Theoretical morphological analysis of differential morphospace occupation patterns for terrestrial and aquatic gastropods.

Despite the morphological diversity of organisms, they only occupy a fraction of the theoretically possible spectrum (i.e., morphospace) and have been studied on several taxa. Such morphospace occupation patterns are formed through evolutionary processes under multiple constraints. In this study, we discovered a differential morphospace occupation pattern between terrestrial and aquatic gastropods and subsequently attempted to quantitatively understand these differences through morphospace analysis. These differential occupation patterns between terrestrial and aquatic species were observed in the morphospace of spire height and aperture inclination, including a bimodal distribution of shell height in terrestrial species alongside the absence of high-spired shells with high aperture inclination. Although terrestrial species were distributed along optimal lines of shell instability and shell hindrance to locomotion, aquatic species were distributed not only along this line but also within a suboptimal region of the low spire with low inclination. Based on numerical simulation and biometric analysis, here we propose the hypothesis that this difference was caused by the aquatic species being able to adopt a posture with the growth direction perpendicular to the substrate due to reduced functional demands. Our results provided an ultimate explanation for the differential occupation patterns between habitats alongside an overview of the morphospace.