A hectocotylus (plural: hectocotyli) is one of the arms of the male of most kinds of cephalopods that is modified in various ways to effect the fertilization of the female's eggs. It is a specialized, extended muscular hydrostat used to store spermatophores, the male gametophore. Depending on the species, the male may use it merely to store and transfer sperm to the female, or he may wrench it off and present it to the female. Males generally form a new hectocotylus in each new season.
Among Decapodiformes (ten-limbed cephalopods), generally either one or both of arms IV are hectocotylized; in incirrate octopuses it is one of arm pair III. Many coleoids lack hectocotyli altogether. The shape of the tip of the hectocotylus has been much used in octopod systematics. In many species it is considerably elaborated. However, in the males of some species, such as the Seven-arm Octopus (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.
The term is also used to specifically refer to the greatly modified arm of Argonauta and allied genera. In argonauts, the male transfers the spermatophores to the female by putting its hectocotylus into a cavity in the mantle of the female. This mantle cavity is known as the pallial cavity. This is the only contact the male and female have with each other during copulation, and it can be at a distance. During copulation, the hectocotylus breaks off from the male. The funnel–mantle locking apparatus on the hectocotylus keeps it lodged in the pallial cavity of the female.
The name hectocotylus was devised by Georges Cuvier, who first found one embedded in the mantle of a female argonaut; supposing it to be a parasitic worm, Cuvier gave it a generic name. The hectocotyl arm was first described in the biological works of Aristotle, and it was widely disbelieved until its rediscovery in the nineteenth century.
Generalized anatomy of squid and octopod hectoctyli:
Hectocotyli are shaped in many distinctive ways, and vary considerably between species. Some examples are shown below.
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