Recruitment of the sea urchin Heliocidaris erythrogramma and the distribution and abundance of inducing bacteria in the field

Bacterial biofilms induce settlement in many marine invertebrate organisms. However, there remains a lack of understanding regarding the specific components of biofilms in situ that are responsible for the high inducing activity of some biofilms. The correlation of field recruitment patterns with laboratory settlement preferences for particular biofilms is also not well understood. We investigate if recruitment of the sea urchin Heliocidaris erythrogramma is related to the field distribution of highly inducing bacterial genera. Newly recruited sea urchins were found in highest numbers on the coralline algae Amphiroa anceps and Corallina officinalis and in low numbers on rubble and the brown alga Ecklonia radiata. Most bacterial genera that induce high levels of larval settlement of H. erythrogramma had previously been identified by 16S rRNA sequencing as Pseudoalteromonas, Vibrio or Shewanella. Oligonucleotide probes were developed for each of these genera to enable their quantification in environmental samples via catalysed reporter deposition-fluorescence in situ hybridisation (CARD-FISH). The probes were applied to biofilms on several algal species from the adult urchin habitat. All 3 genera were found in biofilms on all species of macroalgae examined. Pseudoalteromonas were found in highest numbers on the coralline alga C. officinalis and in higher numbers on red algae in comparison to brown algal species. Shewanella strains were also found in highest densities on C. officinalis. This is the first example demonstrating that bacteria that are able to induce larval settlement in the laboratory are also present in the field in the juvenile recruitment habitat.