Immune Resilience and Subcellular Adaptation of Oyster Hemocytes under Hypoxic Stress

Oysters inhabit estuarine environments where oxygen levels frequently fluctuate and are often under hypoxic stress. As bivalves rely solely on innate immunity to cope with environmental challenges, the responses of their hemocyte subpopulations to hypoxia remain unclear. In this study, we investigated the immune responses of an estuarine oyster <i>Crassostrea hongkongensis</i> under moderate (2.0 mg/L) and severe (0.2 mg/L) hypoxic exposure. Flow cytometry and fluorescence imaging analyses revealed that granulocytes, the dominant immune effector subtype, exhibited dynamic structural and functional adaptations. Moderate hypoxia initially enhanced phagocytic activity at 48 h, whereas severe hypoxia suppressed phagocytosis, followed by partial recovery. Granulocytes accumulated reactive oxygen species (ROS) in a time- and severity-dependent pattern, accompanied by progressive depletion of glutathione (GSH). Mitochondrial dysfunction was evidenced by decreased mitochondrial membrane potential (MMP) and increased morphological fragmentation. Meanwhile, lysosomal activity was elevated in terms of higher quantity and area. Increased colocalization of mitochondria and lysosomes indicated the activation of mitophagy; however, under severe hypoxia, the lysosomal degradation capacity was compromised due to membrane instability. These findings indicated that granulocytes deployed compensatory organelle-level mechanisms to mitigate hypoxic stress, although persistent severe hypoxia may overwhelm these responses. Our study offers insights into immune-organelle crosstalk underlying immune plasticity in hypoxia-tolerant bivalves.