The octopus ‘arm slap’: Understanding the motor nature of hydrostat leveraging (#189)
Octopuses produce a variety of movement patterns with their soft body using principles that govern muscular hydrostats. They use highly stereotypical movements to reduce the number of variables required to execute a task. The kinematics of two goal directed patterns have been well stereotyped – ‘reaching’ and ‘fetching’. Reaching involves extension of the arm by propagating a bend along a linear plane of the arm. Fetching uses quasi-articulated structures to combine localized stiffening with multiple bend points that behave like temporary joints. These quasi-articulated systems use principles of levers to maximize either displacement or force depending on location of the pivot or joint. All of these descriptions have been made under lab controlled conditions specifying a stationary target goal. We observed similar movement patterns in the field while studying Abdopus sp. in Okinawa, Japan. Individuals used arm extensions to ‘punch’ goby fish away from their territory. Mather and Mather (1994) described this behavior as ‘arm slap’ and is meant to deter rather than capture the fish. One arm or two arms, either lateral or anterior, were recruited for punching. The arms started held up in an upright posture with tips completely curled. Upon extension from the base of the arm to the tip, the proximal segment stiffened, acting like a rigid body. This was followed by unfurling of the tips starting medially by bend propagation. This punching pattern appears to use principles from both the reaching and fetching patterns, creating bend propagation at the tip and a quasi-joint medially adjacent to a stiffened base, but maybe optimized for force amplification more than movement amplification. Kinematic analysis of the data and finite element simulation will be shown to point to an intertwining of different mechanical amplification principles in one behavior.