Male Japanese pygmy octopuses take extra care to safeguard their third right arm, the hectocotylus, which plays a key role in reproduction. Researchers at Nagasaki University observed that males resist touching this arm and use it less for risky tasks than females do. The findings highlight an evolutionary adaptation to protect this vital appendage.
Keijiro Haruki at Nagasaki University in Japan led a study on this behavior after noticing a male octopus pull back strongly when he touched one specific arm. This third right arm, denoted as R3, differs anatomically from the others and serves as a hectocotylus to deliver sperm during mating. Males produce sperm in a single testis within the mantle and store it in spermatophores; the small penis cannot reach the female alone, so the hectocotylus inserts its tip, curls into a tube, and pushes the spermatophore with water pressure. Haruki's team collected 32 male and 41 female Japanese pygmy octopuses, or Octopus parvus. Among them, 13 females had lost their R3 arm, but only one male had. This disparity suggested males protect it more effectively. In experiments, females used their R3 arm more often to explore a lead sinker in their tank, while males hesitated. When frozen shrimp were placed in a box, males relied on their other seven arms first, minimizing risk to the hectocotylus. Haruki explained that specializing one arm this way evolved because the protection cost is lower than enlarging the penis. Losing the arm halts mating until it regrows over several months, though few males lose it, indicating protection is manageable. The study appears in Ethology (DOI: 10.1111/eth.70073).
