Gladius (cephalopod)

: 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).

Composed primarily of chitin, it lies within the shell sac, which is responsible for its secretion.

[1][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] Gladii are shaped in many distinctive ways and vary considerably between species, though are often like a feather or leaf.

Gladius, showing measurement of rachis and vane
Fossilised gladius of Trachyteuthis
Fossilised gladius of Teudopsis
Paralarva chiroteuthids (such as the Chiroteuthis veranyi pictured) are unusual in that they possess a greatly elongated gladius extending well beyond the fins; this supports a long, trailing tail-like structure.