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

For the male octopus, there is one appendage he cannot afford to lose. This is his third right arm, which has a specialized role in sex. Therefore, they pay increased attention to its protection.

A new study led by Keijiro Haruki of Nagasaki University in Japan has revealed the lengths octopuses will go to in order to keep their most valuable arm safe from damage or bites from a predator.

Haruki was inspired to conduct research after gently touching the arm of a male octopus with his finger. “He would fight back hard if I touched one particular arm and pulled it back to his body,” Haruki says. “This behavior made me realize 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 shrink.”

The third right arm – referred to as R3 – in the male octopus is called the hectocotylus and is anatomically distinct from the other seven. R3 has the task of delivering sperm from a penis that is so small that it cannot reach the woman on its own.

Male octopuses have one testicle, 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 hectocotyl into the female. Before ejaculation, males coil the hectocotylus to form a tubular structure into which they push water to push the spermatophore from the penis into the female.

To find out how the octopus protects its third right arm, Haruki and his colleagues collected 32 male and 41 female Japanese dwarf octopuses (Octopus parvus).

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

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

Then the frozen shrimp were placed in a box in their tank. Males spent much more time exploring their other seven arms before compromising their hectocotylus.

Haruki says that this complicated system of using the arm as a sex tool probably evolved “because the cost of specializing one of the eight arms as a hectocotylus and protecting it is less than the cost of enlarging the penis”.

If R3 is lost, a man’s sex life ends until a new one grows, which can take several months, he says. “But in reality, since very few individuals lose their hectocotylus, it is likely that protecting a particular arm from loss is not particularly difficult for males.”

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