Octopus hydrostat movement systems (#191)
Octopuses have a complex many-unit movement system without any fixed skeleton, relying instead on muscular hydrostats, and a distributed nervous system with more neurons in the arms than in the brain. The result is a wide choice of potential actions and a question of how the resulting large number of degrees of freedom is reduced. They appear to have the same combination of controls for the movement systems that is seen in animals with solid skeletons—central commands producing general output patterns, peripheral neural networks adjusting to immediate sensory input and environmental demands, and bias, in the sense that actual actions are a subset of potential ones. Output patterns may change flexibly with environmental demands as, like fishes, octopuses change gaits with increasing demand for speed, due to recruitment of different body parts into action. The radial symmetry of the group of eight arms results in equipotentiality for all arms to be involved in actions such as walking, yet no apparent sequence of arm movement. Bias in arm use means posterior ones are more likely to be used for walking and movement to be in the anteriorward direction; anterior arms are used more for exploration, but each animal has a ‘favourite’ one and neighbouring ones are recruited for assistance. To reduce the number of degrees of freedom in arm movement, neural firing patterns result in a simplified combination of muscle stiffening and action, resulting in temporary ‘joints’ for reaching and bringing food to the mouth. While this is not a conventional system of motor control, it appears well adapted to the octopus’ marine benthic habitat.