Two Legs Good for Tiptoeing Octopus

Eight legs good, two legs better - if you are a tropical octopus faced with a hungry predator, that is.

US biologists have found two species of octopus that can stroll along the seabed using just two of their available legs.

Faced with a threat, the creatures wrap the other six tentacles around their bodies and gently tiptoe backwards in a curious manoeuvre that lets them stay blended in with the background.

Crissy Huffard, a marine biologist at the University of California, Berkeley, said octopuses camouflaged themselves by changing their colour and shape, but had to give up their disguise when startled into moving quickly.

"This bipedal behaviour allows them to get away and remain cryptic," she said. "This is the first underwater bipedal locomotion I know of."

Scientists had thought walking on two limbs was restricted to creatures with muscles attached to bones or other skeletal structures.

An octopus is basically a water-filled balloon that uses fluid pressure to maintain its shape.

The creatures usually jet backwards through the water or crawl along the seabed by pushing and pulling their sucker-covered tentacles.

Dr Huffard first noticed the unexpected behaviour in a coconut octopus - which has a body the size of an apple and lives on the sandy seabed among sunken coconuts - while helping a film crew shoot footage off the island of Sulawesi in Indonesia.

The octopus used the outer halves of its two back legs in a similar way to tank treads, alternately laying down a sucker edge and rolling it along the ground - making it look like a drifting coconut.

Then she noticed the strange walking in a second species, the smaller Octopus aculeatus - which camouflages itself as algae - while working in Australia.

"It seemed like it was walking on little conveyor belts," she said.

Dr Huffard has now captured the strolling octopuses on video and publishes her discovery today in the journal Science.

Robert Full, a professor of biology who helped to write the study, said that understanding how octopuses walked could help make a new generation of "soft, squishy" robots. "New artificial muscles that stiffen at will could reproduce this walking behaviour, Professor Full said.

"The wonderful thing about soft robotics is that it's infinitely adaptable."