Physiological and Morphometric Responses of Cultured Scallops (Chlamys farreri and Argopecten irradians) to Summer High-Temperature Period Across Water Depths in Tongyeong Coastal Waters, Korea

Summer thermal stratification and elevated seawater temperatures pose critical physiological challenges for cultured bivalves in coastal ecosystems. This study evaluated the depth-dependent impacts of thermal stress on physiological acclimation capacity, shell morphometric changes, individual wet-weight responses, and oxidative stress responses in temperate Chlamys farreri and subtropical Argopecten irradians. Scallops were cultured across three depth strata (5, 15, and 25 m) during the peak summer period. In the 5-m surface layer, prolonged exposure to elevated water temperatures was associated with high cumulative mortality and altered growth-related traits among surviving scallops. Mortality patterns were descriptive because net-specific mortality data were unavailable, preventing statistical comparisons among depths. Although individual body weight increased among surviving scallops at 5 m, this pattern may have been influenced by survivorship bias and mortality-induced reductions in stocking density rather than favorable growth conditions. Elevated hemolymph levels of hydrogen peroxide and malondialdehyde, together with reduced total antioxidant capacity (TAC), suggested increased circulating oxidative stress and altered antioxidant capacity. Among the three depths evaluated at this study site, 15 m exhibited a comparatively moderate thermal environment and relatively balanced physiological performance. At 25 m, lower seawater temperatures may have alleviated thermal stress, whereas potential differences in food availability and other depth-dependent environmental factors may have limited individual body weight. These findings suggest that culture-depth management may have potential for mitigating physiological stress during summer high-temperature periods, although its effectiveness requires further validation. However, dissolved oxygen, chlorophyll-a, pH, and other potentially relevant environmental variables were not measured, limiting the interpretation of depth-dependent responses. Differences in initial body size, experimental starting dates, sampling intervals, and hemolymph processing between the two species preclude direct interspecific comparisons of physiological responses and intrinsic thermal tolerance.