A Renewed Understanding of Shell-shape Diversity among Marine Gastropod Species: Invariance and Covariance between Geometrical Parameters in Conispirally Coiled Shells

Abstract

Approximately conispirally coiled shells make the bulk of shell structure among marine gastropod species, apart from those shells with strongly overlapping whorls (typical, in particular, of Cones, Cowries and the like). By selecting appropriately the set of geometrical parameters appropriate to describe the interspecific variations of shell-shape, an original pattern of covariances and independences was unveiled between these parameters, providing, in turn, a partly renewed understanding of the interspecific diversification of shell-shapes among conispirally coiled marine gastropods. This novel approach should be considered as complementary, rather than alternative, to the traditional and well established models by Raup and others. Among the four selected descriptive parameters (the whorl expansion ratio, the degree of whorl compression, the number of whorls of fully blown shell and the shell apical angle), four strong covariances and two mutual independences are empirically documented and analysed theoretically. Two covariances (apical angle covarying positively with the whorl expansion ratio and negatively with the degree of whorl compression) are imposed by geometrical constraints exclusively; one covariance (negative between the number of whorls and the whorl expansion ratio) is likely attributable to purely biological causes, tentatively associated to either developmental or functional constraints; while the last covariance (negative between apical angle and number of whorls) involves both geometrical and biological determinisms. Thus, the conispiral coiling mode, in marine gastropods, involves quite an intricate interplay of covariances among the parameters describing shell-shape. While limiting thereby the range of occupation of the potential morphospace, this pattern of covariances introduces enhanced shell shape complexity. Highlighting this underlying complexity may, in turn, contribute to a more thorough and fundamental understanding of the ontogenetic aspects involved in the profuse diversity of shell-shapes among marine gastropods.