Settlement of generalist marine invertebrate herbivores in response to bacterial biofilms and other cues

Most marine invertebrates have a complex life cycle involving a benthic adult phase, and a planktonic larval phase. The process whereby tiny larvae are able to locate, settle and metamorphose in a habitat where juveniles are then capable of successfully establishing themselves is a key stage in the life cycle and a central theme of current marine research. Bacterial biofilms are an important settlement cue for many larvae, and it appears that particular strains within environmental communities may be responsible for the inducing ability of some biofilms. The focus of this thesis is the importance of biofilms for larval settlement of the blacklip abalone Haliotis rubra and the sea urchin Heliocidaris erythrogramma. Larval development of the sea urchin Centrostephanus rodgersii was also examined, but this species was problematic for a larval settlement study. H. rubra larval settlement occurred in response to several macroalgal species. Biofilmed (but otherwise abiotic) surfaces did not induce settlement of H. rubra larvae and reduction of surface films of bacteria and diatoms on inducing algae did not reduce the settlement response. Macroalgae, particularly green algal species, may play an important role in the recruitment of H. rubra larvae in the field and can be used to induce settlement in hatcheries. H. erythrogramma settled in response to a range of surfaces with highest settlement on coralline algae. Settlement was reduced by autoclaving plants and treating plants with antibiotics. Molecular and culture based analysis revealed a shift in microbial community structure between plants treated with antibiotics and unmanipulated plants. Many bacterial strains, dominated by the genera Pseudoalteromonas, Shewanella and Vibrio, induced larval settlement. Three probes targeting Pseudoalteromonas, Shewanella and Vibrio were developed for CARD-FISH, enabling quantification of these genera in biofilms on algae. The three genera were found in highest numbers on coralline algae and in variable numbers on the surfaces of other algae. Recruitment of H. erythrogramma also occurs in highest numbers on coralline algae, and in low amounts on co-occurring algae. This is the first example demonstrating that bacteria that induce settlement in the laboratory are also present in the juvenile recruitment habitat