A hectocotylus (plural: hectocotyli) is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female. Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, or he may wrench it off and present it to 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 hectocotyli:
- Many coleoids lack hectocotyli altogether.
- 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. Rare examples of double and bilateral hectocotylization have also been recorded in incirrate octopuses.
- In male Seven-arm Octopuses (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 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, 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.
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