Cephalopod beak

The beak of a giant squid, surrounded by the buccal mass and limbs

All extant cephalopods have a two-part beak, or rostrum, situated in the buccal mass and surrounded by the muscular head appendages. The dorsal (upper) mandible fits into the ventral (lower) mandible and together they function in a scissor-like fashion.^[1][2] Composed primarily of chitin and cross-linked proteins,^[3][4][5][6] beaks are more-or-less indigestible and are often the only identifiable cephalopod remains found in the stomachs of predatory species such as sperm whales.^[7] They can be used to estimate the mantle length and total body weight of the original animal as well as the total ingested biomass of the species.^{[8][9][10][11][12][13][14]} Cephalopod beaks gradually become less stiff as one moves from the tip to the base, a gradient that results from differing chemical composition. In hydrated beaks of the Humboldt squid (Dosidicus gigas) this stiffness gradient spans two orders of magnitude.^[15]

Fossilised remains of beaks are known from a number of cephalopod groups, both extant and extinct, including squids, octopuses, belemnites, and vampyromorphs.^{[16][17][18][19][20][21][22]} Aptychi – paired plate-like structures found in ammonites – may also have been jaw elements.^{[23][24][25][26]}

The beak may also be referred to as the mandibles or jaws.^[19]

Side view of the lower beak of Chiroteuthis picteti (3.6 mm LRL, 160 mm ML (estimate))[1] 3D red cyan glasses are recommended to view this image correctly.  Side view of the upper beak from the same specimen (2.7 mm URL)[1]

Measurements

Giant squid beak and associated muscles with hand for scale

The abbreviations LRL and URL are commonly used in teuthology to refer to lower rostral length and upper rostral length, respectively. This is the standard measure of beak size in Decapodiformes; hood length is preferred for Octopodiformes.^[7]

Lower rostral length  Upper rostral length

References

Wikimedia Commons has media related to Cephalopod beaks.

Further reading

Cephalopod anatomy
Shell	Shell types External shells: Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone Internal shells: Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet External features Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus Internal features Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls
Mantle	External anatomy Funnel Dermal structures Fins (wings) Mantle locking-apparatus Internal anatomy Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands
Head and limbs	Brachial crown Arms Hectocotylus Tentacles Buccal region Aptychus Beak Odontophore Radula Spadix and antispadix Other parts Eyes Statocysts
General	Chromatophores Photophores Nervous system Squid giant axon Squid giant synapse
Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell)