{"id":370090,"date":"2026-04-08T20:44:20","date_gmt":"2026-04-08T20:44:20","guid":{"rendered":"https:\/\/www.newsbeep.com\/nz\/370090\/"},"modified":"2026-04-08T20:44:20","modified_gmt":"2026-04-08T20:44:20","slug":"the-one-arm-an-octopus-will-never-risk-in-a-fight-its-sex-arm","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/nz\/370090\/","title":{"rendered":"The One Arm an Octopus Will Never Risk in a Fight\u2014Its Sex Arm"},"content":{"rendered":"

\"\"<\/a>Webfoot octopus (Amphioctopus fangsiao) male with its mating arm curled up (upper left). Credit: Keijiro Haruki<\/p>\n

For an octopus, exploring the ocean floor with eight highly sensitive arms is a constant game of roulette. These limbs are essential for tasting the world and grappling with prey, but their constant, blind probing makes them a vulnerable target. As a result, it is not uncommon for an octopus to lose one of these appendages to hungry crabs or aggressive fish. Luckily, they have the remarkable ability to regenerate a lost arm over time.<\/p>\n

Yet researchers have discovered that male octopuses draw a strict, biological line when it comes to one specific appendage. The right third arm, known as the hectocotylus, serves as a specialized reproductive tool, and males will go to extreme, risk-averse lengths to shield it from harm. <\/p>\n

This guarded behavior reveals a hidden evolutionary strategy where a male will eagerly risk life and limb for a meal\u2013except for one limb in particular that safeguards his future fatherhood.<\/p>\n

A Sensitive Matter<\/p>\n

Male octopuses possess only one testis, which is tucked away inside the mantle\u2014the muscular body cavity behind the head that houses most internal organs.<\/p>\n

Inside the mantle, the testis produces sperm and packages them into neat, delivery-ready bundles called spermatophores. But the male has a mechanical problem: his penis is too small to reach a female on its own.<\/p>\n

That is where the right third arm comes in. This limb is uniquely modified to deliver the sperm. During mating, the male uses the hectocotylus to transfer spermatophores into the female\u2019s mantle cavity. The arm has specialized anatomy for holding and guiding those sperm packets.<\/p>\n

Keijiro Haruki, a researcher at Nagasaki University, first noticed the animal\u2019s profound attachment to this limb through simple observation.<\/p>\n

\u201cHe would strongly resist when I touched one particular arm and pull it back towards his body,\u201d Haruki told New Scientist<\/a>. \u201cThanks to this behavior, I realized that there is an arm that is particularly important for males.\u201d<\/p>\n

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Better Safe Than Sorry<\/p>\n

To test exactly how far males will go to protect this anatomical treasure, Haruki and his colleagues turned to the Japanese pygmy octopus, Octopus parvus. They collected 32 males and 41 females from the wild.<\/p>\n

Life in the ocean is perilous, and 78% of the captured octopuses were missing at least one arm due to encounters with predators. However, the location of those missing arms revealed a pattern. Thirteen of the females had lost their right third arm. Among the males, only one individual was missing his hectocotylus.<\/p>\n

The researchers designed two laboratory experiments to observe how the animals navigate risky situations. In the first test, they dropped a heavy lead fishing sinker into the center of the tanks. This unfamiliar object demanded investigation.<\/p>\n

Females eagerly reached out, using their right third arm just as frequently as any other limb to feel the cold metal. Males behaved entirely differently. Not a single male used his hectocotylus to touch the mysterious sinker.<\/p>\n

The team then escalated the stakes. They placed a tasty piece of frozen shrimp inside a dark, narrow box. To get a meal, the octopus had to reach inside a blind hole\u2014a scenario that mimics hunting in rock crevices where dangerous crabs often hide.<\/p>\n

Females plunged their arms in without hesitation. Males, however, spent significantly more time probing the dark hole with their other seven arms first. Only after thoroughly sweeping the area for danger would a male carefully deploy his mating arm as well to grab the food.<\/p>\n

Eight Arms, Distinct Jobs<\/p>\n

Do octopuses usually show such extreme preference for specific limbs? Scientists have long debated whether cephalopods divide labor among their arms. In a separate 2025 study <\/a>analyzing octopus movements in the wild, researchers found that the animals are incredibly adaptable.<\/p>\n

According to Kendra Buresch of the Marine Biological Laboratory, octopuses can perform deformations like twisting, bending, and elongating with all eight arms simultaneously. Yet, they do organize their tasks.<\/p>\n

\u201cIn general, we did see that for most actions the octopuses used their front arms more often than their back arms,\u201d Buresch told the Guardian<\/a>.<\/p>\n

The animals favor their front limbs for reaching and grabbing, while utilizing their back limbs to act as stilts or conveyor belts to walk across the seafloor.<\/p>\n

\u201cThis means that octopuses can be very flexible and adaptable in many different environments and tasks,\u201d Buresch added.<\/p>\n

The Evolutionary Trade-off<\/p>\n

\"\"<\/a>One of the Japanese pygmy octopus from the experiment holding its hectocotylus curled up. Credit: Keijiro Haruki<\/p>\n

Yet, for all their famous flexibility, male Japanese pygmy octopuses draw a hard line when it comes to their mating arm. While a lost limb will eventually grow back, the regeneration process takes two to three months.<\/p>\n

For a creature that lives for only about a single year, spending a quarter of your life waiting for your reproductive organ to regrow is a catastrophic delay. Losing the hectocotylus could essentially knock the male out of the breeding pool.<\/p>\n

Evolution tends to favor the most efficient path to survival and reproduction. Instead of growing a longer penis, the octopus repurposed an existing limb and developed deep-seated behavioral instincts to keep it intact.<\/p>\n

\u201cBecause the cost of specializing one of the eight arms as a hectocotylus and protecting it is lower than the cost of enlarging the penis,\u201d Haruki explained. \u201cBut in fact, since very few individuals lose their hectocotylus, it is likely that protecting a specific arm from loss is not particularly difficult for males.\u201d<\/p>\n

Their study was published in the journal Ethology<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"Webfoot octopus (Amphioctopus fangsiao) male with its mating arm curled up (upper left). Credit: Keijiro Haruki For an…\n","protected":false},"author":2,"featured_media":370091,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[26],"tags":[17798,193680,18581,153,193681,111,139,69,19234,193682,19087,147,406,5295],"class_list":{"0":"post-370090","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-wildlife","8":"tag-animal-behavior","9":"tag-arm-regeneration","10":"tag-cephalopods","11":"tag-marine-biology","12":"tag-mating-behavior","13":"tag-new-zealand","14":"tag-newzealand","15":"tag-nz","16":"tag-octopus","17":"tag-octopus-parvus","18":"tag-reproduction","19":"tag-science","20":"tag-wildlife","21":"tag-zoology"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/370090","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=370090"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/370090\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/370091"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=370090"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=370090"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=370090"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}