Novel underwater walk helps creatures slip away unnoticed.
Two tiny species of tropical octopus have demonstrated a remarkable disappearing trick. They adopt a two-armed 'walk' that frees up their remaining six limbs to camouflage them as they slink away from trouble.
"When we noticed one was walking, I thought my gosh, this is amazing. It's the first underwater bipedal locomotion I know of," says Christine Huffard of the University of California, Berkeley, who captured the behaviour on video.
Huffard's team filmed the apple-sized Octopus marginatus in the tropical waters of Indonesia. Instead of its usual sprawling crawl, O. marginatus fled from divers by striding on two arms, with the rest of its arms wrapped around its body, giving it the appearance of a walking coconut.
By rolling its rearmost arm out along the sea bed, and then repeating the action using a second arm, the octopus walks as if it is on a conveyor belt. "It's like a backward walk," says Huffard, who reports the discovery in Science1.
Looking like a coconut may help O. marginatus to go unseen, Huffard believes. There is an abundance of coconuts on the sea floor in the area, she points out.
The related species Octopus aculeatus, which has a body the size of a walnut, was also filmed walking, this time along the floor of an aquarium tank. With its arms raised about its head, the octopus looked like a clump of algae.
Bipedal locomotion was thought to require muscle pulling against hard skeleton. But the octopuses walk using opposing muscle movements, a technique the researchers think requires little brain control.
"There is so much to learn about octopus behaviour. It is completely possible that they may walk another way and may not be the only octopus species to do it," Huffard says.
HuffardC., BonekaF. & FullR.Science, 307. 1927 (2005).