Abstract

Vibrio infection severely threatens the sustainability of the clam aquaculture industry. As the primary defenders in bivalve mollusks, hemocytes are crucial for immunity, yet their subpopulations and functional specialization are not fully investigated. In the present study, we classified clam hemocytes in Meretrix petechialis and investigated their immune responses under Vibrio parahaemolyticus infection. Three major types of hemocytes were identified in the hemolymph, which we termed as granulocyte, semi-granulocyte and hyalinocyte. After V. parahaemolyticus infection, total hemocyte counts (THC) significantly increased at 3 days post infection (dpi), while decreased significantly at 10 dpi. Hemocytes exhibited high levels of phagocytosis, apoptosis, lysosomal, and ROS in early stages after infection (1 dpi and 3dpi), and showed low production of NO, suggesting hemocytes actively initiate immune defense. We found that granulocytes were the main phagocytes, while semi-granulocytes were the main producers of lysosomes, ROS, and NO. A significant positive correlation was revealed between apoptosis and THC as well as lysosome content, while phagocytosis was primarily correlated to ROS and NO levels. Accordingly, apoptosis and phagocytic activity may represent candidate parameters for assessing the clam's health status. In conclusion, our results provide descriptive evidence for differential immune-associated responses among hemocyte subpopulations and offer a preliminary basis for health assessment in bivalves.