Octopuses 'always attack prey with same tentacle'


When wrapping its prey in a swirling mass of tentacles, it is not immediately obvious that the octopus favours any one of its eight 'arms'.

But by tracking their hunting tactics, scientists have discovered that octopuses are either right or left 'tentacled'.

In a series of filmed experiments, researchers from the University of Minnesota discovered octopuses always attack their prey using the second arm from the middle of their body.

The scientists studied the California two-spot octopus, which lives for around two years and grows to the size of tennis balls.

The team dropped different types of prey, including crabs and shrimps, into underwater tanks while the octopuses hid in ornamental "Spongebob-style" dens with one eye facing outwards. The team then recorded in slow-motion each octopus lunging forward, entrapping and eating their prey.

The octopuses were found to use slightly different hunting tactics for crabs and shrimps, which move at different speeds.

When hunting crabs, octopuses pounced on the prey from above with a cat-like movement, leading with the second arm.

Yet for shrimps, which can flick their tails to escape quickly, the octopuses adopted a more stealthily approach. They led with their second arm and after making contact, they used their first and third arms to entrap the shrimps.

The octopuses were also found to use their arms on the same side as the eye viewing the prey.

'The hunting behaviour was exceedingly repeatable'

Flavie Bidel, the lead author of the study - which was published in the journal Current Biology - said she was shocked by the predictability of the behaviour.

"For creatures whose movement appears unpredictable, the hunting behaviour was actually exceedingly repeatable," she said. "One of the next steps is to study how neurons facilitate the arm movements."

Trevor Wardill, an assistant professor at the College of Biological Sciences, who studies octopuses and other cephalopods, added: "Normally when you look at an octopus for a short while, nothing is repeatable.

"The surprising thing with octopuses that the sort of general public may not understand is they hunt with just one eye. So there's one eye looking out into the world in one direction, and one eye looking in the other direction.

"And so the eye that spies the food item... will then be directing arms towards the prey. And they'll always, and I mean always, use arms on the side that the eye is pointing towards the food item."

Findings could help design highly dexterous robots

Mr Wardill said the team’s findings could be used to help design new and highly dexterous robots.

"Octopuses are extremely strong. For them, to grasp and open a door is trivial, given their dexterity. If we can learn from octopuses, then we can apply that to making an underwater vehicle or soft robot application," he said.

"We now know [controlling] them is a little less erratic than what you would see if you just were a naive observer of an octopus.

"And so we're hoping that that will inspire, you know, engineers to make fancier vehicles that maybe do underwater rescue or, you know, surgeons that could have a very highly coordinated arm system to do keyhole surgery or something like that."