The gladius (
pl.: 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
Romanshort 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.[1][2] Some species, like the
bigfin reef squid, still has a gladius with some degree of mineralization.[3]
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.
Paralarvachiroteuthids (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.
^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.
doi:
10.1016/0305-0491(81)90071-7.
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) (Ph.D.) (in Russian and English). Moscow: Institute of Oceanology, SSSR Academy of Sciences. 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.). Coral Gables, Florida: University of Miami. 390 pp.
Toll, R.B. (1998). "The gladius in teuthoid systematics". Smithsonian Contributions to Zoology. 586 (1): 55–67.