By Christopher Chapman
An investigation was undertaken into the cause of Pacific Oyster (Crassostrea gigas) larvae epizootics occurring in a commercial hatchery located at Bicheno, Tasmania. An extensive monitoring survey was conducted at the hatchery to characterise the microbiological environment in the immediate vicinity of the larvae associated with different production outcomes. It was shown that microbial communities of the larvae and water column varied primarily with larvae age and sampling period and that the most likely cause of variability with sampling period was variability in the seawater. Altered culture conditions changed the microbial communities of the water column but larvae communities were shown to be largely resistant to change experienced in the water column. Larvae microbial communities were also shown to be closely related to the indigenous microbial communities of the egg. Thus formation of the indigenous microbial community during spawning and fertilisation may be a control point for management of the microbial composition of the larvae and potentially for managing disease incidence. The presence of predominant non-typical marine species of the genera, Sphingomonas and Ramlibacte, in eggs and larvae samples, indicated a non-marine source of contamination occurring during spawning and fertilisation.
The second investigation characterised the microbial environment associated with the emergence of disease symptoms, and the underlying cause of disease. There was no predominant characteristic microbial community in the larvae or water column associated with disease and no recognised bacterial pathogens were detected using culture-independent methods of assessment. Advanced stages of disease may have been associated with displacement of predominant members of the indigenous microbial community of larvae.
Challenge tests utilising bacterial isolates from diseased larvae indicated that none could be considered particularly pathogenic when compared with the known pathogen Vibrio tubiashii. Vibrio population numbers peaked with the emergence of disease symptoms but remained less than 1% of the total population as indicated by prevalence in 16S rRNA gene clone libraries. Larvae aggregative behaviour near the tank bottom prior to the development of definitive disease symptoms indicated a non-microbiological primary cause of disease, or a microbiological etiology.
Following the monitoring study an investigation was undertaken into the effect of environmental stressors on the susceptibility of larvae to bacterial challenge. Larvae behaviour was modified at copper levels as low as 2.5 ppb, which indicated that behaviour could be used as a sensitive biomarker of copper stress and might also be of use in assessment of other types of chemical stress and susceptibility to bacterial disease. The behavioural response to different concentrations of copper was non-linear and differed with duration of exposure, indicating that behavioural assessments should be made across a range of concentrations and also across a 24 to 48 h time period. A preliminary investigation into heat shock treatment indicated that heat shock could improve larvae performance under bacterial challenge.