Male pygmy octopuses use hectocotylized arm sparingly to avoid loss

A research team including Assistant Professor Yuta Yamate, Associate Professor Takeshi Takegaki, and Doctoral Student Keijiro Haruki from the Graduate School of Integrated Science and Technology at Nagasaki University has demonstrated through tank experiments that male pygmy octopuses refrain from using their "hectocotylus," the arm essential for reproduction, for exploration when there is a risk of limb loss. The findings were published in Ethology.

Octopuses possess eight arms, which are utilized for movement, exploration, foraging, and reproduction. The hectocotylus is a specialized arm used by males for breeding, and there is only one such arm, located at the third position on the right side (R3 arm). This arm has a specialized structure to deliver "spermatophores" (capsules filled with sperm) to the female during mating. If lost, it cannot be replaced by any of the other arms. Since many octopus species have short lifespans and only one breeding season in their entire lives, the loss of this hectocotylized arm almost always means the male will be unable to produce offspring. Previous studies across various octopus species have noted that the hectocotylus has a lower rate of loss compared to other arms, leading to the hypothesis that males behave in ways to avoid damaging it.

While it was known that males often tuck the hectocotylized arm close to their bodies, the specific behavioral mechanisms used to avoid the risk of losing that arm remained unclear. The research team examined the arm-loss status of pygmy octopuses collected off the coast of Nagasaki Prefecture and confirmed that the male hectocotylus is indeed lost less frequently than the other arms or the corresponding R3 arm in females.

To investigate this, the research team conducted two experiments using these individuals. In the first experiment, the octopuses were presented with an unknown object, a fishing weight placed in the tank, to see which arms they would use to touch it. The results showed that while females used all their arms indiscriminately, the males avoided using their hectocotylized arm entirely.

The second experiment required the octopuses to retrieve food from a hole where the interior was not visible. In this experiment, food was placed inside a small hole large enough for an arm to fit through, and the researchers recorded which arm was inserted into the hole to search for the food, as well as the duration for which each arm was inserted. As a result, there was no significant difference between males and females in the overall frequency of using the R3 arm.

However, a notable difference appeared in the timing of insertion of R3 arm (hectocotylus for males). Males spent significantly more time exploring the hole with other arms before finally inserting the hectocotylus compared to females. In the rocky intertidal zones where pygmy octopuses live, they forage by inserting their arms into numerous crevices and holes. However, predators such as large crabs may lurk in these gaps, posing a risk that an arm might be partially severed if inserted carelessly.

These experiments indicate that to avoid the risk of having the hectocotylus cut by the predator, male pygmy octopuses avoid using the hectocotylus when touching unknown objects or use other arms to confirm safety before using the hectocotylus.

This study demonstrates that male pygmy octopuses do not merely flee from predators but actively exhibit behavior to selectively protect a part of the body essential for future reproductive success. It is expected that this will further the understanding of how animal behaviors evolve to selectively protect specific "vital points" crucial for survival or reproduction.

Journal Information
Publication: Ethology
Title: Male Octopus Avoid Using Hectocotylized Arm Under Situations With Unpredictable Risks
DOI: 10.1111/eth.70073