Establishing a histopathological index to assess health status of oyster Saccostrea cucullata and its implication in seawater quality monitoring.

Coastal monitoring commonly relies on physicochemical measurements, which provide important information on seawater condition but may not fully reflect biological responses in resident organisms. This study established a species-specific histopathological framework for the rock oyster Saccostrea cucullata and evaluated its application for coastal biomonitoring. Oysters were collected from six coastal locations in Thailand during the dry and wet seasons, coordinated with Marine Water Quality Index (MWQI) monitoring by the Pollution Control Department of Thailand. Histopathological alterations in the digestive gland and gills were characterized and integrated into a weighted histopathological index (HI) adapted from established scoring approaches. Total HI differed significantly among locations (Welch's ANOVA, F(5, 78.12) = 146.29, p < 0.001), with comparatively low values at KLA and SAM and greater tissue alteration at CHB and CRM. Season also significantly affected total HI (F(1, 162) = 24.78, p < 0.001), with generally higher values during the wet season, while a significant Location × Season interaction was detected only for digestive-gland HI (F(5, 162) = 2.46, p = 0.035). Across 12 site-season pairs, MWQI was strongly and negatively correlated with total HI (Spearman's ρ = -0.873, p = 0.0002), digestive-gland HI (ρ = -0.852, p = 0.0004), and gill HI (ρ = -0.866, p = 0.0003). Overall, the developed HI provides a practical, species-specific framework for incorporating biological information into coastal monitoring alongside conventional seawater-quality assessment.