Formation of the sclerites in Solenogastres and Caudofoveata (Mollusca, Aplacophora).

Solenogastres and Caudofoveata are vermiform, shell-less molluscs, with the mantle covered by calcareous scales or spines (sclerites) embedded in a cuticle secreted by the mantle epithelium. Molecular studies recover them as a single clade (Aplacophora), sister of Polyplacophora, and together, they form the sister group to all other molluscs. Within this phylogenetic framework, sclerites emerge as potentially informative characters for reconstructing early molluscan evolution. However, the cellular mechanisms underlying sclerite formation in Aplacophora remain poorly resolved. Previous work has raised two partially independent questions regarding solenogaster sclerite formation (1) whether each sclerite is produced by a single epithelial cell or through several epithelial cells, and (2) whether biomineralization occurs intracellularly or extracellularly (within a crystallization chamber formed by invagination of an epithelial cell membrane). Here, we investigate sclerite formation in five species of Solenogastres and two species of Caudofoveata. In four solenogaster species, sclerites originate extracellularly inside a microvillous crystallization chamber formed by membrane invagination of a single epithelial cell. In contrast, a minute, unidentified solenogaster exhibits microvilli-free crystallization chambers located below the epithelium and originated from prolongation of the intercellular spaces. In the examined Caudofoveata, sclerites also form within microvilli-free chambers situated below the basal lamina but within the muscular layer. The microvillous wall of the crystallization chambers in Solenogastres resembles the formation of the sclerites in Polyplacophora and the shell-forming mantle epithelium of other molluscan classes, like Bivalvia, Gastropoda or Cephalopoda, suggesting retention of a plesiomorphic condition. Conversely, crystallization chamber internalization in Caudofoveata would represent a derived state.