Cirrate shell
Internal shell of cirrate octopuses / From Wikipedia, the free encyclopedia
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:[5]
- Cirroteuthidae
- Cirroteuthis — saddle-shaped, with large wings
- Cirrothauma — butterfly-shaped
- Opisthoteuthidae
- Cirroctopus — V-shaped, lateral wings tapering to fine points
- Cryptoteuthis — U-shaped, each lateral wing ending in broad lobe with pointed projection[6]
- Grimpoteuthis — U-shaped, lateral wings ending bluntly
- Luteuthis — W-shaped
- Opisthoteuthis — U-shaped, lateral wings usually tapering to fine points but termination complex in certain species[7]
- Stauroteuthidae
- Stauroteuthis — U-shaped
The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived.[8] 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.[8] The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.[8]