Effect of transplantation on the gonad development of the freshwater pearl mussel, Margaritifera margaritifera (L.)

In Margaritifera margaritifera (L.) individuals collected from the Ähtävänjoki river in July 1988, the histological structure of the pre-spawn ovaries and testes was neat, and a normal reproductive capacity could be predicted. The sex ratio was even, and no true hermaphrodites were present. The commonly occurring \microhermaphrodite\ gonads with small nests of the germ cells of the opposite sex may show that the mussels are capable of hermaphroditism or sex reversal. The experimental transplantation of freshwater pearl mussels for conservation purposes 10 km upstream in the same river in 1987 did not disturb the mussels' gonad development. This was assessed from examination of the histological structure of the gonads in July, a year after the transfer. The sizes of testis follicles, the numbers of sperm morulae and oocytes per follicle, the occurrence of microhermaphrodite or out-of-phase follicles were similar both in endemic and transplanted mussels. From these results it could be concluded that transplantation of freshwater pearl mussels can be undertaken in a home river without reducing the gamete producing capacity of the mussels during the breeding season of the next year.