GLOCHIDIUM MORPHOLOGY OF THE FRESH-WATER PEARL MUSSEL (MARGARITIFERA-MARGARITIFERA) AND GLOCHIDIOSIS OF ATLANTIC SALMON (SALMO-SALAR) - A STUDY BY SCANNING ELECTRON-MICROSCOPY

The morphology of glochidia of the freshwater pearl mussel Margaritife ra margaritifera L. and the development of the cyst for 50 days after glochidial infection of wild and hatchery-reared Atlantic salmon (Salm o salar L.) parr were studied using scanning electron microscopy. The microvillar surface of the inner epithelium of the glochidium, which m ay function in nutrition, respiration, and osmoregulation, is describe d. Glochidia were found to have five sensory tufts as well as marginal ciliary bands which are believed to function in directing water curre nts. After artificial infection of salmon parr, cyst formation was the result of shape change and migration of gill epithelial cells, but no t hyperplasia. The process of cyst formation lasted from 9 to 12 h. No mortality of salmon parr occurred during the experiment. Based on our study, glochidia appear to have a negligible effect on the gills of A tlantic salmon, its preferred fish host. These results are discussed i n the context of the possibility of a mutually beneficial coexistence of Atlantic salmon and pearl mussels in northern European rivers such as the Varzuga River in Russia.