Health risk assessment of the hooded oyster, Saccostrea cucullata (Born, 1778) from Libong Island, Thailand

Abstract

Libong Island is an important seagrass bed in Thailand, which has been impacted by the environmental crisis. The environmental changes should have negatively affected the health status of aquatic animals in the area, but few studies are available on this topic. In this study, the health status of a sentinel species, the hooded oyster Saccostrea cucullata, was addressed using organ development of field-collected S. cucullata associated with their shell size distributions, the multi-organ histopathology and apoptotic analysis along with the measurement of heavy metal bioaccumulation. The water quality parameters were not significantly different between the sampling areas except for the water temperature (p<0.05). S. cucullata from the Stone Bridge site had smaller shell lengths and higher condition factors than those from Dugong Tourism by Drones except for the size of the organ. Histologically, the highest mean length of gill lamellae (192.1±3.92 at 2.1-3 cm). The thinnest of the mantle epithelium (73.78±3.08 µm in 4.1-5 cm) of S. cucullata showed and statistically significant difference between the sampled locations (Specific F value=0.1783 and 0.8605, p<0.01, p<0.0001). The density of mucous-secreting cells (Msc) was more prominently distributed in the digestive gland than in other tissues. The 2.1-3 cm group or bigger had mature gonads with protandric characteristics, showing rapid sexual differentiation in the Stone Bridge site. Moreover, gills, digestive glands, gonads, and mantles of S. cucullata were collected from different monitoring a reason this island including the Stone Bridge site and the Dugong Tourism by Drones site. The highest health assessment index HAI values were recorded in the gill, whereas the lowest values were observed in the digestive glands. Histopathological alterations and heavy metal bioaccumulation were found in samples from both sites, but most HAI values and the density of apoptotic cells were significantly higher in samples from the Dugong Tourism by Drones site than those from the Stone Bridge site p<0001. The highest values of heavy metal concentrations were in the digestive gland: zinc 638.38±3.96 mg/kg1 dry weight, copper 190.52±0.44 mg/kg1 dry weight, lead 23.36±0.45 mg/kg1 dry weight, and cadmium 961±0.18 mg/kg1 dry weight. The hazard quotient HQ values were > 1, representing the moderately harmful levels for human consumption. The current knowledge about the use of multi-organ histopathology with the heavy metal bioaccumulation of S. cucullata might be noted in its undisturbed health and suggested that this oyster can be used to monitor marine pollution in Thailand. Long-term monitoring assessment and the environmental protection of Libong Island should be focused on as a more accurate environmental risk assessment.

-