The octopus: sentient and swift, with a bulbous mantle, tiny head fixed on a neckless body and eight nimble arms. These arms, symmetrically radiating outward from its body, would appear to allow the octopus to move in innumerable ways. But with so many possibilities, where and how does the octopus go? And what exactly does it do with so many arms? Although some studies of octopus arm movement had been conducted, how octopuses coordinate all those arms during movement was still unknown. In a recent study published in Current Biology, Guy Levy, at the The Hebrew University of Jerusalem, and his colleagues in Israel sought to understand the dance of the graceful octopus more deeply.
The authors placed nine common octopuses, Octopus vulgaris, in an aquarium with a transparent bottom and enough water for the octopuses to breathe and crawl, but not enough for them to swim. The team then filmed the movement of the octopuses as the animals readily crawled around obstacles and curiously inspected the tank and their observers. Then the scientists analysed the movements of the arms to determine how the octopuses used them and coordinated them to move.
Their study showed that the octopuses used their arms to move much like eight rubber bands with suction cups: scrunch, plant, extend. To the authors’ surprise, they found that although the octopuses preferred to move at angles of about 45 deg relative to the direction that their body was facing they were capable of moving in any direction. So, the octopuses moved in all directions, though they preferred to move diagonally. The animals also preferred to use particular arms and arm pairs when crawling – specifically, those that would send them along their desired diagonal path. The body orientation and crawling direction were independent and the authors suggest that the octopuses coordinate crawling direction with body orientation in a way that maximizes sensory input and feeding.
Most surprisingly, unlike many other animals, octopuses lack rhythm; their arms do not move in a rhythmic way when crawling, they are irregular. Any arm can be used at any time to make an instantaneous movement in any direction.
The authors also found that when arms work in groups (of two to four arms) to move, each arm exerts virtually the same amount of force. For example, the octopuses don't push with twice as much force with one arm as the other arm. So, locomotor decisions are simplified; they don't have to decide how much force to use with each arm to move in a particular direction, they just have to decide which arms to use. This, the authors suggest, combined with the simple worm-like movement of the arms, makes the octopus truly unique. There is a beautiful simplicity in the rhythmless dance of the octopus. And the authors suggest that these specialized movements are the result of extraordinary adaptations of the soft-bodied octopus to its environment.