When an octopus faces a threat from a predator, it can sacrifice any of its eight limbs to escape. However, one of these limbs—the third right arm—holds special significance for males. Losing this particular arm greatly reduces their chances of successful mating until it regenerates. Japanese biologists have studied how male octopuses protect this crucial limb.

Anatomical Features of Octopuses

Octopuses have eight arms, each covered with suckers that can detect tactile and chemical signals. In males, one arm—usually the third on the right (referred to as R3)—is structurally different and is called the hectocotylus. This arm is used to transfer spermatophores—special capsules containing sperm—to the female during mating. The tip of the hectocotylus has fewer suckers but features a groove and unique projections that help transport the spermatophores.

Protective Behavior of the Hectocotylus

Research has shown that octopuses can distinguish functionally important body parts and adapt their behavior to protect them. When touched on different arms, male octopuses are especially quick to shield the hectocotylus, pressing it close to their body and avoiding contact.

Experiments with Japanese Pygmy Octopuses

The study involved 32 male and 41 female Japanese pygmy octopuses (Octopus parvus) collected from the wild. Analysis of their limbs revealed that the third right arm was missing in 13 females but only in one male, indicating that males are more cautious with this limb.

First Experiment

A lead fishing weight was placed in the aquarium to observe which arms the octopuses would use to explore an unfamiliar object. Females often used their third right arm, while males almost never touched the weight with their hectocotylus.

Second Experiment

A frozen shrimp was placed inside a box with a hole, requiring the octopuses to reach in to retrieve the food. Both males and females used R3 with similar frequency, but males spent more time investigating the hole with other arms before using the hectocotylus, showing greater caution. Females did not display this level of care.

Significance of the Results

The quantitative data confirm that male octopuses selectively protect their reproductive organ—the hectocotylus. Prior to this study, such behavior was only hypothesized and had not been confirmed in controlled experiments.

Regeneration of the Limb

Losing the hectocotylus makes mating impossible until the limb is fully regenerated, which can take from several weeks to several months depending on the species. Therefore, males tend to use other arms and rarely risk this vital organ.

Publication of Results

The study’s findings were published in the journal Ethology.