Polydora index and histopathological lesions in the mangrove oyster Crassostrea gasar from the Aby and Ebrié lagoon systems

The mangrove oyster Crassostrea gasar plays a vital ecological and socio-economic role in the Ébrié and Aby lagoons of Côte d’Ivoire. However, it is threatened by infestations of boring worms from the Polydora genus, which cause shell damage that can reduce both its commercial value and physiological health. This study aims to quantify the Polydora Index (PI) and the prevalence of tissue lesions in C. gasar at three sites, Azito and Grand-Bassam on Ébrié Lagoon and Assinie on Aby Lagoon, based on a sample of 1,080 oysters. Infestation levels were classified into five severity classes and histopathological lesions were assessed in the mantle, gills, and gonads. Results show that Assinie had the lowest PI (0.16) and only exhibited gill necrosis (21.1%). Grand-Bassam and Azito had higher PI (0.24 and 0.33, respectively) and a greater prevalence of lesions: necrosis (32.5% to 51.4%), abscesses (12.2% to 41.9%), and neoplasia (14.2% to 28.6%). A clear correlation was observed between shell infestation levels and lesion prevalence, especially in Azito, where up to 85% of infested oysters showed necrosis. The study highlights the need to monitor such infestations as indicators of environmental stress and potential threats to oyster farming.