Supplement 3 (2004)

Bizikov V. A. The shell in Vampyropoda (Cephalopoda): morphology, functional role and evolution. 2004: 1-88.

Comparative functional morphology and microstructure of internal shell vestiges have been studied in 14 species of recent Vampyropoda including the single recent representative of vampire squids, Vampyroteuthis infernalis Chun, 1903, 3 species of Cirrata and 10 species of Incirrata. Relationship between the shell and the soft body was studied on total cross-sections of the mantle at different planes. The study shows that the shell plays different role in different groups of eight-armed Coleoidea, and evolution of the shell was stipulated by evolution of its function. In Vampyroteuthis, an early evolutionary offshoot of the Vampyropoda, the shell represents a typical gladius of the teuthoid type that provides attachment sites for the fins, mantle, funnel and head retractors, and the visceral sac. In cirrate octopods the gladius represents a remnant of cone flags, wings and lateral plates of vampyromorph gladius, which is transformed mostly into a `fin support'. In the Incirrata the shell is reduced to paired rods, stylets, functioning as anchors supporting the funnel retractors. In several advanced lineages of Incirrata the shell has been lost completely.

Each lineage of recent octopodiforms – Vampyromorphida, Cirrata and Incirrata – has its own characteristic type of shell structure and shell-soft body relationships. Wide separation of stellate ganglia in all recent octopods presents morphological evidence that this group evolved from some vampyromorph-like ancestor with wide middle plate of gladius proostracum. Two crucial events took place in evolution of the shell in octopodian lineage: reduction of the middle plate of proostracum and conus resulting in transformation of vampyroteuthoid-like gladius into the gladius of the cirrate type; and reduction of the transversal connection (saddle) in the cirrate gladius resulting in its transformation into paired stylets of the Incirrata. In Incirrata the shell underwent gradual reduction until its complete loss in several evolved forms. This last event, complete disappearance of the shell, has occurred independently in all three principal lineages of Incirrata: Octopodoidea, Bolitaenoidea and Argonautoidea. In all cases the final loss of the shell was accompanied by the reduction of jet-swimming in connection with development of a `walking' habit involving the arms (benthic Octopodoidea) or heavy reliance on passive floatation in pelagic Bolitaenoidea and Argonautoidea. The final loss of the shell vestiges in advanced Incirrata did not change their soft-body design, which remained generally the same as in primitive benthic Octopodoidea.

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