Cirrina or Cirrata is a suborder and one of the two main divisions of
octopuses. Cirrate octopuses have a
small, internal shell and
two fins on their head, while their sister suborder
Incirrina has neither. The fins of cirrate octopods are associated with a unique cartilage-like shell in a shell sac. In cross-section, the fins have distinct proximal and distal regions, both of which are covered by a thin surface sheath of muscle.
The suborder is named for small,
cilia-like strands (cirri) on the arms of the octopus, a pair for each sucker. These are thought to play some role in feeding, perhaps by creating currents of water that help bring food closer to the beak. Cirrate octopuses are noteworthy for lacking
ink sacs.
A
molecular phylogeny based on
mitochondrial and
nuclear DNA marker sequences by
Sanchez et al., 2018, shows that the Cirrina is
paraphyletic, i.e. it is not a single
clade. Instead, a clade containing
Opisthoteuthidae and
Cirroctopodidae is sister to the Octopodida, while a clade containing
Cirroteuthidae and
Stauroteuthidae is sister to the clade that contains those other groups.[2] However, subsequent studies, using a greater coverage of species and genes, have found Cirrata and
Incirrata to be monophyletic clades,[3] consistent with earlier morphological and molecular studies.[4][5]
Classification
The family level clades recognized within Cirrina has changed over time. Currently five families are recognized by the
World Register of Marine Species (
Cirroteuthidae,
Stauroteuthidae,
Opisthoteuthidae,
Grimpoteuthidae, and
Cirroctopodidae),[6] with these groups also recognized in recent molecular work.[3] The families Cirroteuthidae and Stauroteuthidae have been problematic, while they are distinct morphologically, molecular studies using mitochondrial genes revealed a single family (Stauroteuthidae being synonymized under Cirroteuthidae),[4] but more recent analysis using nuclear genes does show separation.[3] The family
Grimpoteuthidae has also at times been synonymized under
Opisthoteuthidae but is currently well supported.[4][7][3][8][9]