A hectocotylus (
pl.: hectocotyli) is one of the
arms of male
cephalopods that is specialized to store and transfer
spermatophores to the female.[1] Structurally, hectocotyli are
muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, analogously to a
penis 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 knew of its use in mating, he was doubtful that a tentacle could deliver sperm. 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, in 1829 Cuvier gave it a
generic name,[2][3][4][5]
combining the Greek word for "hundred" and Latin word for "hollow thing, cup".
Structure
Generalized anatomy of squid and octopod hectocotyli:
Squid
Octopod
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
octopussystematics.
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.[6] Rare examples of double and bilateral hectocotylization have also been recorded in incirrate octopuses.[7][8]
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.
^Thompson, D'Arcy Wentworth (1913). On Aristotle as a biologist, with a prooemion on Herbert Spencer. Being the
Herbert Spencer Lecture before the University of Oxford, on February 14, 1913. Oxford University Press. p. 19.
^Robson, G.C. 1929. On a case of bilateral hectocotylization in Octopus rugosus. Journal of Zoology99(1): 95–97.
doi:
10.1111/j.1469-7998.1929.tb07690.x