Bacteria, Granulocytomas, and Trematode Metacercariae in the Digestive Gland of Mytilus edulis: Seasonal and Interpopulation Variation

Abstract

During the period October 1983–September 1984, mussels were sampled at monthly intervals from three populations in Denmark. The mussels were prepared for histopathological studies and 20 individuals were randomly chosen from each month and each population for a study of the prevalence and variation of the histological changes. The aim of the study was to test the hypothesis that the prevalence of parasites, bacteria, and granulocytoma differs among blue mussel populations during a year, due to the reproductive cycle and the environment. The only bacterial infection found was Chlamydia sp. and this was rare. Neither sex, month, nor population was an important determinant of the prevalence of bacteria. Metacercariae of the trematode Renicola roscovita had a prevalence of almost 54% in a population from Lilla Baelt but only 4% in a population from Isefjorden. A statistically significant interaction between month and population indicated that the prevalence varied over months but the pattern of variation was different among populations. No other parasites were found. The prevalence of granulocytomas within populations was positively correlated with the degree of anthropogenic contamination of the localities, which might suggest a functional connection. Neither month nor sex had a significant effect.