Cirrate shell

Dorsal (left) and ventral aspects of a mature female *Grimpoteuthis angularis* (73 mm ML). source ↗

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.¹

The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms.²³ It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to cartilage.⁴ Each of the eight extant cirrate genera is characterised by a distinct shell morphology outlined below⁵ (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 species⁶
- 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).⁷
  - Luteuthis — W-shaped, lateral wings ends expanded (with offset spike present).
  - Cryptoteuthis — U-shaped, each lateral wing ending in broad lobe with pointed projection.⁸

The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived.⁹ 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.⁹ The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.⁹

References

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. doi:10.1111/j.1475-4983.2008.00797.x
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.
Vecchione, M., K.M. Mangold & R.E. Young (2010). Cirrata Grimpe, 1916. Tree of Life Web Project.
Collins, 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.
Villanueva, R., R.E. Young & M. Vecchione (2008). Opisthoteuthis Verrill 1883. Tree of Life Web Project.
Verhoeff, Tristan Joseph; O’Shea, Steve (2022-01-02). "New records and two new species of Grimpoteuthis (Octopoda: Cirrata: Grimpoteuthididae) from southern Australia and New Zealand". Molluscan Research. 42 (1): 4–30. doi:10.1080/13235818.2022.2035889. ISSN 1323-5818.
Collins, M., R. Villanueva & R.E. Young (2008). Cryptoteuthis Collins, 2004. Tree of Life Web Project.
Bizikov, V.A. (2004). Evolution of shell in Octopodiformes (Cephalopoda) Archived 2013-10-21 at the Wayback Machine. 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.

References

Further reading