Comparative Health Assessment of Crassostrea belcheri from Breeding and Farming Sites in Thailand: Histopathological, Apoptotic, and Molecular Evidence

Oyster health is important for aquaculture productivity and sustainability. In Thailand, the white scar oyster, Crassostrea belcheri, is being promoted for cultivation, yet its health status has not been compared between research breeding and community farming sites. This study evaluated histopathological features, ultrastructure, apoptosis, and defender against apoptotic death 1 (dad1) gene expression in sexually mature C. belcheri collected from these two sites. Gill tissues were examined by histology, transmission electron microscopy, TUNEL assay, and gene expression analysis, while organ condition was assessed using a Health Assessment Index (HAI). The proportion of TUNEL-positive cells in the gills and mantle differed significantly between sites (p < 0.05), with higher levels in oysters from the farming site. In contrast, TUNEL-positive cells in the digestive gland did not differ significantly between sites, although brown cells were observed only in the digestive gland of oysters from the breeding site, suggesting possible physiological stress. To assess the expression level of dad1 in oysters cultured under different conditions, RT-qPCR revealed no significant difference between the two sites. The breeding site also had lower temperature and salinity than the farming site. Overall, these findings suggest that site-specific environmental conditions may influence gill health and stress-related responses in C. belcheri, providing baseline information for oyster health assessment and aquaculture management.