In most octopus males, the tip of one of their eight arms is used to pass spermatophores, little packages filled with sperm, to females during mating. Thompson and Voight found the extra modification in one species of shallow-water octopus.
Thompson, who called the finding weird, said biologists have looked for erectile tissue in other molluscs but not found them.
It is not surprising that no-one has noticed this before, Thompson said.
Octopuses, known for their intelligence and complex behaviour, are shy animals. Observing their mating is difficult and often the females attack and eat the males during courtship.
And you thought baboon males have it bad. It was almost certainly a Liliputian octopus. Blefuscan octopuses are far too well-behaved.