Histopathological analysis of soft-shell clams Mya arenaria collected from 2 northwest Russian locations disclosed high prevalence of 2 pathological gill conditions. One involved the occurrence of more or less extended gill areas in which the branchial filaments showed hyperchromatic (basophilic) epithelium with some hypertrophied nuclei, which were considered presumptive signs of viral infection. Another pathological condition involved abnormal proliferation of the branchial epithelium, which lost the main differential features of the normal branchial epithelium (ciliated and simple cell layer structure), becoming non-ciliated, pseudostratified or stratified hyperchromatic epithelium with abundant mitotic figures and frequent apoptotic cells. The most complex cases involved loss of the normal branchial filament architecture, which was replaced with tumour-like growths consisting of branching, convoluted epithelial projections with a connective stroma. Images suggesting migration (invasion) of cells from the abnormally proliferating epithelium to the subjacent connective tissue, which would involve malignancy, were observed in one individual. The occurrence of both pathological conditions in clams from both locations and their co-occurrence in one clam suggest the possibility of a common, possibly viral, aetiology. Furthermore, the high prevalence of the abnormal proliferative disorder in non-polluted areas suggests an infectious aetiology. Additional studies are needed to assess a viral aetiology for the nuclear hypertrophy and/or the abnormal epithelial proliferation as well as the malignancy of the latter condition.
Keywords: Bivalve mollusc; Cancer; Disease; Gills; Histopathology; Tumour.