Tumor-like lesions in the mantle of the mussel Modiolus difficilis from the Sea of Japan

Two inner growths in the mantle beneath the epithelium were found in 1 of 1 000 mussels Modiolus difficilis from Amursky Bay, Sea of Japan, within the city precincts of Vladivostok. Both growths were about 2000 mu m in maximal diameter in section and elevated slightly above the mantle surface. The ma ntle epithelium near the growths formed deep invaginations, and clusters of mucous cells were numerous beneath the epithelium. Histological and histoc hemical methods were employed. Two different kinds of growth were revealed. The off-white growth consisted of cells with thin granular or vesicular cy toplasm containing glucosaminoglycans, proteins and a small amount of neutr al polysaccharides. Growth cells were pure white in color after treatment o f preparations with 1% H2SO4 and differed markedly from the mantle cells. T he yellow growth consisted of large granular cells with neutral polysacchar ides and proteins. Although growths were composed of different kinds of cel ls, they seemed to be derived from subepithelial mucous cells. This was sup ported by histological and histochemical staining reactions of some tumor a nd mantle epithelial cells. Mitotic indices (MI) of growths and subepitheli al mucous cells were zero, MI of ciliated mantle epithelium reached 0.07 %. The lesions were areas of strongly altered mucous cells of mantle epitheli um and were non-neoplastic.