{"word_count": 586, "character_count": 3845}
Résumé de la communication de Laure BONNAUD-PONTICELLI (Museum National d'Histoire Naturelle, Paris) donnée à l'occasion du séminaire Les constructions en milieu marin du 14 au 19 mars 2016All metazoan groups present mineralized structures, and we have chosen the mollusk group as an illustration. The molluscs are characterized by an external shell and the diversity of shells show the different mechanisms that have led to this high diversity of shapes and structures: spicules, several plates, unique valve, tooth-like, bivalves, coiled, chambered shells and sometimes internalized and regressed as a chitinous plate, their evolution regard to the body plan organization is quite exciting in a developmental approach. The component is either aragonite or a mix of aragonite and calcite (in Bivalvia and Gastropoda). The mantle synthetizes the shell and the different layers, prismatic (calcitic) and nacreous (aragonitic) show very different microstructures between the different groups. The organic component are very diverse and often specific to the species. If there is "basal and universal" components they remain to be determined. There is cleary no "basic" component or structures common to all mollusk groups.In cephalopods, the evolution of the shell is quite difficult to reconstruct. If there is a good fossil record for the ectocochleate forms (external shell), there is very poor remains for the present coleoids lineage showing an internal shell: the shell is still calcified in cuttlefish and spirulids but the aragonitic composition does not allow good fossilization and the shell is reduced to chitinous blade in squid and to rudiments in octopods. To have a more precise idea of the evolution of shell, we have a developmental approach: the idea is that the developmental sequences could help to approach the evolutionary mechanisms that led to a particular shell. The embryonic shell is aragonitic as in the adult. There is no change in the composition, suggesting that aragonite was the main component in the cephalopod lineage. The embryonic shell show a particular organization between the chambers: there is no continuity between the support pillars suggesting maybe an ancestral state.These pillars are actually found in adult fossils of sepiid lineage. Genes involved in the mineralization and/or controlling its development must be deeper explored to have a better understanding of the selective constraints on the shell. Actually, even if constrained because of its basic role of protection, the shell is very plastic and many of shell characters are not robust and cannot be used as phylogenetically informative characters, in cephalopods as in other mollusk groups. The shell shape and ornamentation are probably dependent of the environment. The process of mineralization could be impacted by external modifications. Unlike other molluscs in which the pH decrease leads to a fragile and breakable shell in adult and abnormalities during development, the shell is overcalcified in cuttlefish probably as a result of the physiological regulation of bicarbonate ions in the closed circulatory system towards the shell sac. This particularity could be of interest to explain the evolution of extinct lineages in a specific paleoenvironment. Finally, in cephalopods, a stress (mechanical or chemical) leads to inclusion of melanin during growth. Actually the secreting mantle is responsible of the inclusion of colors and this process is very precise and controlled but the molecular and cellular mechanism remain to be determined.In conclusion the diversity of mollusk shell in shapes and component as well as in term of molecular and physiological processes is a huge and exciting field of research in a comparative approach even if the evolutionary meaning of such a diversity could probably never be found.Summary