A male webfoot octopus (amphioctopus fangsiao) with its hectocotyle folded (upper left)
kejiro haruki
For the male octopus, there is one appendage he cannot afford to lose. This is its third right arm, which has a special role in sex. Therefore, they take extra care to protect it.
A new study led by Keijiro Haruki at Nagasaki University in Japan has revealed the lengths to which octopuses will go to ensure that their most precious arm remains safe from being damaged or bitten by a predator.
Haruki was inspired to research after gently touching each arm of a male octopus with his finger. “He would protest strongly when I touched a particular hand and pulled it back toward his body,” says Haruki. “Thanks to this behavior, I realized that there is a hand that is especially important for men. Because even human men, who are evolutionarily quite far from octopuses, feel fear and their penises and testicles shrink.”
The third right arm of the male octopus – denoted as R3 – is called the hectocotylus, and it is physically different from the other seven. The job of R3 is to deliver sperm from a penis that is too small to 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 hectocotyle into the female. Before ejaculation, males fold the hectocotyl to form a tube-like structure, into which they pour water, in order to push sperm from the penis into the female.
To find out how protective octopuses are over their third right arm, Haruki and colleagues collected 32 male and 41 female Japanese pygmy octopuses (Octopus parvus).
Thirteen females had lost their R3 arm, but only one male had lost this arm. The team then conducted two experiments to compare how males and females use their R3.
a male octopus parvus Hectocotylus bent (lower right), during an experiment
kejiro haruki
First, the team placed a lead sinker in the middle of the tank to see how the octopuses would use their limbs to figure out what the sinker was. Significantly more women than men used their R3 arm to detect the unfamiliar object.
Then, the frozen shrimp were placed inside a box in their tank. Males spent more time exploring with their other seven arms before exposing their hectocotylus to danger.
Haruki says that this complex system of using the arm as a sexual aid probably evolved “because the cost of specializing and protecting one of the eight arms as a hectocotylus is less than the cost of enlarging the penis”.
If they lose R3, a man’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 hectocotyle, it is likely that it is not particularly difficult for males to avoid loss of a specific arm.”
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