The effects of salinity stress on Crassostrea hongkongensis gill morphology, cell proliferation, and apoptosis

Mass mortality induced by high salinity has negatively impacted the cultivation of the Hong Kong oyster (Crassostrea hongkongensis), a commercially valuable bivalve, in the South China Sea; however, limited knowledge exists regarding its salinity response mechanisms. To obtain a basic understanding of this species' cellular response to salinity stress, changes in gill morphology, cell proliferation, and apoptosis were examined after transfer from a salinity of 18‰ to 6‰ low salinity and 30‰ high salinity conditions. More precisely, histological and electron microscopic observations were performed to evaluate the gill morphology of oysters sampled at 6 h, 24 h, and 48 h at the tested salinities. Proliferating cell nuclear antigen (PCNA) analysis and terminal deoxynucleotidyl (TdT)-transferase dUTP nick-end labeling (TUNEL) assays were used to detect cell proliferation and apoptosis, respectively. According to histological observations, low salinity stress resulted in swelling of gill filaments, whereas high salinity stress caused shrinkage, as confirmed by scanning electron microscopy. Moreover, the surface of non-ciliated epithelial cells in the gill filaments flattened after 24 h of exposure to low salinity, and a few cells ruptured at 48 h. When exposed to high salinity, obvious shrinkage could be observed on the external surface of non-ciliated epithelium cells at 24 h. Additionally, the number of non-ciliated cells per gill filament increased at 48 h, as well as the number and size of cavities on the filaments surfaces, compared with that under a salinity of 18‰. Transmission electron microscopy revealed no significant difference in the intracellular ultrastructure of the gills under different salinity conditions, but two types of cells were identified as potential ionocytes involved in ion transportation. Furthermore, compared with the control, cell proliferation was enhanced only in oysters exposed to low salinity for 6 h. An increase in apoptosis occurred after 6 h in high-salinity oysters and after 48 h in low-salinity oysters, indicating a slower response under low-salinity conditions. This study provides novel insights into the cellular response of C. hongkongensis to salinity stress and promotes understanding of the regulatory mechanisms of the oyster salinity response.