Bacterial challenge undermines the innate immune response in Hyriopsis cumingii

The ecologically and economically important bivalve species Hyriopsis cumingii is affected by numerous bacterial diseases, but the effects of these harmful bacteria on the antioxidant responses and histological alterations of H. cumingii are unclear. This study evaluated the effects of common pathogenic infections on immune function in H. cumingii. A series of antioxidant enzyme activities, total haemocyte counts, and histopathological changes were measured at 24, 48, and 96 h post infection (hpi) with three pathogens (Aeromonas hydrophila, Streptococcus agalactiae, and Pseudomonas fluorescens). All enzyme activities sharply decreased or remained constant at 24 hpi, except for acid phosphatase and alkaline phosphatase activities in the P. fluorescens-infected group. However, peroxidase activity significantly increased at 24 and 48 hpi in all treatments (p < 0.05), followed by a time-dependent decrease. The lysozyme, superoxide dismutase, and glutathione peroxidase activities decreased first and then gradually approached the control levels at 96 hpi. In our observations, there were five major types of haemocytes in H. cumingii, including mainly granulocytes and hyalinocytes. Bacterial infection increased total haemocyte counts and especially the proportion of granulocytes. Moreover, infection resulted in tissue oedema, columnar cell disarrangement, and epidermal detachment. Overall, our data provide a new perspective on the initiation of innate immunity after bacterial challenge in H. cumingii.