The impact of UV-B radiation on pacific oyster Crassostrea gigas health and pathogen Vibrio aestuarianus development

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UV-B (λ280–315 nm) radiation is an important aquatic ecosystem regulator which influences animal motility and orientation, immune health, and mating behaviour. However, UV-B can also induce adverse genotoxic effects on microbial, plant and animal life, including on surface water pathogens such as Vibrio species which can be pathogenic to humans and aquatic animals e.g., V. cholerae, V. splendidus, and V. aestuarianus. Thus, UV-effects on organisms are multifaceted, and can be positive or negative. Pacific oysters Crassostrea gigas are typically cultured in the intertidal zone and experience extreme fluctuations in environmental conditions including increased variations in exposure to UV radiation during the diurnal tidal flow. C. gigas stocks experience mortality events during the summer months, and these are typically associated with environmental stressors such as elevated sea and air temperature, high salinity, low nutrient availability and pathogens such as protozoa, viruses and bacteria including from the Vibrio genus such as V. aestuarianus and V. splendidus. The role of UV-B in such mortality events or in contributing to underlying physiological pathologies is unknown. To assess the impact of UV-B exposure this study used lab-based experiments whereby C. gigas seed were exposed to i) a short, high intensity UV-B dose while submerged in seawater or ii) a longer, low intensity UV-B exposure while the oysters were aerially exposed. Impacts on oyster survival and development of Vibrio aestuarianus and related and unrelated pathologies were measured. V. aestuarianus infected oysters exhibited pathologies commonly associated with vibriosis such as diapedesis, haemocyte accumulation, tissue degradation and digestive gland atrophy. UV-B exposure did not affect oyster tissue structure and morphology. Yet, results from both low and high intensity exposure trials showed that UV-B exposure increased the rate of mortality in oyster seed, with highest mortality in the smaller seed. Prevalence and intensity of V. aestuarianus infection transiently decreased in oysters exposed to UV-B. Prevalence and intensity of V. aestuarianus infection were most reduced in oysters submerged in seawater during the UV exposure treatment. Thus, UV-B caused more C. gigas death, despite a decrease in Vibrio infection. The data identify a causative factor for, thus far, unexplained mortality events during the summer. These findings will inform husbandry management of this commercially important species.