Octopus arm flexibility facilitates complex behaviors in diverse natural environments.

Octopus arms are among the most flexible of biological structures, yet the full range of arm flexibility has not been investigated in detail, for example in varied benthic marine habitats where structural complexity far exceeds that of lab trials. This field study quantified arm flexibility with a hierarchical analysis of octopus behaviors, arm actions, and arm deformations used throughout diverse natural habitats. Twenty-five videos of naturally behaving octopuses were analyzed from 5 Caribbean sites and 1 site in Spain. Octopus behaviors were delineated into 12 arm actions, consisting of 4 possible arm deformations (shorten, elongate, bend, or torsion). Overall, 3,907 arm action occurrences demonstrated that all arms could execute each action. Anterior arms performed more actions than posterior arms, while there were no differences between left and right arms. Furthermore, 6,871 arm deformation occurrences indicated that all 4 arm deformations were used across all actions; however, the frequencies of these deformations varied by arm region (proximal, medial, distal). The combination of deformations and arm actions implemented to achieve complex behaviors illustrates extreme arm flexibility and coordination during a wide range of arm functions. Such demonstrations of flexibility may help inform ethologists, sensory ecologists, neuroscientists, and engineers designing soft robotic appendages.