The morphology of glochidia of the freshwater pearl mussel Margaritifera margaritifera L. and the development of the cyst for 50 days after glochidial infection of wild and hatchery-reared Atlantic salmon (Salmo salar L.) parr were studied using scanning electron microscopy. The microvillar surface of the inner epithelium of the glochidium, which may function in nutrition, respiration, and osmoregulation, is described. Glochidia were found to have five sensory tufts as well as marginal ciliary bands which are believed to function in directing water currents. After artificial infection of salmon parr, cyst formation was the result of shape change and migration of gill epithelial cells, but not 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 Atlantic salmon, its preferred fish host. These results are discussed in 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.

Populations of the ninespine stickleback with the complete loss of the pelvic girdle are described from Russia. In some areas in northern Karelia, these populations are reproductively isolated from parapatric populations of the 'wild type' with pelvic girdle, while in adjacent areas, populations are polymorphic and have intermediate girdle phenotypes. This seems to reflect the initial stage of divergence. Experiments in large ponds show a selective advantage of the loss of the pelvic girdle under pressure from insects predators and a selective disadvantage under predatory fish. Mate choice tests indicate reproductive isolation between pelvic phenotypes.

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The results of genetical studies of osteal plate polymorphism of threespine stickleback Gasterosteus aculeatus reported to date (Munzing 1959; Hagen and Gilbertson 1973; Avise 1976) are discussed and proposed inheritance models are considered. Results of crosses between the morphs of G. aculeatus from the White Sea are presented. An attempt has been made by the author to devise a unifying model for the inheritance of plate morphs within the whole range of G. aculeatus. The role of polymorphism in the microevolutionary processes taking place within this complex species is discussed.