Induction of larval settlement in the polychaete Hydroides elegans by marine biofilms:: an investigation of monospecific diatom films as settlement cues

Larval settlement in the fouling polychaete Hydroides elegans Haswell (1883) is mediated by natural biofilms, a complex consortium of marine bacteria, benthic diatoms, protozoa, fungi and adsorbed organic matter. Currently, the inductive effect of natural biofilms on larval settlement is attributed to the bacterial component. In this paper, we present results of an investigation of the diatom component of natural biofilms with respect to induction of larval settlement in H. elegans. Thirty-two diatom strains were isolated from natural biofilms, identified and individually investigated in still-water laboratory settlement assays. A multiple pairwise comparison of larval settlement rates grouped the diatoms into 3 statistical categories: (1) 'inductive'; same percentage of larval settlement as in the positive control (glass slide with natural biofilm); (2) 'no effect'; same percentage of larval settlement as in the negative control (clean glass slide); and (3) 'weakly inductive'; percentage of larval settlement significantly higher and lower than in the negative and the positive control, respectively. Out of 32 diatom strains, 3 (10%) fell into the first, 19 (59%) into the second and 10 (31%) into the third category. The variability in larval settlement was not correlated with the density of diatoms in the monospecific films. Despite sterile measures and the use of antibiotics, diatom films under investigation were not axenic, with bacterial abundances ranging from 200 to 8000 cells mm(-2). No quantitative correlation between the bacterial abundance in diatom films and the percentage of larval settlement was observed. To evaluate the qualitative impact of the bacterial fraction in the mixed diatom-bacteria films on the induction of larval settlement, 6 representative diatom strains, comprising the 3 categories stated above, were selected to create diatom-free microbial films with the bacterial contaminants of each culture. Whilst the bacterial abundance in the non-axenic diatom films and the diatom-free control films was statistically the same, the percentage of larval settlement towards these treatments differed significantly for the diatom strains belonging to category (1) and (3). These categories comprised 4 different genera of diatoms, indicating that the observed inductive effect on larval settlement was not restricted to a certain diatom genus. Our results suggest that in addition to microbial films, benthic diatoms may significantly contribute to the mediation of larval settlement behaviour in H. elegans by marine biofilms.