The One Arm an Octopus Will Never Risk in a Fight—Its Sex Arm
For an octopus, exploring the ocean floor with eight highly sensitive arms is a constant gamble. These limbs help them taste the environment and capture prey, but the constant, blind probing leaves them vulnerable. It’s not unusual for an octopus to lose an arm to hungry crabs or aggressive fish. Fortunately, they can regenerate lost limbs over time.
Researchers, however, have found that male octopuses take special care with one particular arm. The right third arm, called the hectocotylus, is a specialized reproductive tool, and males go to great lengths to protect it. This behavior highlights an evolutionary strategy: males will risk almost everything for a meal—except the one limb that secures their future reproduction.
A Sensitive Matter
Male octopuses have a single testis inside the mantle, the muscular body cavity that holds most internal organs. The testis produces sperm and packages it into spermatophores, small bundles ready for transfer. But males face a mechanical challenge: their penis is too short to reach females.
The right third arm solves this problem. This limb, the hectocotylus, is adapted to deliver spermatophores directly into a female’s mantle cavity during mating. It has specialized anatomy to hold and guide the sperm packets. Keijiro Haruki, a researcher at Nagasaki University, first noticed the octopus’s attachment to this arm during simple observation.
“He would strongly resist when I touched one particular arm and pull it back toward his body,” Haruki told New Scientist. “That’s when I realized there is an arm that is particularly important for males.”
Better Safe Than Sorry
To understand how far males would go to protect the hectocotylus, Haruki and colleagues studied the Japanese pygmy octopus (Octopus parvus). They collected 32 males and 41 females from the wild. Ocean life is hazardous, and 78% of the octopuses had already lost at least one arm to predators. However, the pattern was clear: 13 females had lost their right third arm, but only one male had lost his hectocotylus.
The researchers ran two lab experiments. First, they dropped a heavy lead sinker into the tanks. Females explored it freely with all their arms, including the right third. Males, by contrast, never used the hectocotylus to touch the unfamiliar object.
Next, they placed a piece of shrimp inside a dark, narrow box, simulating hunting in a crevice where crabs might lurk. Females immediately reached in with all arms, but males probed cautiously with seven arms first, only using the hectocotylus after ensuring the area was safe.
Eight Arms, Distinct Jobs
Do octopuses usually specialize their arms? Scientists have long studied whether cephalopods assign different tasks to each limb. A 2025 study of wild octopuses found that while all eight arms are highly flexible and capable of twisting, bending, and elongating, they still show preferences.
Kendra Buresch of the Marine Biological Laboratory explained that octopuses tend to use front arms for reaching and grabbing, and back arms for walking or moving objects. “This shows they are flexible and adaptable in many environments and tasks,” she said.
The Evolutionary Trade-off
Despite their flexibility, male Japanese pygmy octopuses draw a strict line regarding the hectocotylus. While lost arms will regenerate, the process takes two to three months. For a species with an average lifespan of about one year, losing the mating arm could remove a male from reproduction for a significant portion of his life.
Evolution favors the most efficient path to survival and reproduction. Rather than evolving a longer penis, octopuses repurposed an existing limb and developed strong behavioral instincts to protect it.
“Specializing one of the eight arms as a hectocotylus and guarding it costs less than enlarging the penis,” Haruki explained. “Since few males actually lose this arm, keeping it safe is a strategy that works well.”
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