Octopus arm ‘slap’ in situ: A syntactic analysis of a complex motor action

Abstract An octopus, Abdopus sp., can use rotation and translation of its hydrostatic arms, and combine these kinematic behaviors serially and in parallel, on different arms, to ‘slap’ at fish in the wild. Different motor programs may be used in multiple arms producing complex actions. The movements analyzed in this work show how complex the movements of the octopus, in situ, can be, furthering knowledge of this animal’s behavior, as well as furthering understanding of the structure of animal motor control. Stiffening of the flexible muscular hydrostatic arms was found to be important to both primitives of translation and rotation. By combining these kinematic primitives, the octopus is able to maintain flexibility while controlling only a few factors, or degrees of freedom, a concept we term ‘flexible rigidity’. The slapping action of the octopus of interest, Abdopus sp., therefore, gives support for Flash and Hochner’s embodied organization view of motor behavior, as well as their idea that motor primitives can combine syntactically to form a complex action. Our results suggest that the octopus’s ability to use sensory feedback from the position of a moving fish target, along with the feed-forward motor primitives, allows for the building of complex actions at dynamic equilibrium with the environment. Overall, these findings lead to a more realistic view of how a complex behavior allows an animal to coordinate with its environment.