Clever octopus sheds light on arm evolution

The octopus may have flexible arms, but it uses them in the same three-jointed way as vertebrates, a finding that sheds intriguing light on how limbs evolved, a new study says.

An Israeli research team filmed octopuses as they stretched out an arm from a hidey-hole in an aquarium to grab a piece of food with their tentacles and bring it to their mouths.

The octopuses, filmed about a hundred times, used a vertebrate-like strategy to carry out the complex movement.

Even though their arms are supple and rubbery, the creatures stiffened the limbs through muscle control and articulated them in a way eerily like that of animals with rigid skeletons, the scientists found.

To carry out the fetching movement, the octopus flexes its arm to form three "joints," located in similar locations to the shoulder, elbow and wrist in humans.

The middle "joint" divides the octopus' arm into two main segments of equal length, roughly like the upper arm and forearm among humans.

This similarity is not an accident, the scientists report in Thursday's issue of Nature, the weekly British science journal.

Limbed species may be very different in physiology, but they each face the same challenge in locating food, seizing it and bringing it their mouths.

Millions of years of evolutionary pressure has determined that the triple-jointed arm is the simplest and most efficient way of achieving this, the study suggests.

"Fetching seems to be an example of evolutionary selection of solutions that are similar even though they are based on quite different mechanisms," the scientists suggest.