Thompson, J.T. and J.R. Voight. 2003. Erectile tissue in an invertebrate animal: the Octopus copulatory organ. Journal of Zoology 261: 101-108.

Erectile tissues are rarely seen among invertebrates. However, in male Octopus bimaculoides erectile tissues have been found at the tip of the reproductive arm. How does erectile tissue in the arm of Octopus bimaculoides assist in reproduction and what is its significance?

Male octopodes mate using a special arm known as the heterocotylus. The tip is known as the ligula and is used to deposit spermatophores into the oviduct of the female. Generally, a larger ligula is more advantageous, as it allows more spermatophores to be passed. Octopus bimaculoides lives in shallow-water and possesses a fairly small ligula that is light-colored on the ventral face.

Thompson and Voight studied Octopus bimaculoides through a histological approach. Tissue slides were made from Octopus bimaculoides heterocotylus specimens, as well as those of two other deep-water species. The ligula in Octopus bimaculoides display a different microanatomy than that of other octopodes; the tissues are highly vascularized and possess many open cavities, it is supplied by its own blood vessel beds. Upon erection of the ligula, these spaces become engorged with blood.

The authors concluded that the presence of the erectile tissue in the heterocotylus of Octopus bimaculoides assists in reproduction by extending the ligula and increasing its ability to deposit spermatophores into a female. It can be retracted when not in use, as the light-color makes the octopus a target for visual predators. The similarity to the mammalian penis in structure and function suggests "evolutionary convergence between the reproductive organs of cephalopods and vertebrates."

(250 words)

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(posted March 19, 2009)