Delayed maturation in the genus Vestia P. Hesse (Gastropoda: Pulmonata: Clausiliidae): a model for clausiliid lifecycle strategy

We studied the development of the reproductive system of two iteroparous land snails with determinate growth, Vestia gulo and V. turgida, in various stages of the life cycle: juvenile snails at five different stages, subadult snails during formation of their closing apparatus and adults. Maturation of the reproductive system is similar in both species but delayed in relation to the shell growth. Moreover, gonad development is faster than that of the remaining reproductive organs. The juvenile gonad contains only numerous dividing cells, gradually appearing spermatocytes and growing previtellogenic oocytes. Spermiogenesis starts during formation of the closing apparatus; 1 month after growth completion the number of mature spermatozoa packets, early vitellogenic and vitellogenic oocytes increases. Six months after growth completion, the gonad is histologically identical to that of reproducing individuals. During formation of the closing apparatus, the hermaphrodite duct becomes distinct as a slightly folded structure, the primordial spermatheca and mucous gland appear, and primordia of penis, oviduct and vagina as well as primordial albumen gland all become well visible. Three months after growth completion, all the organs are well developed and the reproductive system is morphologically mature. This is the time when the snails start to mate. Five to 6 months after growth completion, the snails are capable of retaining fertilized, developing eggs in their oviducts. Both clausiliids thus have a developmental strategy with a delay in maturation. This may decrease the time required to build the closing apparatus which reinforces the shell.