Effects of dissolved oxygen changes on the gill tissue structure of Ruditapes philippinarum

The Ruditapes philippinarum mainly inhabits estuaries, inner bays and coastal mudflat. Affected by tides, rainfall and seasons, the dissolved oxygen in their living environment is prone to change, which will affect their growth, survival and metabolism.The gills of bivalves are both respiratory organs and feeding organs used for filtering food. In order to investigate the effect of dissolved oxygen changes on the gill tissue structure of R. philippinarum and provide parameters for their aquaculture management, we designed three modes of dissolved oxygen changes, namely, continuous maintenance of regular dissolved oxygen C treatment, regular dissolved oxygen-rapid hypoxia for 24 h-rapid reoxygenation for 4 h AHR treatment, and regular dissolved oxygen-slow hypoxia for 48 h-slow reoxygenation for 8 h CHR treatment. Then tissue sections and immunohistochemstry methods were used to analyze the effect of dissolved oxygen changes on the gill tissue structure. The results of tissue sectioning showed that dissolved oxygen changes would affect the morphology and structure of gill tissue in R. philippinarum. Hypoxia caused gill filaments to widen, surface epithelial cells to damage, cilia to fall off, the sponge like blood cavity tissue to become loose, gill lumen gaps to become larger, and there were cell fragments inside. Hypoxia reoxygenation significantly changed the morphology of the gill flap, and the damage to the tissue cells of the outer gill flap was more severe than that of the inner gill flap. Slow hypoxia for 48 h and slow reoxygenation for 8 h caused more severe damage to the gill tissue structure than quick hypoxia for 24 h and quick reoxygenation for 4 h. The immunohistochemical results showed that both rapid hypoxia for 24 h and slow hypoxia for 48 h increase the ROS level in the gill tissue, while reoxygenation only reduced the ROS level a little in the gill tissue. There was no significant repair effect on the damage caused by hypoxia in a short period of time. The gill tissue cells damaged by slow hypoxia for 48 h and reoxygenation for 8 h were numerous and diffuse, with unclear nuclear cytoplasmic boundaries, the degree of damage to gill tissue cells was higher than that of rapid hypoxia for 24 h and reoxygenation for 4 h. In summary, this experiment found that both slow hypoxia for 48 h and rapid hypoxia for 24 h could cause damage to the cellular structure of the gill tissue. The damage to reoxygenated tissue cells could not be repaired in a short period of time. Slow hypoxia for 48 h caused more severe damage to gill tissue structure than fast hypoxia for 24 h. The tissue cell damage continues to worsen even after 8 hours of slow reoxygenation. The results of this study suggested that the management of R. philippinarum farming need to consider the stable control of dissolved oxygen, and efforts should be made to avoid changes in dissolved oxygen, especially the damage to the gill tissue of clams caused by long-term chronic hypoxia stress. This study provides parameters for dissolved oxygen control in the process of R. philippinarum aquaculture.