Theoretical Modelling of the Molluscan Shell: What has been Learned From the Comparison Among Molluscan Taxa?

Abstract

The molluscan shell has been the subject of extensive theoretical work for over a century. From geometrical models to mechano-chemical models, a wide range of models exist that highlight the principles underlying coiling and the ornamentation in molluscan shells. Here, I review these theories, focusing on the comparison between different clades of molluscs with an external shell, namely cephalopods, bivalves, and gastropods. The models using a fixed axis -qualified as ``form models''- facilitate the analysis of morphospace occupation and functional/ developmental constraints at a high phylogenetic level, whereas moving reference models---qualified as ``growth models''- facilitate the quantitative study of allometry and phenotypic plasticity at lower phylogenetic levels. This review highlights the advantages and limitations of each framework and shows that comparative theoretical and experimental studies are very informative with regards to the evolutionary conserved rules underlying accretionary growth.