Octopuses have preferred arms – just like us

Researchers studying octopuses to enhance knowledge about how to build useful underwater robots have come up with a surprise finding: cephalopods prefer some arms over others.

University of Minnesota scientists watched octopuses for hours: “Normally when you look at an octopus for a short while, nothing is repeatable,” said Trevor Wardill, an assistant biology professor who studies octopuses and other cephalopods.

“They squirm around and just look weird in their exploratory movements.”

The new study, published in Current Biology looked at 10 California two-spot octopuses (Octopus bimaculoides), a yellow, tennis-ball sized creature which lives for around two years.

In an observing arena with just a fake plant and den, the octopuses were given live prey – either shrimp or fiddler crabs – and 628 predatory behaviours were captured on video. To make it easier to track, the arms were numbered on each side of the octopus’ body.

From this, the researchers analysed which arms were used more often, as well as the way that the remaining arms were incorporated.

Because crabs move slowly while shrimp can flick their tails to escape quickly, each type of prey potentially requires different hunting tactics.

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The researchers found a couple of surprises. Firstly, the octopuses had a favourite arm – no matter the prey, octopuses seem to prefer using their second arm from the middle.

They also discovered that octopuses would normally use the arm on the same side as the eye viewing the prey.

Finally, to avoid spooking the prey, they would lead with their favourite arm and after making contact with the shrimp, they used neighbouring arms one and three to make sure it didn’t escape.

“Our study corroborates a previous report that side preference for eye and arm use is highly correlated,” the team write in their new paper.  

“We found that the division of labour among the eight arms during monocular attack follow a simple topological rule where proximity of the prey to the viewing eye dictates whether arm recruitment is [on the same side of the body or opposite.]”

Octopuses arms in a strike position
Adjacent arms are preferentially recruited in both sequential and synchronous strikes. Credit: Bidel et al., 2022, Current Biology.

Interestingly, unlike human handedness, the researchers suggest that although there is a slight side bias, the eight arms of an octopus are mostly mirrored, and are “functionally equivalent” to capture prey.

“To some extent, this organization can be seen as the Hominidae fingers where each digit of the left and the right hand are the mirror of each other and skilled for individuated finger movements,” they add.

Wardill and the team are not just looking at this for interests’ sake. They suggest that understanding how this works – down to the neurons – could allow better designed 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.

The research has been published in Current Biology.

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