Securing economically and ecologically significant molluscs, as our oceans warm due to climate change, is a global priority. South eastern Australia receives warm water in a strengthening East Australia Current and so resident species are vulnerable to elevated temperature and marine heat waves. This study tested whether prior exposure to elevated temperature can enhance resilience of oysters to ocean warming. Two Australian species, the flat oyster, Ostrea angasi, and the Sydney rock oyster, Saccostrea glomerata, were obtained as adults and "heat shocked" by exposure to a dose of warm water in the laboratory. Oysters were then transferred to elevated seawater temperature conditions where the thermal outfall from power generation was used as a proxy to investigate the impacts of ocean warming. Shell growth, condition index, lipid content and survival of flat oysters and condition of Sydney rock oysters were all significantly reduced by elevated seawater temperature in the field. Flat oysters grew faster than Sydney rock oysters at ambient temperature, but their growth and survival was more sensitive to elevated temperature. "Stress inoculation" by heat shock did little to ameliorate the negative effects of increased temperature, although the survival of heat-shocked flat oysters was greater than non-heat shocked oysters. Further investigations are required to determine if early exposure to heat stress can enhance resilience of oysters to ocean warming. OPEN ACCESSCitation: Pereira RRC, Scanes E, Gibbs M, Byrne M, Ross PM (2020) Can prior exposure to stress enhance resilience to ocean warming in two oyster species? PLoS ONE 15(4): e0228527. https://doi.

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7Securing economically and ecologically significant molluscs, as our oceans warm and acidify 8 due to climate change, is a global priority. South eastern Australia receives warm water in a 9 strengthening East Australia Current and so resident species are vulnerable to elevated 10 temperature and marine heat waves. This study tested whether oysters pre exposed to 11 elevated temperature or heat stress enhances resilience to ocean warming later in life. Two 12 Australian species, the flat oyster, Ostrea angasi, and the Sydney rock oyster, Saccostrea 13 glomerata, were given a mild dose of warm water or "heat shock" stress in the laboratory 14 and then transferred to elevated temperature conditions where we used the thermal outfall 15 from power generation as a proxy to investigate the impacts of ocean warming. Shell 16 growth, condition index, lipid content and profile and survival of oysters was impacted by 17 elevated temperature in the field, with flat oysters being more impacted than Sydney rock 18 oysters. Flat oysters grew faster than Sydney rock oysters at ambient temperature, but were 19 more sensitive to elevated temperature. Early exposure to heat stress did little to 20 ameliorate the negative effects of increased temperature, although the survival of heat 21 shocked flat oysters was greater than non-heat shocked oysters. Further investigations are 22 required to determine if early exposure to heat stress can act to inoculate oysters to future 23 stress and overall enhance resilience of oysters to ocean warming. 24 45