Cirrate shell

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.^[2]^[3] 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 below^[5] (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 species^[6]
- 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

Further reading

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