Quantification of Photobacterium swingsii and characterisation of disease progression in the New Zealand Greenshell™ mussel, Perna canaliculus.

Greenshell™ mussels (Perna canaliculus) are endemic to New Zealand and support the largest aquaculture industry in the country. Photobacterium swingsii was isolated and identified from moribund P. canaliculus mussels following a mass mortality event. In this study, a challenge experiment was used to characterise, detect, and quantify P. swingsii in adult P. canaliculus following pathogen exposure via injection into the adductor muscle. A positive control (heat-killed P. swingsii injection) was included to account for the effects of injection and inactive bacterial exposure. Survival of control and infected mussels remained 100% during 72-hour monitoring period. Haemolymph was sampled for bacterial colony counts and haemocyte flow cytometry analyses; histology sections were obtained and processed for histopathological assessments; and adductor muscle, gill, digestive gland were sampled for quantitative polymerase chain reaction (PCR) analyses, all conducted at 12, 24, 48 h post-challenge (hpc). The most profound effects of bacterial injection on mussels were seen at 48 hpc, where mussel mortality, haemocyte counts and haemolymph bacterial colony forming were the highest. The quantification of P. swingsii via qPCR showed highest levels of bacterial DNA at 12 hpc in the adductor muscle, gill, and digestive gland. Histopathological observations suggested a non-specific inflammatory response in all mussels associated with a general stress response. This study highlights the physiological effects of P. swingsii infection in P. canaliculus mussels and provides histopathological insight into the tissue injury caused by the action of injection into the adductor muscle. The multi-technique methods used in this study can be applied for use in early surveillance programs of bacterial infection on mussel farms.

Histopathological investigation of four populations of abalone (Haliotis iris) exhibiting divergent growth performance.

The black-foot abalone (paua), Haliotis iris, is a unique and valuable species to New Zealand with cultural importance for Maori. Abalone are marine gastropods that can display a high level of phenotypic variation, including slow-growing or 'stunted' variants. This investigation focused on identifying factors that are associated with growth performance, with particular interest in the slow-growing variants. Tissue alterations in H. iris were examined using histopathological techniques, in relation to growth performance, contrasting populations classified by commercial harvesters as 'stunted' (i.e., slow-growing) and 'non-stunted' (i.e., fast-growing) from four sites around the Chatham Islands (New Zealand). Ten adults and 10 sub-adults were collected from each of the four sites and prepared for histological assessment of condition, tissue alterations, presence of food and presence of parasites. The gut epithelium connective tissue, digestive gland, gill lamellae and right kidney tissues all displayed signs of structural differences between the slow-growing and fast-growing populations. Overall, several factors appear to be correlated to growth performance. The individuals from slow-growing populations were observed to have more degraded macroalgal fragments in the midgut, increased numbers of ceroid granules in multiple tissues, as well as increased prevalence of birefringent mineral crystals and haplosporidian-like parasites in the right kidney. The histopathological approaches presented here complement anecdotal field observations of reduced seaweed availability and increased sand incursion at slow-growing sites, while providing an insight into the health of individual abalone and sub-populations. The approaches described here will ultimately help elucidate the drivers behind variable growth performance which, in turn, supports fisheries management decisions and future surveillance programs.

Interactive effects of elevated temperature and Photobacterium swingsii infection on the survival and immune response of marine mussels (Perna canaliculus): A summer mortality scenario.

The New Zealand Greenshell™ mussel (Perna canaliculus) is an economically important aquaculture species. Prolonged increases in seawater temperature above mussel thermotolerance ranges pose a significant threat to mussel survival and health, potentially increasing susceptibility to bacterial infections. Using challenge experiments, this study examined the combined effects of increased seawater temperature and bacterial (Photobacterium swingsii) infection on animal survival, haemocyte and biochemical responses of adult mussels. Mussels maintained at three temperatures (16, 20 and 24 °C) for seven days were either not injected (control), injected with sterile marine broth (injection control) or P. swingsii (challenged with medium and high doses) and monitored daily for five days. Haemolymph and tissue samples were collected at 24, 48, 72, 96, 120 h post-challenge and analysed to quantify bacterial colonies, haemocyte responses and biochemical responses. Mussels infected with P. swingsii exhibited mortalities at 20 and 24 °C, likely due to a compromised immune system, but no mortalities were observed when temperature was the only stressor. Bacterial colony counts in haemolymph decreased over time, suggesting bacterial clearance followed by the activation of immune signalling pathways. Total haemocyte counts and viability data supports haemocyte defence functions being stimulated in the presence of high pathogen loads at 24 °C. In the gill tissue, oxidative stress responses, measured as total antioxidant capacity and malondialdehyde (MDA) levels, were higher in infected mussels (compared to the controls) after 24h and 120h post-challenge at the lowest (16 °C) and highest temperatures (24 °C), indicating the presence of oxidative stress due to temperature and pathogen stressors. Overall, this work confirms that Photobacterium swingsii is pathogenic to P. canaliculus and indicates that mussels may be more vulnerable to bacterial pathogens under conditions of elevated temperature, such as those predicted under future climate change scenarios.