Male Octopus (Amphioctopus fangsiao) with Curled Hectocotylus (Upper Left)
Photo by Keijiro Haruki
For male octopuses, protecting a specific arm is crucial. This pivotal appendage, known as the hectocotylus, has a distinct role in reproduction, which makes its preservation vital.
Recent research led by marine biologist Keijiro Haruki from Nagasaki University sheds light on the lengths male octopuses go to safeguard this indispensable limb from damage or predation.
Haruki initiated the study by gently prodding a male octopus’s arms. He observed significant resistance from the octopus whenever he touched its hectocotylus: “This reaction indicated how much this arm matters to male octopuses, akin to how human males react when threatened.”
The hectocotylus (designated R3) of male octopuses is anatomically unique compared to the other seven arms, specifically designed for sperm transfer during mating.
Male octopuses possess a single testicle situated within the mantle (the bulbous part at the rear of the head), where sperm are produced and stored in structures called spermatophores.
During mating, the male octopus inserts the tip of his hectocotylus into the female. Before ejaculation, the hectocotylus is rolled into a tubular shape, allowing the male to force water through it and push the spermatophore into the female.
To understand how octopuses safeguard their essential third arm, Haruki’s team studied 32 male and 41 female pygmy octopuses (Octopus Parvas).
Among the sample, 13 females were missing their R3 arm, but only one male lacked this vital appendage. Various experiments compared the usage of R3 between genders.
Male Octopus Parvus Exploring During Experiment (Lower Right)
Photo by Keijiro Haruki
The first experiment involved placing a lead weight in the center of an aquarium to observe how the octopuses used their limbs to investigate. It was found that significantly more females than males employed their R3 arm to explore the novel object.
In a subsequent test, frozen shrimp were presented in a box within the tank. Males exhibited a tendency to explore with their other seven arms far longer before risking the use of their hectocotylus.
Haruki concludes that this elaborate arm usage system likely evolved because safeguarding one specialized arm is less costly than enlarging the male’s penis.
When a male loses R3, his mating activity halts until a new one regenerates, which can take several months. Nevertheless, Haruki notes that such losses are rare, indicating that males effectively protect this crucial arm from harm.
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Source: www.newscientist.com
