Effects of Turbidity on the Physiological Characteristics and Molecular Responses of Meretrix meretrix Juveniles
To clarify the physiological injury characteristics and molecular response mechanisms of juvenile Meretrix meretrix under elevated seawater turbidity caused by sediment disturbance such as mechanized harvesting, juveniles of M. meretrix were used as the experimental animals in this study. Four turbidity levels, namely 2, 10, 50, and 100 NTU, were set, and an intermittent stress regime consisting of 6 h exposure and 18 h recovery per day was adopted. Short-term (7 d) and long-term (60 d) exposure experiments were conducted to systematically evaluate the physiological responses and molecular regulatory characteristics of juvenile clams under turbidity stress by measuring survival, growth, oxygen consumption rate, ammonia excretion rate, antioxidant and immune enzyme activities in gill tissue, energy metabolism-related enzyme activities in the adductor muscle, and histological changes in the gills, combined with dual-tissue transcriptomic analyses of the gill and adductor muscle.The results showed that the 10 NTU group did not differ significantly from the control group in survival, growth, or most physiological indices, indicating that juvenile M. meretrix had a certain tolerance to mild turbidity variation. In contrast, survival rate, specific growth rate of shell length, specific growth rate of wet weight, and condition index all decreased in the 50 NTU and 100 NTU groups, with the strongest inhibition observed in the 100 NTU group. Under short-term stress, SOD and ACP activities in gill tissue, as well as PK and PFK activities in the adductor muscle, all showed significant fluctuations, and recovery remained incomplete under high turbidity, suggesting that persistent particle stimulation had disturbed antioxidant defense, immune status, and energy metabolism. After long-term exposure, oxygen consumption rate and ammonia excretion rate increased in the high-turbidity groups, while histological observation of the gills revealed filament shrinkage, narrowing of lamellae, particle adhesion, and epithelial vacuolation. Transcriptomic analyses further showed that turbidity stress significantly affected the expression of genes related to oxidative stress, immune regulation, energy metabolism, maintenance of cellular structure, and signal transduction in both gill and adductor muscle tissues. Overall changes were limited in the 10 NTU group, became evident in the 50 NTU group with enhanced remodeling during the recovery period, and were most pronounced in the 100 NTU group, which showed stronger oxidative pressure and metabolic reprogramming.These results indicate that the response of juvenile M. meretrix to turbidity stress is characterized by a progressive process from gill interface injury to metabolic imbalance and functional reprogramming of effector tissues. Unlike previous studies that mainly focused on a single tissue or a single phenotypic indicator, the present study revealed the response pattern of juvenile clams under intermittent turbidity exposure from multiple levels, including physiology, biochemistry, histological structure, and dual-tissue transcriptomics, and further showed that the adductor muscle is not merely a passively injured tissue, but also an important effector tissue actively involved in energy regulation and stress execution. These findings provide a reference for understanding the ecological risk and physiological basis of juvenile M. meretrix under coastal sediment disturbance.