If you happen to be SCUBA diving off Australia, and you see a clump of algae drift by, look again. It may be a stealthy octopus. Researchers have discovered that two octopus species, including the Australian species Octopus aculeatus, can walk along the seafloor with two arms, using the other six to keep up a disguise.
Octopuses are masters of deception. They can change their shape and coloring to blend in with their backgrounds or even impersonate other sea creatures (ScienceNOW, 29 August, 2001). Such skills are real lifesavers for an animal that is essentially "a soft-bodied, yummy hunk of protein that everybody is trying to eat," says Roger Hanlon of the Marine Biological Laboratory in Woods Hole, Massachusetts. But most guises only fool predators when an octopus is sitting still. As soon as it moves, it looks like a mobile morsel.
That's why the behavior of O. aculeatus is so fascinating, says Christine Huffard, a graduate student at the University of California, Berkeley, who led the new study. The octopus can scurry along the seafloor on the tips of two arms--planting one "foot" before the other in a motion surprisingly reminiscent of human walking--while it keeps its other six arms extended in a fairly convincing algae imitation (see links below).
In the 25 March issue of Science, Huffard and colleagues also report that an Indonesian octopus, O. marginatus, performs a similar trick, walking on two arms with its other arms balled up beneath its body. The researchers interpret this behavior, perhaps somewhat imaginatively, as an impression of a rolling coconut. Both octopuses can move slightly faster in stealth mode than they can when crawling with several arms, another possible advantage, Huffard says.
"It's a spectacular observation," says Hanlon, who was not part of the research project. He adds that two-arm walking is no simple feat for an octopus. "It's like me telling you to stand on your head and move around with only one arm," he says. "The interesting thing about this is the combination of a sophisticated behavior of locomotion with an equally sophisticated cryptic body pattern." William Kier, a biologist at the University of North Carolina, Chapel Hill, agrees: "It's another really excellent example of the range and diversity of behaviors that these animals are capable of."