Gladius (cephalopod) explained

The gladius (: gladii), or pen, is a hard internal bodypart found in many cephalopods of the superorder Decapodiformes (particularly squids) and in a single extant member of the Octopodiformes, the vampire squid (Vampyroteuthis infernalis).^[1] It is so named for its superficial resemblance to the Roman short sword of the same name, and is a vestige of the ancestral mollusc shell, which was external.^[1] The gladius is located dorsally within the mantle and usually extends for its entire length. Composed primarily of chitin, it lies within the shell sac, which is responsible for its secretion.^[2] Some species, like the bigfin reef squid, still has a gladius with some degree of mineralization.^[3]

Gladii are known from a number of extinct cephalopod groups, including teudopseids (e.g. Actinosepia, Glyphiteuthis, Muensterella, Palaeololigo, Teudopsinia, Teudopsis, and Trachyteuthis), loligosepiids (e.g. Geopeltis, Jeletzkyteuthis, and Loligosepia), and prototeuthids (e.g. Dorateuthis, Paraplesioteuthis, and Plesioteuthis).^{[4] [5]}

Variability

Gladii are shaped in many distinctive ways and vary considerably between species, though are often like a feather or leaf. Some examples are shown below.

width=33%	Shape of gladius	width=33%	Species	width=33%	Family
	Ancistroteuthis lichtensteini	Onychoteuthidae
	Architeuthis sp.	Architeuthidae
	Bathyteuthis abyssicola	Bathyteuthidae
	Histioteuthis bonnellii	Histioteuthidae
	Histioteuthis reversa	Histioteuthidae
	Illex illecebrosus	Ommastrephidae
	Lepidoteuthis grimaldii	Lepidoteuthidae
	Doryteuthis pealeii	Loliginidae
	Loliolus sumatrensis	Loliginidae
	Lolliguncula brevis	Loliginidae
	Mastigoteuthis agassizii	Mastigoteuthidae
	Onykia ingens	Onychoteuthidae
	Pholidoteuthis massyae	Pholidoteuthidae
	Sepioteuthis lessoniana	Loliginidae
	Taningia danae	Octopoteuthidae
	Taonius borealis	Cranchiidae
	Teuthowenia megalops	Cranchiidae
	Uroteuthis duvauceli	Loliginidae

See also

• Cuttlebone
• Belemnoidea
• Argonaut
• Nautilus
• Spirula
• Mollusc shell

Further reading

• Russian, English. Ph.D. . Bizikov . V.A. . 1991 . Squid gladius: Its use for the study of growth, age, intraspecies structure and evolution (on the example of the family Ommastrephidae) . Institute of Oceanology, SSSR Academy of Sciences . Moscow. 513 pp.
• Toll . R.B. . 1982 . The comparative morphology of the gladius in the order Teuthoidea (Mollusca: Cephalopoda) in relation to systematics and phylogeny . Ph.D. . University of Miami . Coral Gables, Florida. 390 pp.
• Toll . R.B. . 1998 . The gladius in teuthoid systematics . Smithsonian Contributions to Zoology . 586 . 1 . 55–67.

Notes and References

1. Web site: Young, R.E. . Vecchione, M. . Mangold, K.M. . 1999 . Cephalopod gladius terminology . Tree of Life Web Project.
2. Hunt . S. . Nixon . M. . 1981 . A comparative study of protein composition in the chitin-protein complexes of the beak, pen, sucker disc, radula and oesophageal cuticle of cephalopods . Comparative Biochemistry and Physiology B . 68 . 4 . 535–546 . 10.1016/0305-0491(81)90071-7.
3. Messerli . Mark A. . Raihan . M. Jahir . Kobylkevich . Brian M. . Benson . Austin C. . Bruening . Kristi S. . Shribak . Michael . Rosenthal . Joshua J. C. . Sohn . Joel J. . 2019 . Construction and Composition of the Squid Pen from Doryteuthis pealeii . The Biological Bulletin . 237 . 1 . 1–15 . 10.1086/704209 . 7340512 . 31441702.
4. Fuchs . D. . Engeser . T. . Keupp . H. . 2007 . Gladius shape variation in coleoid cephalopod Trachyteuthis from the Upper Jurassic Nusplingen and Solnhofen Plattenkalks . Acta Palaeontologica Polonica . 52 . 3 . 575–589.
5. Web site: Fuchs, D. . 2010 . Teudopseina . Tree of Life Web Project.