How to hunt like an octopus

In its small glass aquarium, an octopus placidly curls up in its lair.

Then a crab falls into the tank.

The octopus is quick to pounce on the crab, looking less like a finely honed killing machine than a little child who has seen a cracker, engulfing its clawed prey in a cloud of suction-cup arms and legs.

Yet there is order in this limb chaos, according to a paper published Tuesday in the journal Current Biology. Scientists who observed how the octopus hunts found that the creature almost always uses its second arm from the middle to grab prey, and when it needs support, it’s the arms closest to that second appendage that come into play. The researchers also found that different prey are hunted with different tactics: crabs get a move scientists call “parachuting,” while shrimp, which are more skittish and sensitive to large movements, are caught with a stealthy limb.

Octopuses seem to hunt more by touch than by sight. They are often found searching for crabs and other prey rather than chasing them.

“They roam the reef and speculate by sticking their arms into the holes,” said Trevor Wardill, an ecology professor at the University of Minnesota who studies the vision of octopuses and other cephalopods and is an author of the new paper.

But sometimes the octopuses see a tasty creature running and catch it. Dr. Wardill and Flavie Bidel, a postdoctoral researcher in their lab, became curious about how this less-studied form of hunting worked, so they set up high-speed cameras around aquariums with octopuses and tossed live crabs and shrimp down a chute to see. what the cephalopods did.

Reviewing hundreds of action videos of octopuses on prey, they identified a number of moves used over and over again. There is parachuting, for example, where the octopus jumps, spreads the net between its limbs to contain the prey and uses its limbs to bring food to its mouth. There’s the snap trap, where two sets of arms extend like a pincer around the prey.

There was also a strange little maneuver that the octopuses displayed when confronted with a shrimp, which the scientists called “saluting.”

If a shrimp detects movement in the nearby water, it will flick its tail and move away, in what Dr. Wardill calls a “ballistic, get the hell out of here” motion. In the videos, the octopuses would often snake an arm in front of them, waving it gently, before wrapping it tightly around a shrimp’s antennae and going for the kill. He speculates that this wave motion could be a way to spoof the shrimp’s sensors: To the shrimp, it might feel like a piece of seaweed rippling nearby, thus hiding the octopus’s actions.

The researchers were also interested to see that octopuses tended to use the second arm from the midline of their body, the side facing prey, to lead an attack.

“There must be a hierarchy in the octopus brain,” a way of controlling the order in which the limbs move, Dr Wardill said.

Octopus limbs “are very flexible to a really amazing degree,” he added, and are capable of almost infinite arrangements and shapes to a degree that can seem random.

But each limb could have a specific role during a visual hunting match, the study suggests. All that swirling could obscure, for the average viewer without a high-speed camera, a carefully choreographed response.

To understand what’s going on inside the octopus’s brain and limbs, the team would like to be able to record what’s going on in the animal’s nervous system when it goes into action. Does the brain recruit arms in specific groups or in a certain order? Does the octopus react differently in low light?

For Dr. Wardill, Dr. Bidel, and their colleagues, the answers lie ahead, on the other side of experiments where countless octopuses dance in a hail of snacks.

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