Interactive effects of temperature and salinity on physiology, microbiome and histopathology of the Akoya oyster (Pinctada fucata Gould 1850)

Environmental fluctuations such as temperature and salinity changes pose significant stress to aquaculture species, necessitating physiological resilience for survival and production. This study investigated the impact of thermal and salinity stress on survival, physiological condition, and histological responses in Pinctada fucata (Akoya oyster) over a 21-day trial. Akoya were exposed to combinations of warm (24 °C) and cold (16 °C) temperatures across three salinities (25, 30, and 35 ppt). Body condition index (BCI) and tissue moisture content were used as physiological indicators, while histological assessments of the gill and gut were performed to evaluate tissue-level stress responses. Cold stress led to 100 % mortality before Day 21 and was associated with marked gill histopathology including cilia loss, increased mucocyte hypertrophy, and vacuolisation, suggesting compromised feeding and ion-regulatory capacity. Gut histology revealed significantly increased lamina propria thickness and altered villus thickness and lamina propria thickness ratios under cold exposure, indicating inflammatory responses potentially linked to observed gut microbial dysbiosis. Microbiome profiling showed a reduction in beneficial lactic acid bacteria and a concurrent rise in opportunistic pathogenic genera under stress.These findings demonstrate that cold temperature combined with low salinity induces pronounced physiological and histological stress in Akoya, compromising both gill and gut function and ultimately leading to mortality. This study underscores the value of integrated histological and microbiome analyses in understanding stress responses in bivalves.