First report of mass mortality event caused by Eomarteilia granula in Manila clam Ruditapes philippinarum reared in subtidal and bottom cages on the south coast of Korea

Abstract

During a comparative experiment assessing the growth performance and health of the Manila clam Ruditapes philippinarum in subtidal suspended cage and intertidal bottom cage culture system, we observed an unexpected decline in condition index alongside a sharp increase in mortality. Histological examination revealed a progressive surge in infection prevalence of Eomarteilia granula in the digestive glands, coinciding with the mortality trends. As the infection advanced, histological sections showed progressive tissue alterations within the digestive gland tubules, including epithelial thinning, necrosis, and complete architectural collapse in severely affected individuals—indicative of profound structural disruption. To assess the pathological impact of this parasite, we applied a histopathological condition index (HPI), calculated exclusively for digestive gland tubules due to the parasite's strict tissue tropism. HPI values increased markedly toward the end of the experiment, indicating severe tissue damage. These findings suggest that E. granula-induced impairment of digestive gland function likely contributed to the observed mortality. This study represents the first documented case of E. granula-associated mass mortality in Manila clams. While the prevalence of E. granula has remained low in natural Asian environments, our results highlight that intensive culture systems may amplify infection severity and associated mortality risks.