Male octopuses have a favourite arm that they mostly use for sex

For a male octopus, there is one appendage it cannot afford to lose. This is its third right arm, which has a specialised role in sex. Therefore, they take extra care to protect it.

A new study led by Keijiro Haruki at Nagasaki University, Japan, has revealed the lengths octopuses will go to ensure their most precious arm stays safe from getting damaged or bitten off by a predator.

Haruki was inspired to conduct the research after softly touching each of a male octopus’s arms with his finger. “He would strongly resist when I touched one particular arm and pull it back towards his body,” says Haruki. “Thanks to this behaviour, I realised that there is an arm that is particularly important for males. Because even human males, who are evolutionarily quite distant from octopuses, feel fear and their penises and testicles shrivel.”

The third right arm – denoted as R3 – on a male octopus is called a hectocotylus, and it is anatomically different to the other seven. R3 has the job of delivering sperm from a penis that is so small that it cannot reach the female on its own.

Male octopuses have one testis, located in the mantle – the balloon-shaped part behind the head. Sperm are produced here and then stored in packages called spermatophores.

During mating, the male inserts the tip of the hectocotylus into the female. Before ejaculation, males curl up the hectocotylus to form a tube-like structure, which they force water into, in order to push the spermatophore from the penis into the female.

To find out how protective octopuses are over their third right arm, Haruki and his colleagues collected 32 male and 41 female Japanese pygmy octopuses (Octopus parvus).

Thirteen of the females had lost their R3 arm, but only one of the males had lost this arm. The team then conducted two experiments to compare how males and females use their R3.

First, the team placed a lead sinker in the middle of the tank to see how the octopuses would use their limbs to find out what the sinker was. Significantly more females used their R3 arm to explore the unfamiliar object than the males.

Then, frozen shrimp were placed inside a box in their tank. Males spent much more time exploring with their other seven arms before putting their hectocotylus at risk.

Haruki says this complicated system of using an arm as a sex aid probably evolved “because the cost of specialising one of the eight arms as a hectocotylus and protecting it is lower than the cost of enlarging the penis”.

If they do lose R3, a male’s sex life is over until a new one grows back, which can take several months, he says. “But in fact, since very few individuals lose their hectocotylus, it is likely that protecting a specific arm from loss is not particularly difficult for males.”