Planet Earth

Deep-sea squid can break off all its arms onto an enemy

Not Exactly Rocket ScienceBy Ed YongAug 1, 2012 1:00 PM

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If you grew up on a diet of 1980s cartoons, as I did, you will have seen many a giant robot shoot many a rocket-propelled fist into many a big monster. Sadly, there are no rocket punches in the real world, but I can give you the next best thing: a squid that can grabs its enemies with flashing, writhing, self-amputating arms. The squid in question is Octopoteuthis deletron, a beautiful red animal with hook-lined arms, which grows to around five inches in length. I’ve written about it before – the males have a tendency to indiscriminately implant members of both sexes with sperm. [embed width="610"]http://youtu.be/-SxLl6Sj6s0[/embed] Stephanie Bush from the Monterey Bay Aquarium Research Institute has been studying the squid for the last decade. She used remotely operated underwater vehicles to film 84 of them, and reviewed archival footage of 21 more individuals. The squids normally have eight arms that taper to a point, but a quarter of the animals that Bush saw had at least one short, blunt-ended arm. Some had lost several. And on a few dives, the vehicle saw disembodied tips, cleanly severed and slowly sinking. Fishermen have made similar reports; they’ve caught O.deletron specimens with blunt-ended arms. These sightings suggested that the squid could willingly break off its arms, and Bush confirmed that ability when she brought some back to her lab. Even while the sub was collecting specimens, one of the squids broke off two arms. Back in the lab, 7 of the 11 captured squid did the same thing. One particular individual had a flair for theatrics:

“It grasped the textured bottom (rubber topped with fabric) of its holding container with the arm hooks, somersaulted, and re-leased ink as it autotomized [detached] part of all 8 arms.”

The breaks can happen at different points along the arm, but they always need some tension. Either the squid has to grab something, or something has to grab it. Once broken, the tips flail about for at least 10 seconds, while glowing vigorously from the light-producing organs at their tips. The glow is bright enough that Bush could see it under full laboratory lighting. The obvious interpretation is that the squid jettisons its body parts to confuse and distract a predator. But it can also use this ability offensively. When Bush threatened the animals with a bottle brush, several of them attacked it. Five of them broke off their arms while they were grabbing the brush. The arms flashed away while continuing to hold their grip, while the squids jetted off. It’s called “attack autotomy”. I imagine it would be quite off-putting if you were a fish or a bottle brush. While some octopuses have the ability to break off their arms, O.deletron is the first squid that’s known to do so. This dramatic ability is just part of its defensive arsenal, which also include camouflage, jet-propulsion, ink, and its distracting light-producing arm tips. Self-amputation seems like a rather drastic measure. The arms can grow back, but it takes a while to do so. In the meantime, the squid may have trouble grabbing food (especially since it possibly uses the glowing tips as fishing lures). To minimise these costs, the squid practices what Bush calls “economy of autotomy” – it breaks off its arm just above the point where it has grabbed or been grabbed. That’s very different to octopuses, which break off their arms at a pre-set point at their base. Instead, the squid only amputates what it has to. Many animals can willingly amputate parts of their body. Lizards are the most famous example – many of their can break off their tail if they are caught by a predator. Several other species, including the males of some spiders and octopuses, can break off their genitals inside their partners to continue fertilising them from afar. Hat tip toAmy West at Science for alerting me to the paper. Reference: Bush. 2012. Economy of arm autotomy in the mesopelagic squid Octopoteuthis deletron. MARINE ECOLOGY PROGRESS SERIES http://dx.doi.org/10.3354/meps09714

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