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.

The second arm from the middle is the attacking arm.

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.

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.