Cirrate octopuses possess a well-developed internal shell that supports their muscular swimming fins. This is in contrast to the more familiar, finless, incirrate octopuses, in which the shell remnant is either present as a pair of stylets or absent altogether.Fuchs, D., C. Ifrim & W. Stinnesbeck (2008). A new Palaeoctopus (Cephalopoda: Coleoidea) from the Late Cretaceous of Vallecillo, north-eastern Mexico, and implications for the evolution of Octopoda. Palaeontology 51(5): 1129–1139.
The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms.Young, R.E., M. Vecchione & K.M. Mangold (1999). Cephalopoda Glossary. Tree of Life Web Project.Young, R.E., M. Vecchione & K.M. Mangold (2002). Cirrate Shell Terminology. Tree of Life Web Project. It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to cartilage.Vecchione, M., K.M. Mangold & R.E. Young (2010). Cirrata Grimpe, 1916. Tree of Life Web Project. Each of the eight extant cirrate genera is characterised by a distinct shell morphology outlined belowCollins, M.A. & R. Villanueva (2006). Taxonomy, ecology and behaviour of the cirrate octopods. In: Gibson, R.N., R.J.A. Atkinson & J.D.M. Gordon (eds.) Oceanography and Marine Biology: An Annual Review. Aberdeen University Press, Aberdeen. pp. 277–322. (below taxonomy updated per WoRMS):
Superfamily Cirroteuthoidea Cirroteuthidae Cirroteuthis — saddle-shaped, with large wings Cirrothauma — butterfly-shaped Stauroteuthidae Stauroteuthis — U-shaped Superfamily Opisthoteuthoidea Opisthoteuthidae Opisthoteuthis (also Exsuperoteuthis & Insigniteuthis)— U-shaped, lateral wings usually tapering to fine points but termination complex in certain speciesVillanueva, R., R.E. Young & M. Vecchione (2008). Opisthoteuthis Verrill 1883. Tree of Life Web Project. Cirroctopodidae Cirroctopus — V-shaped, lateral wings tapering to fine points Grimpoteuthidae Grimpoteuthis — U-shaped, lateral wings ends expanded in a broad lobe (with offset spike present or absent). Luteuthis — W-shaped, lateral wings ends expanded (with offset spike present). Cryptoteuthis — U-shaped, each lateral wing ending in broad lobe with pointed projection.Collins, M., R. Villanueva & R.E. Young (2008). Cryptoteuthis Collins, 2004. Tree of Life Web Project.
The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived.Bizikov, V.A. (2004). Evolution of shell in Octopodiformes (Cephalopoda) . In: Abstracts of the conference Mollusks of the Northeastern Asia and Northern Pacific: Biodiversity, Ecology, Biogeography and Faunal History. October 4–6, 2004, Vladivostok, Russia. pp. 21–23. The shell of Cirroctopus appears transitional in form between those of incirrate octopuses and other cirrates, and resembles the reduced shell of the Late Cretaceous Palaeoctopus newboldi. The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.
References
Further reading
Bizikov, V.A. (2004). The shell in Vampyropoda (Cephalopoda): morphology, functional role and evolution. Ruthenica 3: 1–88. Haas, W. (2002). The evolutionary history of the eight-armed Coleoidea. Abhandlungen der Geologischen Bundesanstalt, Wien 57: 341–351.