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Octopuses ‘walk’ on 2 arms to get by predators

From the journal Science: Two species of octopus can walk on two arms and use their six non-walking arms to camouflage themselves as plant material in order to hide from lurking predators.
/ Source: Science

While pigs are not yet flying, coconuts are walking and clumps of algae are tiptoeing — sort of. The coconuts and clumps of algae are really octopuses walking on two arms and using their six non-walking arms to camouflage themselves as plant material in order to hide from lurking predators.

These octopuses are the first animals without a hard skeleton known to walk on two limbs. Octopuses normally travel along the ocean floor using all or many of their eight arms in a sort of crawl. Their muscles are supported by fluid and not bone. Using underwater video, the scientists analyzed the strides of Octopus marginatus and Octopus aculeatus. For both species, each walking arm stayed in contact with the sandy ocean floor for more than half of the stride, qualifying the pitter patter of two octopus arms as official walking.

These findings appear in the 25 March, 2005 issue of the journal Science published by AAAS, the nonprofit science society.

“It wouldn’t surprise me if other octopus species also walk,” said Science author Christine Huffard, from the University of California, Berkeley.

Huffard observed octopuses seemingly impersonating coconuts in Indonesia and octopuses walking like floating algae in Australia. She explained that many octopus species around the world have strong, muscular back legs that could be used for walking on two limbs.

'Impersonating' plants
By walking on only two arms, the other six arms can be used to transform their bodies into clumps of algae or rolling coconut shells that may not interest predators. Clumps of algae and coconut shells are commonly found in the shallow, coastal seawaters that these octopuses call home.

Octopus predators such as sharks, sting rays, predatory reef fish and flounder scan the sea floor for creatures doing the traditional “octopus crawl” which involves the octopus pushing and pulling its body along the ocean floor using many of its eight arms.

By shifting to a camouflaged walk, the octopus may slip past their predators’ octopus-detection schemes without having to stand still. Camouflaged walking -- always in the backward direction and on the back pair of legs -- is probably hardwired into their brains as well as their arms.

When threatened by a predator, something kicks in and the octopuses start walking without consciously thinking, “I need to move the right arm and the left arm,” Huffard explained.

From octopus to soft robot
A walking octopus whose legs require limited guidance from the brain excites Science coauthor Robert Full from the University of California, Berkeley.

This discovery adds to our growing understanding of how soft-limbed creatures perform complex behaviors without too much communication between limb and brain. A better understanding of how walking octopus arms work could help scientists design better artificial materials and improved soft robots, Full said.

“This discovery provides true inspiration for the beginning of a new age of soft robotics. The videos are almost unbelievable,” Full said.

A walking octopus also provides another chance for scientists to study the interactions animals with soft limbs have with their environment, explained Full, who is also intrigued by how elephants use their soft trunks.

Algae and coconuts
The “algae octopus” is no stranger to life in the algae impersonation business. When they are not moving, their long arms and walnut-size bodies look like algae. This is the first time, however, scientists have seen an octopus take this algae impersonation show on the road.

Not surprisingly, Octopus marginatus has prior coconut experience. This species is known to crawl inside empty coconut shells for shelter, pulling the two sides of the shell together with the suckers on their arms.

According to local diving lore from Indonesia, octopuses inside real coconut shells will stick their arms out of the shells and walk around, Huffard said. While this may be just a story, the scientists caught plenty of curious octopus muscle activity on video.

A muscle beach without ego
The most amazing muscles at the beach are not flexed by the people playing sand volleyball.

Bands of muscle in the octopus arms are oriented in three directions. The muscle contractions start near the top of the walking arms and move down to the tips of the arms. These waves of muscle contractions give the arms the flexibility they need for walking, the authors suggest. The fluid filling the muscles provides stability.

When the “coconut octopus” rolls along the sand on its back two arms, these fluid-filled muscular limbs serve as conveyor belts. At least one of these conveyor belts is on the ground at all times, which qualifies the movement as walking.

The arms of the “algae octopus” move in a somewhat similar fashion but they hold the other six arms in a different manner. Imagine a ragged head of broccoli running along the bottom of the ocean.

Huffard hopes to return to Indonesia and Australia in the near future to continue studying these walking octopus species.