Histopathological assessment of the health status of Mytilus chilensis (Hupé 1854) in southern Chile

Mussel (Mytilus chilensis) farming is an important economic and social activity in the south of Chile. With a landing of 302,642 tons in year 2016, Chile is the second mytilid producer after China, and the first exporter worldwide. To be better prepared to protect this industry, the health status of a total number of 478 mussels (M. chilensis) from the Los Lagos Region, in the south of Chile, was assessed. Baseline information on symbionts, parasites and pathological conditions of both cultivated and natural mussels was collected using histology. Organisms detected included: intracellular microcolonies of bacteria (IMC), located in the epithelium of the gill and of digestive gland tubules; a microsporidian resembling Steinhausia mytilovum hosted by oocytes; two gill ciliates; a copepod; and a digenean trematode. Pathological conditions included neoplasia, hemocyte aggregates within digestive epithelia, lipofuscin-like pigments in various tissues, and gill epithelium desquamation. The prevalence of each finding was assessed and compared statistically between sites and between cultured and natural populations. The infection intensity of them was low, and no OIE listed parasite was detected. Of all the findings of this study, those that could be of concern are the IMC, neoplasia, and the Steinhausia mytilovum-like microsporidian. This study provided baseline data necessary for the design of a larger, seasonal sampling, which will allow to assess the feasibility of a permanent monitoring program to protect the huge mussel cultivating industry of the Los Lagos Region in Chile.