“Obstetrical dilemma” in viviparous snails

Viviparous gastropods keep embryos within their genital tract, hidden as the whole soft body, within a parental shell. Consequently, during birth, shelled offspring pass through the aperture of the parental shell, which results in a trade-off between the dimensions of the aperture and the embryo size. The mechanical obstacles involved in this process mirror the selection of obstetrics experienced by humans, as well as the pelvic constraints recorded in other tetrapods. The repeated evolution of viviparity in Clausiliidae, the family of land snails, which produce complex apertural barriers, gave us the opportunity to explore different ways of overcoming this evolutionary conflict. Most viviparous species are adapted by broadening of the shell canal and/or reduction of embryonic width. Some species reduce apertural barriers to the extent that they are no longer functional, whilst others produce unusual flexible, “soft shelled” embryos. The latter adaptation, recorded here for the first time , allows to overcome the “obstetrical dilemma” with minimal change in the outline of the adult shell and apertural barriers. The thickness and organomineral structure of flexible embryonic shells were investigated, compared to the typical “hard” shell of clausiliid embryos, which are rigid and unpliable already in the genital tract of the parent.