Comparison of embryonic and adult shells of Sepia officinalis (Cephalopoda, Mollusca)

Development and evolution of the shell in cephalopods is difficult to establish as there is few species with a calcified shell that could be fossilized (stable in geological time). Internal cuttlebone of sepiids is so particular that homologies are difficult to find. The developmental sequence in embryos give some response elements by comparison with adult cuttlebone. The macro and microstructure of adult shell is well known but an approach at nanostructural level allows to determine structure and composition of the two main parts, the dorsal shield and chambered part. We evidence in the embryonic shell, mainly organic, a light calcification of the shell, which occurs directly as aragonite, as it is all along the formation of the shell and whatever the parts. In embryonic shell, the prismatic and/or lamellar layers, present in adult, are not differentiated and the dorsal shield grows progressively, from posterior to anterior. Despite microstructural differences, all layers of both chambered part and dorsal shield are composed of rounded nanogranules (between 50 and 100 nm), similar to what is found in other mollusc shells. Finally, the presence of pillars evidenced in embryo suggests either that their absence in extinct lineages of sepiids is the result of a diagenetic process or that they are a novelty in present sepiid species.