Octopoteuthis deletron, a species of squid found deep in the cold waters of the Pacific Ocean, has many natural predators: elephant seals, giant grenadier fish and the mysterious Perrin’s beaked whale.
To protect itself, the squid has developed a a rather unusual defensive mechanism, recently discovered by cephalopod researcher Stephanie Bush of the University of Rhode Island: When attacked, the squid plants its arms in its predator and then breaks them off. While seemingly counterproductive, there’s a reason for this tactic.
“If a predator is trying to attack them, they may dig the hooks on their arms into the predator’s skin. Then the squid jets away and leaves its arm tips stuck to the predator,” Bush explains. “The wriggling, bioluminescing arms might give the predator pause enough to allow the squid to get away.” In the squid’s extremely dark habitat—anywhere from 1,300 to 2,600 feet below the surface—this distracting, flashing “disarmament” could be the difference between staying alive and getting eaten.
Scientists have known for some time that lizards and other land-based species can voluntarily detach their appendages to elude predators, a tactic they call “arm autonomy.” But Bush’s discovery, revealed in a paper published this month in the journal Marine Ecology Progress Series, is the first ever documented case of a squid engaging in the practice.
Bush says she first became interested in looking into the phenomenon when she was working as a researcher at the Monterey Bay Aquarium Research Institute and noticed that many wild squid had extremely blunt arms that seemed to be in the process of regenerating. Scientists had speculated that damage caused by researchers’ nets was the underlying reason, but Bush wasn’t so sure. So she and her colleagues sent a remotely-controlled submersible equipped with a video camera deep into the waters of the Monterey Bay Submarine Canyon, found a squid and poked it with the control arm of the vehicle.
“The very first time we tried it, the squid spread its arms wide and it was lighting up like fireworks,” she says. Because the metal control arm was smooth, though, the squid’s arms slid off of it without detaching.
The team then came up with a makeshift solution: They attached a brush used to clean their laboratory glassware to the control arm of the vehicle and then used that to poke the squid. “It then came forward and grabbed the bottlebrush and jetted backwards, leaving two arms on the bottlebrush,” recounts Bush. “We think the hooks on its arms latched onto the bristles of the brush, and that was enough for the arms to just pop off.” Luckily, the team caught the fascinating encounter on camera for us to enjoy.
Bush later found other squid of the same species and repeated the test. Although some were more hesitant to discharge their arms than others, fighting back against the fearsome bottlebrush at first, all engaged in the unusual tactic after sufficient provocation. None of the other squid species she tested did the same. The species appeared to discharge their arms efficiently: Looking under a microscope afterward, Bush saw that most arms were torn as close as possible to the stress point, minimizing the amount of tissue lost.
The squid can regrow their arms, but that takes energy, and swimming around without an arm or two could make capturing food and mating more difficult (the bioluminescent organ on the arms’ tips are used to attract mates). Still, the strategy is a smart one under sufficiently dire circumstances. “There is definitely an energy cost associated with this behavior,” Bush says, “but the cost is less than being dead.”