Keijiro Haruki
A male webbed octopus (Amphioctopus fangsiao) with curled hectocotyl (top left)
The right third arm of male octopuses performs a specialized function in mating, and these animals take extra care to avoid damaging it or losing it to a predator.
For a male octopus, there is one appendage that he cannot afford to lose. And you third right armwhich has a specialized function in sex. Therefore, protect it with special care.
A new study, led by Keijiro Harukifrom Nagasaki University in Japan, revealed the lengths to which octopuses are willing to go to ensure that their most precious arm does not get damaged nor is it bitten off by a predator.
Haruki decided to carry out this investigation after touch gently with his finger on each of the arms of a male octopus. “He resisted strongly when I played on a particular arm and pulled him close to his body”, says Haruki.
“It was thanks to this behavior that I realized that there is a particularly important arm for males. Because even human males, which are evolutionarily quite distant from octopuses, when they feel afraid see the penis and testicles shrink”.
The third right arm on a male octopus, identified as R3is called hectocotyl and is anatomically different from the other seven. R3 has the function of transport sperm from a penis — which is so small that, by itself, can’t reach the female.
Male octopuses have a single testicle, located in the mantlethe balloon-shaped part behind the head. This is where sperm is produced and then stored in shells called spermatophores.
During mating, the male introduces the tip of the hectocotyl into the female. Before ejaculation, males curl the hectocotyl to form a tubular structure, into which they force water to enter, in order to push the spermatophore from the penis into the female.
To understand the extent to which octopuses protect their third right arm, Haruki and his colleagues collected 32 male and 41 female Japanese pygmy octopuses (A small octopus).
Thirteen of the females had lost R3, but among the males only one I was without this precious member. The team then carried out two experiments to compare how males and females use R3., explains to .
First, researchers placed a fishing lead in the center of the aquarium to see how the octopuses would use their limbs to understand what that object was. One significantly more females used R3 to explore the unknown object than males.
Afterwards, they placed frozen shrimp inside a box in the aquarium. Males spent much more time explore with the other seven arms before putting the hectocotyl at risk.
Haruki says this complex system, in which an arm is used as a sexual aid, likely evolved “because the cost of specializing one of the eight arms like hectocotyl and protect it is less than the cost of enlarging the penis”.
If they lose R3, a male’s sex life is suspended until a new one grows, which can take several months, says the researcher. “But in fact, because very few individuals lose the hectocotyl, it is likely that protecting a specific arm from loss is not particularly difficult for men.”
The results were presented in an article published last week in the journal Ethology.
