How Does an Octopus Move a Limb With No Bones? Muscle Against Muscle

A limb normally works by muscles pulling on rigid parts that pivot at joints, which constrains movement to particular directions and supplies something for the muscle to act against. An octopus arm has nothing rigid in it at all, and it solves the problem by being packed with muscle and fluid at essentially constant volume, so that contracting muscle in one direction forces expansion in another. Shortening the arm makes it thicker, squeezing it makes it longer, and contracting muscle on one side while the arm resists shortening makes it bend. The whole limb is therefore its own skeleton, with the incompressible tissue providing the resistance that bone normally supplies, and the arrangement is called a muscular hydrostat.

Three sets of fibres running in different directions produce every movement:

• •Longitudinal fibres running along the arm, which shorten it and produce bending when acting on one side
• •Transverse fibres running across it, which narrow it and therefore lengthen it
• •Oblique fibres wrapping helically, which produce twisting
• •Combinations of the three generate every movement the arm can make
• •The same three-way arrangement appears in a tongue, an elephant trunk and a snail's foot
• •Suckers along the arm have their own musculature and operate independently

The nervous system is organised unlike a vertebrate's and that organisation is the most discussed feature of these animals. A very large share of the neurons sit in the arms rather than in the central brain, with each arm containing a nerve cord and clusters of neurons that handle local processing, so an arm can execute complex movements and respond to what it touches without instruction from the centre. Severed arms continue to reach and to manipulate objects for a time. The evidence suggests the central brain issues goals rather than detailed commands, leaving the arms to work out the execution, which is one solution to the enormous problem of controlling a limb with effectively unlimited ways to bend. How much the animal knows about where its arms are remains an open question.

Each arm carries rows of suckers that are far more capable than the name suggests. A sucker grips by sealing its rim against a surface and then contracting muscle to reduce the pressure inside, which holds it by the difference between that and the surrounding water pressure, and the grip is strong enough that detaching one by force damages the tissue. The rim is flexible enough to seal against irregular and rough surfaces where a rigid cup would fail. Each sucker moves independently, can be pointed, and passes objects along the arm towards the mouth in a coordinated relay. They also carry dense concentrations of chemical and touch receptors, so the animal effectively tastes what it touches, and recent work has identified receptor types specialised for detecting molecules that do not dissolve well in water, which suits examining surfaces rather than sampling the surroundings.

The arrangement solves a problem that rigid robots have, which is operating safely in cluttered and unpredictable spaces, and it has been copied directly. Soft robotic arms built from elastomers with embedded channels inflate to bend and elongate, reproducing the same principle with air pressure rather than muscle, and they can squeeze through gaps, conform around objects of unknown shape and contact people without injuring them. Prototypes explicitly modelled on octopus arms have been built and demonstrated for underwater work and for surgical instruments that must navigate through a body. The control problem carries over as well, and the biological solution of delegating execution to the limb has influenced how such devices are commanded. The main engineering difficulties are supplying force and knowing precisely where the limb is.

An arm packed with muscle at constant volume is its own skeleton, since contracting in one direction forces expansion in another, so shortening thickens it and squeezing lengthens it. Longitudinal, transverse and oblique fibres together produce every movement, the same arrangement as a tongue or a trunk. Most of the neurons sit in the arms, which handle execution locally.