Octopus arms exhibit exceptional flexibility

Abstract

The octopus arm is often referred to as one of the most flexible limbs in nature, yet this assumption requires detailed inspection given that this has not been measured comprehensively for all portions of each arm. We investigated the diversity of arm deformations in Octopus bimaculoides with a frame-by-frame observational analysis of laboratory video footage in which animals were challenged with different tasks. Diverse movements in these hydrostatic arms are produced by some combination of four basic deformations: bending (orally, aborally; inward, outward), torsion (clockwise, counter-clockwise), elongation, and shortening. More than 16,500 arm deformations were observed in 120 min of video. Results showed that all eight arms were capable of all four types of deformation along their lengths and in all directions. Arms function primarily to bring the sucker-lined oral surface in contact with target surfaces. Bending was the most common deformation observed, although the proximal third of the arms performed relatively less bending and more shortening and elongation as compared with other arm regions. These findings demonstrate the exceptional flexibility of the octopus arm and provide a basis for investigating motor control of the entire arm, which may aid the future development of soft robotics.

The Case for Octopus Consciousness: Unity

“…By far the most common action was bend, less common in the proximal 1/3 and more likely in the oral-aboral directions. The first and second pairs of arms were more likely to produce bend actions, which is logical as they are more likely to be the exploratory pairs, since the octopus crawls forward more often using pairs 3 and 4 [43]. Elongation-shortening and torsion were more commonly carried out all along the arm.…”

“…Of course, all the degrees of freedom cannot be realized, and there must be some constraints on actions. Kennedy et al [43] observed a very large set of arm actions and found some probabilities. By far the most common action was bend, less common in the proximal 1/3 and more likely in the oral-aboral directions.…”

Energy-shaping control of a muscular octopus arm moving in three dimensions

“…The control problem is to obtain stabilizing control inputs u for the system (3.9) so as to achieve some predefined tasks. Here, the tasks are motivated by stereotypical motions observed in octopuses [11,12]. Two of these motions are reaching and grasping.…”

Lessons for Robotics From the Control Architecture of the Octopus

“…Likewise, as the longitudinal muscles contract and the arm shortens, the cross-sectional area increases. The interplay of the three muscle groups and the lack of inherently rigid structure equips the arm with its remarkable flexibility ( Kier and Smith, 1985 ; Kier and Stella, 2007 ; Kennedy et al, 2020 ).…”

“…If the suckers find prey during foraging, the suckers will recruit their neighbors to capture and immobilize the animal ( Rowell, 1963 ; Altman, 1968 ; Gutfreund et al, 2006 ; Zullo et al, 2011 ). Surface conformation: as suckers recruit their neighbors toward encountered surface features, the arm’s shape conforms to that of the surface ( Altman, 1968 ; Kennedy et al, 2020 ). Reaching: using visual information the brain determines the horizontal and vertical angle (yaw and pitch) of the arm.…”