Dr Stephanie Bush, a postdoctoral researcher at the University of Rhode Island, has discovered a never-before-seen defensive strategy used by a small species of deep-sea squid in which the animal counter-attacks a predator and then leaves the tips of its arms attached to the predator as a distraction.

“When the foot-long octopus squid Octopoteuthis deletron found deep in the northeast Pacific Ocean ‘jettisons its arms’ in self-defense, the bioluminescent tips continue to twitch and glow, creating a diversion that enables the squid to escape from predators,” explained Dr Bush, who authored the paper in the July issue of the journal Marine Ecology Progress Series.

“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. The wriggling, bioluminescing arms might give the predator pause enough to allow the squid to get away,” she said.

Scientists had speculated that squids may release their arms, just as lizards can release their tails when attacked, but no one had seen it happen. Using a remotely operated vehicle in the Monterey Bay Submarine Canyon off the coast of California, Dr Bush poked at a squid with a bottlebrush.

“The very first time we tried it, the squid spread its arms wide and it was lighting up like fireworks,” she said. “It then came forward and grabbed the bottlebrush and jetted backwards, leaving two arms on the bottlebrush. 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.”

The squid are able to re-grow their missing arms. ”There is definitely an energy cost associated with this behavior, but the cost is less than being dead,” Dr Bush said.

In further experiments, Dr Bush found that some octopus squid appeared hesitant to sacrifice their limbs, but some did so after being prodded several times. When she provoked seven other squid species similarly, none dropped their arm tips.

“Scientists had assumed that squid living in the deep-sea would not release ink as a defensive measure, but all the species I’ve observed did release ink,” she said. “They assumed that because they’re in the dark all day every day that they’re not doing the same things that shallow water squids are doing. They also assumed that deep-sea squid don’t change color because of the dark, but they do.”

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Bibliographic information: Bush SL. 2012. Economy of arm autotomy in the mesopelagic squid Octopoteuthis deletron. Mar Ecol Prog Ser 458: 133-140; doi: 10.3354/meps09714