Effect of dissolved oxygen concentration changes on the ultrastructure of blood cells in Manila clam (Ruditapes philippinarum).

Manila clam (Ruditapes philippinarum) inhabits environments in which dissolved oxygen (DO) concentrations are prone to changes. Hemocytes are important participants in shellfish immune responses and are sensitive to environmental changes. To investigate the effect of DO concentration changes on the ultrastructure of R. philippinarum hemocytes and provide parameters for healthy breeding and management of this species, three DO concentration change modes were designed: normoxia C treatment; normoxia followed by acute hypoxia for 24 h and acute reoxygenation for 4 h, and normoxia followed by chronic hypoxia for 48 h and then chronic reoxygenation for 8 h. Hemocytes were classified as amoebocytes (AMCs), secretory cells (SECs), thrombocytes (THCs), spherical cells (SPCs), and macrophage-like cells (MACs). Granules were present in AMC, SEC, and SPC cytoplasm, and were rarely observed in THCs and MACs. The granule function and number and morphology of various organelles differed among hemocyte types. SPCs lost their spherical structure and their intracellular ultrastructure was severely damaged, leading to apoptosis. AMCs and SECs also suffered ultrastructural damage and apoptosis. AMCs exhibited weak deformability, decreased ability to engulf and remove foreign substances, and decreased secretion ability of SECs. Acute hypoxia damaged the THC ultrastructure and acute reoxygenation triggered coagulation reactions. Chronic hypoxia and reoxygenation severely affected MAC morphology and ultrastructure. Effects of DO concentration changes on the ultrastructure and function of five hemocyte types in Manila clams were clarified, providing a cellular basis for revealing the response of Manila clams to DO concentration changes.