Disseminated neoplasia in flat oysters Ostrea edulis from Galicia (NW Spain): occurrence, ultrastructural aspects and relationship with bonamiosis.

Disseminated neoplasia (DN) was one of the most important pathological conditions found in cultured flat oysters (Ostrea edulis) from different geographical origins grow in Galicia (NW Spain), during a two years selective breeding programme to produce oysters less susceptible to bonamiosis. Histological characteristics observed in oysters affected by DN included intense infiltration of connective tissue of various organs (gills, stomach, digestive gland and gonad) by large undifferentiated cells, with a large nucleus and a high nucleus-to-cytoplasm ratio. The main ultrastructural features were predominance of euchromatin over heterochromatin that was arrayed in small clumps in the nucleus, prominent granular nucleolus, swollen mitochondria with few cristae and high number of free ribosomes in the cytoplasm. A seasonal pattern of DN prevalence was detected, with higher values in spring-summer, but there were no significant differences between geographic origins or families within these origins. However, the intensity of the disease was significantly different between origins; oysters originating outside of Galicia (particularly those originating from Ireland) were more susceptible to develop advanced DN. DN (8%) and bonamiosis (4.9%) were found concurrently in oysters. The nature and significance of this association warrants more investigation to determine its importance, if any.