The shell structure and chamber production cycle of the cephalopod Spirula (Coleoidea, Decabrachia)

The endocochleate coleoid cephalopod Spirula spirula , the only present-day representative of the order Spirulida, secretes a coiled shell consisting of a series of chambers divided by septa and connected by a siphuncle. It is the shell closest to those of Recent and extinct ectochleate cephalopods: nautiloids, ammonoids. Therefore, its study may help to understand which characters remained unchanged or became transformed during the evolution of endocochleates. We have carried out detailed observations on the different structures composing the Spirula shell, with the aim of reconstructing their morphology, distribution, and mutual relationships. Alongside this, we also review the previous profuse terminology. Taking into account the additional information provided by growth lines and crystal orientations, we propose mechanisms for the secretion of the shell structures. All these mechanisms are integrated in a consistent way into a general model of chamber formation. The periostracum is secreted within a distinct periostracal groove. The outer shell layer is secreted externally to the periostracum by the soft tissues lining the shell externally. The inner shell layer is produced by the shell wall mantle, whereas the septa and the siphuncle are made periodically by a differentiated septal/siphuncular mantle. The most adoral septal mantle edge changes from secreting septal to inner shell wall material to produce the mural flap. The adapical ridge is formed by passive precipitates from cameral fluid residues trapped by surface tension, whereas the fibrous prismatic deposits of the connecting ring are biominerals produced remotely within mantle secretions. Homologies with Nautilus and Sepia are discussed.