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
- Cirroteuthidae
- 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]
- Opisthoteuthidae
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]
