KIER, W.M.*; SMITH, A.M.: The Structure and Adhesive Mechanism of Octopus Suckers

Octopus suckers employ suction adhesion to perform a variety of functions including locomotion, anchoring the body and holding prey, collecting and manipulating objects, chemotactile recognition, behavioral displays, and cleaning maneuvers. The suckers consist of a tightly packed three-dimensional array of muscle with three major muscle fiber orientations: 1) radial muscles that traverse the wall; 2) circular muscles arranged circumferentially around the sucker; and 3) meridional muscles oriented perpendicular to the circular and radial muscles. The sucker also includes inner and outer fibrous connective tissue layers and an array of crossed connective tissue fibers embedded in the musculature. Adhesion is achieved by reducing the pressure inside the sucker cavity. Contraction of the radial muscles thins the wall, thereby increasing the enclosed volume of the sucker. If the sucker is sealed to a surface, however, the cohesiveness of water resists this expansion. Thus, contractile activity of the radial muscles reduces the pressure of the enclosed water. The meridional and circular muscles antagonize the radial muscles so that the three-dimensional array of muscle functions as a muscular-hydrostat. The crossed connective tissue fibers may store elastic energy, providing an economical mechanism for maintaining attachment for extended periods. Measurements using miniature flush-mounted pressure transducers show that suckers can generate hydrostatic pressures below 0 MPa on wettable surfaces but cannot do so on non-wettable surfaces. Thus, cavitation, the failure of water in tension, may limit the attachment force of suckers. The attachment force is depth dependent because ambient pressure increases with depth while cavitation threshold is unchanged.