The structure and adhesive mechanism of octopus suckers

Octopus 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 results from reducing the pressure inside the sucker cavity. This can be achieved by the three-dimensional array of muscle functioning as a muscular-hydrostat. Contraction of the radial muscles thins the wall, thereby increasing the enclosed volume of the sucker. If the sucker is sealed to a surface the cohesiveness of water resists this expansion. Thus, the pressure of the enclosed water decreases instead. The meridional and circular muscles antagonize the radial muscles. 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 kPa 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. As depth increases, however, cavitation will cease to be limiting because ambient pressure increases with depth while the cavitation threshold is unchanged. Structural differences between suckers will then determine the attachment force.