The chemical nature of bacteria-derived larval settlement cue(s) for the marine polychaete Hydroides elegans (Haswell)

The process of larval settlement (i.e. attachment and metamorphosis) in the marine polychaete Hydroides elegans has been studied extensively in recent years. Bacteria in marine biofilms are the natural inducers for the settlement process, but the mechanism of induction is poorly known. The main goal of this thesis research was to determine the chemical nature of the bacteria-derived larval settlement cues) for H. elegans, and the specific objectives were: (1) to investigate the species-specificity of bacterial induction of larval settlement in H. elegans; and (2) to determine the bacterial components) and/or products) which triggers) the settlement process.; About 60% of bacterial isolates from natural biofilms could induce larval settlement of H. elegans and these bacteria occurred over a wide range of taxa. Moreover, bacterial isolates affiliated to the same genus had tremendously different activities for larval settlement. Therefore, the settlement response of larvae to bacterial induction was not likely to be taxon-specific. The diversity of bacterial types that induce larval settlement of H. elegans suggests that the settlement cues) may be a substances) commonly produced by many bacteria, unless larvae have a highly versatile set of chemoreceptors to cope with different types of cues from different bacteria. Many bacterial isolates, even for generally inductive ones, evoked variable levels of larval settlement among trials of bioassays, suggesting that many factors influence the optimal expression of settlement cue(s).; Extracellular polymers (exopolymers) of bacteria induced a pre-attachment behavior in the larvae of H. elegans. This effect was not likely induced by the polysaccharide components of exopolymers. In contrast, waterborne metabolites of bacteria induced the larvae to undergo a complete settlement process. The presumptive settlement cues) were non polar, <0.5 kD in molecular size, and likely be volatile. Bacterial isolates that appeared to be non-inductive for larval settlement of H. elegans in laboratory bioassays also produced the presumptive cue(s). Despite the presumptive cues) being waterborne metabolites, direct contact with a bacterial film is a prerequisite for the induction of larval settlement. It was suggested that exopolymers on bacterial cell surface mediated larval settlement by entrapping and concentrating the presumptive cues) for larval chemoreception.