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.
Highlights
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 set out to examine this assumption by calculating how diverse the arm deformations are in Octopus bimaculoides, while keeping in mind the assumption that each arm is likely to perform all four deformations based on published literature on anecdotal behavioural observations and muscle morphology[6,10,17]
Bending was the most common arm deformation of octopuses in our study: more than 11,000 examples were seen in the videos, the vast majority occurring in the anterior arms—i.e., arm pairs 1 and 2 (8,227 anterior arm bends, 2,847 posterior arm bends; Fig. 3A; X2 = 252.56, p = 0.006)
Summary
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 set out to examine this assumption by calculating how diverse the arm deformations are in Octopus bimaculoides, while keeping in mind the assumption that each arm is likely to perform all four deformations based on published literature on anecdotal behavioural observations and muscle morphology[6,10,17]. Does this hold for all four arm pairs in this species, and does it apply along the length of each arm? Does this hold for all four arm pairs in this species, and does it apply along the length of each arm? At least one p aper[10] found structural variants among arm muscle groups in O. vulgaris suggesting
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