3D-reconstruction of the freshwater snail Borysthenia naticina, Menke 1845 (Heterobranchia: Valvatidae) with a closer look on the genital system.

Among the - basal heterobranch gastropods (the paraphyletic Allogastropoda, Haszprunar 1985) the Valvatidae (Gastropoda, Ectobranchia) are exceptional by their overall freshwater habitat. Most data on the anatomy of the family are based on the large and most common Valvata piscinalis (O.F. Müller, 1774). In contrast, the likewise quite large (max. 5.5 mm) Borysthenia naticina (Menke 1845) is only poorly known and insufficiently described. Lindholm (1927) first reported an ovoviviparous kind of reproduction for this species, whereas Valvata species produce regular spawn. Accordingly, the genital apparatus of Borysthenia naticina was the main focus in our work. By accident, it was possible to gain living animals from the Bavarian Danube river. These specimens spawned in captivity: the eggs included quite well developed embryos (contrary to cleaving eggs in Valvata) and hatchlings occurred already after three days (vs. weeks in Valvata). Thus, Lindholms suggestion of an ovovivipariuos development needs at least to be widened, although we cannot exclude that true ovovivipary occurs in other populations or under certain environmental circumstances. Contrary to our expectations, there were no special structures within the whole genital system correlated with such an accelerated ontogeny within the parent´s body.

For detailed analysis of the genital tract and other structures, we used 3D surface and volume rendering techniques using AMIRA® based on histological semi-thin slices. This method suits perfectly for small and twisted animals like gastropods. For a detailed view on fertilization and development Borysthenia naticina was investigated in two different stages (juvenile and adult) so it was possible to create a 3D-study over time. As in all valvatids a taenioglossate radula can be found and we provide the first SEM-photos of the radula, for Borysthenia naticina to complete the description of this poorly known species.

With this work the detailed description of a second valvatid genus aside of Valvata could be completed. Future research of our group will focus on other ectobranch families, as happened with the still rhipidoglossate Hyalogyrinidae (Haszprunar et al. 2011), to aim a complete morphological and perhaps molecular survey of this early heterobranch offshoot.