Octopuses are the first soft-bodied animal found to have erectile tissue. Credit: © J. Forsythe

Octopuses can get erections, US researchers have discovered. They are the first soft-bodied animal found to have erectile tissue.

The inflatable organ, called the ligula, lies at the tip of a male octopus' mating arm. When it's not aroused, the two-spot octopus (Octopus bimaculatus) "has an exceptionally tiny ligula that's very hard to see", says Janet Voight of the University of Chicago1.

But Voight glimpsed a rather different ligula while watching a failed mating. "It was quite prominent," she recalls.

Its structure is remarkably similar to mammal penises and clitorises, Voight and her colleague Joseph Thompson found. It has cavities that fill with blood held together by collagen.

Male octopuses produce a packet of sperm and insert it into their mate using this specialized arm. When all goes according to plan, the ligula is deployed inside the female, obscuring its function and size. It might help to transfer sperm, or it might scrub out the sperm of previous mates.

The organ is bright white, lacking the colour-changing cells that camouflage the rest of the octopus. In the two-spot, which hunts by day, this might be a beacon to predators. Shrinking it away might minimize this risk.

"For defensive purposes, you want a tiny copulatory organ, but you also want to transfer large quantities of sperm," says Voight. In most octopuses, which are nocturnal or live in the deep sea, the ligula is muscular, like a continuation of the arm, and comes in just one size.