Physiological and Immuno-Antioxidant Responses of Mercenaria mercenaria to Salinity Stress

This study evaluated the effects of salinity stress on survival, gill morphology, and immune-, osmoregulatory-, and antioxidant-related enzyme activities in Mercenaria mercenaria. Nine salinities (0–50, practical salinity units, PSU) were tested, with 30 as the control. Survival was monitored for 168 h, and median lethal salinities under hypo- and hypersaline conditions were estimated. Gill histology was examined at 24, 72, and 168 h under salinities 15, 30, and 50, while ACP, AKP, SOD, CAT, and Na+/K+-ATPase activities were measured at five time points. M. mercenaria showed substantial tolerance to salinity stress. After 168 h, cumulative mortality was 20% at salinity 45 and 22% at salinity 20, with median lethal salinities of 52.232 and 15.097, respectively. Both low and high salinity caused gill damage, including filament shrinkage at 24 h and wider inter-filament spaces after 72 h. Enzyme activities generally increased initially, then declined and stabilized, although response timing differed among enzymes. These results indicate strong early phase physiological plasticity (<72 h), whereas prolonged stress (>72 h), which could increase mortality risk, may reflect either impaired osmoregulation, antioxidant capacity and immune defense, or the establishment of physiological acclimation. The findings support pond culture management and breeding of salinity-resilient strains.