Octopuses are rather odd. The sophisticated brains of these hunters are encased in a soft, squishy body that’s encircled by eight long, flexible arms. Now, researchers conducting the first kinematic analysis of their arm coordination reveal how it doesn’t matter which way their head is pointed, they can crawl in any direction. And that’s thanks to simple shortening and elongating movements. The findings were published in Current Biology this week.
"Octopuses use unique locomotion strategies that are different from those found in other animals," Binyamin Hochner from the Hebrew University of Jerusalem says in a news release. "This is most likely due to their soft molluscan body that led to the evolution of 'strange' morphology, enabling efficient locomotion control without a rigid skeleton."
Surprisingly, we’re still not sure how they coordinate their “hyper-redundant” arms during locomotion. Like us, their bodies are bilaterally symmetrical, with one side being more or less a mirror image of the other; their two eyes and optic lobes are located on the sides of their head, for instance. Their eight legs, however, are radially symmetrical around their bodies -- that is, evenly spaced, like a pie or an umbrella.
To figure out how octopuses control the movement of their bodies, Hochner and colleagues recorded videos of Octopus vulgaris crawling over objects in a tank. As they studied the footage frame-by-frame, they noticed that octopuses can crawl in any direction relative to their body orientation, despite their bilateral symmetry. It doesn’t matter which way their head is facing: The orientation of the body and crawling direction are independently controlled, and their crawling lacks any clear rhythm.
The secret to their maneuverability comes from the radial symmetry of their arms, as well as the simple pushing-by-elongation mechanism that gives their crawl its thrust. As one leg elongates, it pushes the body away from where that leg touches down on the ground. With its radially symmetric legs, choosing a direction is as simple as choosing which legs to stretch, Science explains: To move right, stretch the legs on the left, and to move forward, stretch the legs in the back.
This video shows how an octopus can crawl in any direction relative to its body orientation and never needs to readjust its head (green arrow for body orientation, blue arrow for crawling direction):
In this video, you can see how the orientation of the body and crawling direction are independently controlled, and how its crawling lacks any apparent rhythmical patterns in limb coordination:
According to the Embodied Organization concept, the body and its motor control strategies evolved together within the context of the environment. Octopuses likely evolved from clam-like animals with a hard outer shell and not much movement, or a snail with its single foot. "During evolution, octopuses lost their heavy protective shells and became more maneuverable on the one hand, but also more vulnerable on the other hand," says study co-author Guy Levy also of the Hebrew University of Jerusalem. "Their locomotory abilities evolved to be much faster than those of typical molluscs, probably to compensate for the lack of shell."
Videos: Levy et al./Current Biology 2015