Induction of larval settlement in the polychaete Hydroides elegans by surface-associated settlement cues of marine benthic diatoms

Abstract

Larval settlement in the polychaete Hydroides elegans is mediated by marine biofilms, which are complex agglomerates of bacteria, diatoms, fungi and protozoa. The induction of H. ele- gans larval settlement by marine biofilms has been mainly attributed to bacteria and diatoms. In con- trast to bacteria-derived settlement cues, the nature and origin of diatom-derived settlement cues is poorly understood. In this study, we present the first investigation on the nature and origin of larval settlement cues produced by marine diatoms. Diatoms with inductive (i.e. Achnanthes sp. and Nitzschia constricta) and non-inductive (i.e. Amphora tenerrima and N. frustulum) effects on larval settlement of H. elegans were selected as model strains in this investigation. Larval settlement bio- assays with a choice between monospecies diatom films and unfilmed substratum revealed that the diatom-derived settlement cue was water-insoluble and associated with the film surface. There was a clear correlation between the surface coverage of diatoms in films and their inductive effect on larval settlement. In the case of Achnanthes sp., even the lowest surface coverage of 1.8% induced larval settlement significantly more than the control of filtered seawater. The interstitial distance between diatom cells at this low film density was within the body size range of H. elegans larvae (100 ± 30 µm). The inductive effect of diatom films on larval settlement prevailed even after heat treatments (121°C for 1 h) that completely killed the diatoms, as verified with the vital stain fluores- cein diacetate. These results suggest that the induction of larval settlement by diatoms is not related to their viability, and that, contrary to marine bacteria, diatom-induced larval settlement cues are composed of heat-stable surface components, such as capsular extracellular polysaccharides. These results not only demonstrate that different components of biofilms play inductive and inhibitive roles on larval settlement, but also that their relative space occupation deserves consideration regarding their roles as mediators of larval settlement.