A hectocotylus (: hectocotyli) is one of the

arm In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between ...

s of male

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s that is specialized to store and transfer

spermatophore A spermatophore, from Ancient Greek σπέρμα (''spérma''), meaning "seed", and -φόρος (''-phóros''), meaning "bearing", or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especiall ...

s to the female. Structurally, hectocotyli are

muscular hydrostat A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid ...

s. Depending on the species, the male may use it merely as a conduit to the female, analogously to a

penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...

in other animals, or he may wrench it off and present it to the female. The hectocotyl arm was first described in Aristotle's biological works. Although

Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...

knew of its use in mating, he was doubtful that a tentacle could deliver sperm. The name ''hectocotylus'' was devised by

Georges Cuvier Jean Léopold Nicolas Frédéric, baron Cuvier (23 August 1769 – 13 May 1832), known as Georges Cuvier (; ), was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...

, who first found one embedded in the mantle of a female

argonaut The Argonauts ( ; ) were a band of heroes in Greek mythology, who in the years before the Trojan War (around 1300 BC) accompanied Jason to Colchis in his quest to find the Golden Fleece. Their name comes from their ship, ''Argo'', named after ...

. Thinking it to be a

parasitic worm Parasitic worms, also known as helminths, are a polyphyletic group of large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other par ...

, in 1829 Cuvier gave it a generic name (''Hectocotyle''), which is a New Latin term combining the Greek words for "hundred" (''hec(a)to(n)'') and for "hollow thing, cup" (''

cotyle :''The cotylae are also features on the proximal end of the radius and of the ulna in birds.'' In classical antiquity, the ''cotyla'' or ''cotyle'' () was a measure of capacity among the Greeks and Romans: by the latter it was also called ; by ...

'').

Structure

Generalized anatomy of squid and octopod hectocotyli:

Variability

Hectocotyli are shaped in many distinctive ways, and vary considerably between species. The shape of the tip of the hectocotylus has been much used in

octopus An octopus (: octopuses or octopodes) is a soft-bodied, eight-limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like oth ...

systematics Systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: phylogenetic trees, phylogenies). Phy ...

. * Many coleoids lack hectocotyli altogether. * Among

Decapodiformes Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one ...

(ten-limbed cephalopods), generally either one or both of arms IV are hectocotylized. * In incirrate octopuses, it is one of arm pair III.Young, R.E., M. Vecchione & K.M. Mangold (1999)
Cephalopoda Glossary

Tree of Life Web Project The Tree of Life Web Project (ToL) is an Internet project providing information about the diversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world. The s ...

. Rare examples of double and bilateral hectocotylization have also been recorded in incirrate octopuses.Palacio, F.J. 1973. ''The Nautilus'' 87: 99–102. * In male

seven-arm octopus The seven-arm octopus (''Haliphron atlanticus''), also known as the blob octopus or sometimes called septopus, is one of the two largest known species of octopus; the largest specimen ever discovered had an estimated total length of and mass o ...

es (''Haliphron atlanticus''), the hectocotylus develops in an inconspicuous sac in front of the right eye that gives the male the appearance of having only seven arms. * In

argonauts The Argonauts ( ; ) were a band of heroes in Greek mythology, who in the years before the Trojan War (around 1300 BC) accompanied Jason to Colchis in his quest to find the Golden Fleece. Their name comes from their ship, ''Argo'', named after it ...

, the male transfers the spermatophores to the female by putting its hectocotylus into a cavity in the mantle of the female, called the ''pallial cavity''. This is the only contact the male and female have with each other during

copulation Sexual intercourse (also coitus or copulation) is a sexual activity typically involving the insertion of the erect male penis inside the female vagina and followed by thrusting motions for sexual pleasure, reproduction, or both.Sexual inte ...

, and it can be at a distance. During copulation, the hectocotylus breaks off from the male. The

funnel–mantle locking apparatus The funnel–mantle locking apparatus is a structure found in many cephalopods that connects the mantle and hyponome (funnel) and restricts their movement relative to each other.Young, R.E., M. Vecchione & K.M. Mangold (1999)Cephalopoda Glossar ...

on the hectocotylus keeps it lodged in the pallial cavity of the female.

Table of hectocotyli

References

{{Cephalopod anatomy Cephalopod zootomy Sex organs Animal male reproductive system