How to Hunt Like an Octopus

In his small glass aquarium, an octopus lies peacefully curled up in his cave.

Then a crab falls into the tank.

The octopus attempts to fling itself over the crab, looking less like a finely-honed killing machine and more like a toddler who’s spotted a cookie and is devouring its clawed prey in a cloud of sucking arms and legs.

However, there is order in this limb chaos, according to an article published Tuesday in the journal Current Biology. Scientists who watched the octopus hunt found that the creature almost always uses its middle second arm to reach for prey, and when it needs support, it’s the arms closest to that second appendage that they bring into play. The researchers also found that different prey are hunted with different tactics: crabs are given a move scientists call “skydiving,” while shrimp, which are shyer and more sensitive to large movements, are caught with a stealthy limb.

Octopuses seem to hunt by feel rather than sight. They often seek out crabs and other prey instead of hunting them.

“They wander around the reef and speculate by sticking their arms in holes,” said Trevor Wardill, a professor of ecology at the University of Minnesota who studies the vision of squid and other cephalopods and is the author of the new work.

But sometimes octopuses see a tasty critter racing by and grab it. dr Wardill and Flavie Bidel, a post-doctoral researcher in his lab, were curious about how this less-explored form of hunting works, so they placed high-speed cameras around aquariums of squid and dropped live crabs and shrimp down a chute to see what the cephalopods were doing did it.

They reviewed hundreds of videos of octopus-on-prey actions and identified a number of moves that were used over and over again. For example, when skydiving, the octopus swoops, spreads the webbing between its limbs to hold prey, and uses its limbs to bring food into its mouth. There is the snap trap, in which two groups of arms stretch around the prey like pincers.

There was also a weird little maneuver the squid performed when facing a shrimp that the scientists dubbed “waving.”

When a shrimp senses movement in the water nearby, it flicks its tail and darts away, which Dr. Wardill calls a “ballistic, get the hell out of here” move. In the videos, squid would often stretch out an arm in front of them and wave gently before wrapping it tightly around a shrimp’s antennae and going on the hunt. He speculates that this undulating motion could be a way to manipulate the shrimp’s sensors: To the shrimp, it could feel something like a piece of seaweed moving nearby, thus obscuring the octopus’ actions.

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

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

Octopus limbs “are highly flexible at really amazing levels,” he added, and are capable of nearly infinite configurations and shapes to a degree that may seem random.

However, the study suggests that each limb might play a specific role during a sight hunt. All that swirling might obscure a carefully choreographed response from the average viewer without a high-speed camera.

To understand what’s going on in the octopus’ brain and limbs, the team wants to be able to record what’s happening in the animal’s nervous system as it goes into action. Does the brain recruit arms in specific groupings or in a specific order? Does the octopus react differently in low light?

for dr Wardill, Dr. Bidel and her colleagues have the answers before us, on the other side of experiments that see myriads of squid dancing in a shower of snacks.