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posted: 8/5/2012 8:47 AM

The mystery of deep-sea squids' missing arms

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  • Many deep-sea squid dispel an ink cloud to flee a predator, but one species goes a step farther: It ensures a getaway by counterattacking and then ditching the tips of its arms. These detached bits can continue to twitch and emit bioluminescent light -- likely providing a vital distraction.

      Many deep-sea squid dispel an ink cloud to flee a predator, but one species goes a step farther: It ensures a getaway by counterattacking and then ditching the tips of its arms. These detached bits can continue to twitch and emit bioluminescent light -- likely providing a vital distraction.

 
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Many deep-sea squid dispel an ink cloud to flee a predator, but one species goes a step farther: It ensures a getaway by counterattacking and then ditching the tips of its arms. These detached bits can continue to twitch and emit bioluminescent light -- likely providing a vital distraction. By catching this strange maneuver on camera, scientists have established Octopoteuthis deletron as the only known squid to drop portions of its arms in self-defense, much as lizards drop their tails before escaping.

O. deletron inhabits depths of 500 to 600 meters. Little is known about the biology of these gelatinous deep-dwellers, but recently they have begun to yield their secrets -- including some bizarre mating behavior -- thanks to powerful video cameras mounted on robotic submersibles operated by researchers at Monterey Bay Aquarium Research Institute in Moss Landing, Calif.

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Viewing some of this footage, Stephanie Bush, a postdoctoral fellow at the University of Rhode Island, Kingston, noticed many individuals with arms of different lengths and suspected that these cephalopods lost their arms during an attack.

To investigate, Bush collaborated with researchers at MBARI. With a bit of luck, the team found squid off the coast of California and tried poking them with the control arm of the submersible. The creatures attacked the vehicle but never held on, perhaps because they couldn't grip its smooth metal surface, Bush said.

Eventually, the researchers resorted to attaching the bottlebrush they used to wash their laboratory glassware to the submersible. When they nudged the next squid they encountered, it attacked the brush and immediately left behind parts of two arms. Fortunately, the team caught the action with a high-resolution camera. Bush said that as the scientists erupted into cheers in the control room, she wondered why she hadn't tried this earlier.

Not all individuals were so hasty to drop their arm tips -- it often took several provocations -- but squid always struck back before ditching their arm tips, Bush reports this month in Marine Ecology Progress Series. This distracting counterattack and the subsequent loss of limbs has been documented only in some crabs, Bush said. When she prodded seven other squid species from similar depths in Monterey Bay, none sacrificed their arms.

Much of the previous work on O. deletron relied on nets to capture them, and researchers assumed squid with missing arm tips were damaged by the nets, said squid biologist Michael Vecchione of the Smithsonian National Museum of Natural History in Washington. Vecchione praised Bush for using a variety of methods to unravel this deep-sea behavior.

When Bush viewed muscle tissue under the microscope, she found the muscle could tear at many places along the arm but only did so very near to where the arm was strained. That makes sense, she said, because it minimizes the amount of tissue lost.

Octopuses, close relatives of squid, can ditch entire arms, but O. deletron's more frugal tissue loss may give it an advantage, said Mark Norman, a biologist at the Museum Victoria in Melbourne, Australia, who specializes in octopus behavior.

"In the deep-sea environment where you have a lean nutritional budget, you are not going to give up unnecessary body parts when it might be six months between feedings," Norman says.

Losing limbs can create difficulties in swimming and capturing food, however. It also means surrendering the light-emitting organ on the squid's arm tip, which scientists believe could lure prey or mates. But it's a loss worth taking: "It doesn't do you any good to look for a mate if you are already dead," said biologist Richard Young from the University of Hawaii, at Manoa, who first described the species 40 years ago.

"This paper is a classic example of a hidden world and hidden strategies," Norman said. "In the deep sea our window on natural behavior observations is so tiny that it's as exciting as outer space and alien hunting. You come across these creatures rarely, and when you do, you get a tiny glimpse into their lives."

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