The ontogeny of the lower reproductive tract of the landsnail Helix aspersa (Gastropoda: Mollusca)

Abstract

We provide a size-based ontogenetic sequence of the development and differentiation of the lower reproductive tract of the heterobranch gastropod Helix aspersa (Muller 1774). Twelve development stages distributed among nine size classes were recognized based on readily visible changes in morphology and changes in tissue density. Geometric morphometrics was used to calculate the deformation between stages as represented by thin-plate spline bending energies. The developmental stages and sequence of developmental events are also compared to previously published scenarios for the evolution of stylommatophoran and other pulmonate reproductive tracts. These comparisons suggest that heterochronies, which include both acceleration and retardation, are operating in the morphological evolution of the pulmonate lower reproductive tract. This supports previous observations that largest number of developmental changes coincides with the transition to sexual maturity, which is also seen in the exponential curve of bending energies we observed in Helix aspersa. The belated organogenesis makes the ontogeny of the complex hermaphroditic reproductive system of pulmonates readily observable in size-friendly juveniles. This observation, coupled with the ease of raising individuals in the laboratory, recommends Helix aspersa as a potential model laboratory system for investigating molluscan evolutionary development.