On the histology, ultrastructure and function of the spermoviduct and free oviduct in egg-retaining door snails (Pulmonata: Clausiliidae)

Abstract

We studied the anatomical, histological and ultrastructural changes of the spermoviduct and free oviduct in three closely related clausiliid snails (Baleinae) in relation to their different reproductive strategies: oviparity in Laciniaria plicata, egg retention in Vestia gulo and viviparity in Alinda biplicata. Since egg retention occurs in the lower parts of the spermoviduct and free oviduct, these organs were studied in detail to determine their structural diversity. The results of anatomical studies showed that the length proportions of the free oviduct and spermoviduct are different between oviparous snails and egg-retaining/viviparous snails. The allospermiduct of the free oviduct displays significant morphological and ultrastructural dissimilarities in snails that differ in reproductive strategies. These differences mainly concern the type of subepithelial secretory cells and their secretory products. In egg-retaining snails, the subepithelial cells are serous and their secretory products create a suitable environment (oncotic pressure) for retained eggs, whereas in oviparous snails the serous cells are absent.