The clam that lost its shell

Octopuses probably evolved from animals similar to clams, with a protective outer shell and almost no movement to speak of, and developed a unique motor control strategy that matches their unusual body shape, a new study has found.

“Octopuses use unique locomotion strategies that are different from those found in other animals,” says Binyamin Hochner of The Hebrew University of Jerusalem, who led the research.

“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.”

During evolution, octopuses lost their heavy protective clam-like shells. They “became more manoeuvraable on the one hand, but also more vulnerable on the other hand”, says study co-author Guy Levy. “Their locomotory abilities evolved to be much faster than those of typical molluscs, probably to compensate for the lack of shell.”

The evolution of something like a snail’s foot into long and slender arms gave octopuses extraordinary flexibility. Excellent vision, together with a highly developed and large brain and the ability to colour camouflage, made cephalopods very successful hunters.

The researchers,  who set out to discover how the creatures manage to co-ordinate their eight, long flexible arms, studied videos of octopuses in action, frame by frame.

they found that, despite the octopus’ bilaterally symmetrical body, it can crawl in any direction relative to its body orientation.

The orientation of its body and crawling direction are independently controlled, and its crawling lacks any apparent rhythmical patterns in limb coordination. Their manoeuvrability comes from the radial symmetry of their arms and the simple mechanism by which the arms create the crawling thrust: pushing-by-elongation.

“These two together enable a mechanism whereby the central controller chooses in a moment-to-moment fashion which arms to recruit for pushing the body in an instantaneous direction,” the researchers write. The animal needs only to choose which arms to activate in order to determine the direction of locomotion.

The research is published in Current Biology. Previous reports and videos of these remarkable animals here and here.

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