bacterially derive d larval settlement cue by investigating the larval settlement inductive ac tivity of two bacterial strains after various treatments. These two bacteri al strains, Roseobacter sp. and an alpha -subclass Proteobacteria, are high ly inductive to larval settlement of H. elegans. The larvae responded simil arly to Roseobacter and Proteobacteria in all the larval settlement bioassa ys, suggesting that the larval settlement-inducing substances produced by t hese bacteria may share common characteristics. First of all, the larvae di d not settle in the seawater conditioned by the bacteria attached as a film or by the bacteria that were freely suspended in seawater. The results sug gest that the putative larval settlement cue is not released into seawater and, therefore, should be associated with the surface of the bacteria. Seco ndly, formaldehyde treatment entirely eliminated the larval settlement indu ction activity of the bacterial films, and streptomycin treatment reduced t he percentage of larval settlement on the bacterial films in a concentratio n-dependent manner. Since both treatments can kill bacteria with little dam age to the surface chemistry of bacterial cells, the decline in larval sett lement is suggested be due to a reduction of the viable bacterial populatio n in the bacterial films. In fact, the reduction of larval settlement in th e streptomycin treatments coincided with the decrease in viable respective concentrations of streptomycin. These results suggest that the viability of Roseobacter and Proteobacter in is important to their settlement induction effect. Since the larval settlement induction activity of the bacterial st rains appears to correlate with their viability, we suggest that the putati ve larval settlement cue is derived from a metabolic pathway in the bacteri a and that the cue is exported to and concentrated at the extracellular pol ymer matrix of the bacterial cell, at which the larvae establish contact wi th the bacteria. The larval settlement cue may be highly susceptible to deg radation so that a metabolically active bacterial film is needed to maintai n the putative cue at a concentration that surpasses the threshold for indu ction of larval settlement.