Physiological and bioenergetic responses of the undulated surf clam Paphia undulata to rapid salinity change

Rapid salinity change (RSC) is becoming more frequent in coastal waters because of extreme rainfall and freshwater discharge, posing increasing challenges to marine bivalves and ecosystems that support. The undulated surf clam (Paphia undulata), an economically important bivalve species in the South China Sea, is frequently exposed to such events, yet its physiological responses to abrupt salinity change remain poorly understood. Here, we exposed P. undulata to salinities rapidly ranging from 5 to 35 for 96 h to assess survival, gill histology, enzymatic responses, and physiological energetics. Survival declined sharply under severe hyposaline conditions (<10), whereas high survival was maintained between salinities of 25 and 30. The LC₅₀ increased from 13.9 PSU at 24 h to 19.7 PSU at 96 h, indicating a time-dependent decline in hyposalinity tolerance during prolonged exposure Histological examination showed gill filament swelling, ciliary erosion, and epithelial damage under low salinity, indicating substantial structural impairment of the gills. Salinity stress also altered ATPase activities, antioxidant responses, and ACP and AKP activities, consistent with changes in ion-transport-related, oxidative, and nonspecific immune processes. In addition, stressful salinity conditions reduced clearance and respiration rates while modifying nitrogen excretion and the O:N ratio, indicating a shift in energy allocation from growth-related processes toward maintenance. Our findings demonstrate that P. undulata may have a narrow tolerance to RSC and provide a physiological basis for assessing the vulnerability of this species to increasingly frequent salinity fluctuations in coastal aquaculture.