This first report of bipedal behavior in octopuses, written by University of California, Berkeley, researchers, will be published in the March 25 issue of Science.
When walking, these octopuses use the outer halves of their two back arms like tank treads, alternately laying down a sucker edge and rolling it along the ground. In Indonesia, for example, the coconut octopus looks like a coconut tiptoeing along the ocean bottom, six of its arms wrapped tightly around its body.
UC Berkeley graduate student Crissy Huffard clocked the two-legged speed of one coconut octopus at two and a half inches per second, while a second individual zoomed along, backwards, at five and a half inches per second. This is faster than they can crawl, but probably slower than they jet around.
The other type of octopus, which camouflages itself as algae in tropical waters from Indonesia to Australia, looks like a sea monster scooting along the sea floor on two legs. Huffard filmed this creature off Australia's Great Barrier Reef easily rolling over rocks and other obstacles.
"This behavior is very exciting," said Huffard, who first noted it five years ago in the coconut octopus but only recently was able to capture both types of octopuses on film. "This is the first underwater bipedal locomotion I know of, and the first example of hydrostatic bipedal movement."
Huffard and coauthor Robert Full, professor of integrative biology at UC Berkeley, think that this bipedal walking is a strategy octopuses use to backpedal away from predators while remaining camouflaged. Octopuses camouflage themselves by changing both color and shape, but when startled and forced to move quickly, they have to give up their camouflage.
Not so when walking.
"This bipedal behavior allows them to get away and remain cryptic," said Huffard.
An octopus is basically a water-filled balloon, keeping its shape not with an internal or external skeleton but by hydrostatic pressure, sometimes called a hydrostatic skeleton. Normally, it crawls over the bottom of the ocean, pushing and pulling with the suckers on its eight arms, or jets backwards through the water. All these movements are accomplished through muscles that squeeze and bend the fluid-filled arms and body.
Full said he was "blown away" when Huffard showed him video of the octopuses last year. He urged her to obtain more video that could be used to more clearly see how they walk, and encouraged her to publish the observations. Full, who looks at many types of animal locomotion and seeks to determine how animals control such movements, sees a revolutionary new principle in how the octopus uses its arms - one that could be used in making soft, squishy robots.
"Understanding behavior like this could usher in a new frontier of 'soft' robotics," in contrast to the rigid robots common today, he said.
"New artificial muscles that can stiffen at will could reproduce this walking behavior," said Full. "The wonderful thing about soft robotics is that it's infinitely adaptable, unlike the few degrees of freedom of rigid robots."
Huffard first noticed the coconut octopus, Octopus marginatus, dancing along the sand in 2000, while helping a film crew obtain octopus footage off the island of Sulawesi in Indonesia. The octopus, with a head about two inches long, lives on the sandy bottom in water some 20 to 30 meters (60 to 100 feet) deep, among lots of sunken coconuts, and even hides out in the shells of coconuts, drawing two halves around it to hide.
Its weird walking behavior, no doubt noticed by numerous other divers, has apparently never been reported in the scientific literature, she said.
"We know so little about these animals," Huffard said, noting that only 200 of perhaps 300 species of octopus from around the world have been described. She herself is writing up descriptions of five new octopuses, one from Hawaii and four from Tonga.
She filed away her observations about O. marginatus, however, to concentrate on her thesis, which involves the behavior of another Indonesian octopus, Octopus (Abdopus) aculeatus. This creature with a head the size of a walnut inhabits the intertidal zone, foraging along sandy bottoms among grasses and hiding out in tidepools or burying itself in the sand at low tide. To camouflage itself, it sometimes coils its two front arms and raises them in a pose that somewhat resembles algae.
Two years ago, while Huffard was visiting her thesis advisor, UC Berkeley integrative biology professor Roy Caldwell, on Lizard Island 45 miles north of Cairns, Australia, she decided to take a look at local members of that same species. She snorkeled out to capture one and, after putting it in a tank at the research station, was surprised to see it also walking on two arms.
"It seemed like it was walking on little conveyor belts," she said. She suspects that the reason she never saw this behavior in O. aculeatus in Indonesia, despite some thousand hours of snorkeling over five years, is that in Indonesia, the currents are too strong for such behavior.
Both Huffard and Full are interested in how these octopuses learned their unusual form of bipedal locomotion. Recent articles shed light on this. Israeli scientists have reported that octopus arms execute incredibly complex curling and bending motions even when cut off. Apparently a nerve ganglion in each arm can send clock-like signals down the arm to produce rhythmic movements, such as bends propagating down the arm, irrespective of whether there is a head and brain to control them. Similar movements seem to be involved in two-legged walking.
"These are stereotyped movements that don't need feedback from the brain," Huffard said.
"A lot of behavior is built into the ganglia of each octopus arm, so that seemingly complex behavior is really simple," Full added. Similar controls could make a soft robotic arm a lot easier to control than it would seem, and make it feasible to build an octopus robot that walks.
Huffard's research was supported by an American Malacological Society Student Research Grant. Full is supported by the National Science Foundation. A third co-author on the paper is Farnis Boneka of the Department of Fisheries and Marine Science, Universitas Sam Ratulangi, Manado, North Sulawesi, Indonesia.