Ontogenetic dynamics of the shell convolution diameter of the river snail Viviparus viviparus (Gastropoda, Pectinibranchia, Viviparidae)

The dynamics of the shell convolution diameter with regard to the mollusk age was studied for the river snail Viviparus viviparus. It was found that the diameter of the first (juvenile) convolution remained constant during the ontogenetic development. A weak, but significant increase in the diameter of the second and the third shell convolutions (also juvenile) was observed during the first year of postembryonic development. The forth and the fifth shell convolutions were formed during the first year of mollusk life after hatching; their diameters increased gradually during the second year, and remained constant for the remainder of the life span.