Deep-sea squid can ‘drop arms’ in defence tactic

Washington -  A postdoctoral researcher has observed 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.

Stephanie Bush said that 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.

“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,” explained Bush.

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

While Bush was a graduate researcher working with the Midwater Ecology Lab at the Monterey Bay Aquarium Research Institute, she observed that many octopus squid had arms of different lengths.

Scientists had speculated that they might 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, 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,” she said.

The squid are able to re-grow their missing arms.

“There is definitely an energy cost associated with this behaviour, but the cost is less than being dead,” Bush said.

In further experiments, 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.

Bush’s research on squid began in 2003 when she decided to investigate the assumptions that some scientists had made about deep-sea animals.

“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 colour because of the dark, but they do,” she said.

The University of Rhode Island scientist’s current research focuses on a tiny squid that lives in the Gulf of California that migrates every day from the dark depths where there is little oxygen to the surface waters to feed. She is examining their oxygen consumption rates and how increasing water temperatures will affect their survival.

“They’re a really abundant species in the Gulf, so presumably if they are that abundant, they must be feeding on lots of different things and there must be lots of things feeding on them,” Bush said.

“They could be very important to the health of the ecosystem,” she added.

The discovery was published recently in the journal Marine Ecology Progress Series.