Figure 2.
Hypothesis of octopus sucker attachment [13]. In all images, we can identify the pressure of three compartments: Pe, environmental pressure; Pa, pressure in the acetabular compartment of the sucker; and Pi, pressure in the infundibular compartment of the sucker. (a) Sucker in the resting state. (b) First step of the attachment process. The sucker comes into contact with the substrate, and a seal is formed around the infundibulum (black arrows). During this first step, the three pressures (Pi, Pa and Pe) are equal. (c) Second step of the attachment process. The contraction of the acetabular radial muscles (black arrows) applies tension to the water volume in the sucker, inducing a reduction of internal pressure. The grooves present on the infundibular surface (see the black-marked area) allow the low pressure generated in the acetabulum to be distributed across almost the entire attachment area of the sucker. During this second step, the pressure Pi is equal to Pa because the two compartments are still connected via the orifice, but their pressure values are lower than Pe. (d) Third step of the attachment process. The contraction of the acetabular meridional muscles (black arrows) places the acetabular protuberance in contact with the upper surface of the side wall of the orifice, inducing orifice closure. At this step, Pi is still equal to Pa, and both are still lower than Pe, but two different compartments have been formed within the sucker, an acetabular one and an infundibular one. (e) Last step of the attachment process. As all muscles cease contracting, a restoring elastic force (white arrow) induces detachment of the acetabular protuberance from the upper surface of the side wall of the orifice. To maintain attachment at the substrate without muscular force, the restoring elastic force is balanced by the cohesive force of the water within the infundibular compartment (the water volume behaves like a solid under tension; see the grey arrow), and the adhesion force (black arrows) exerted by the dense network of hairs (see the black arrows) is present on the acetabular protuberance's surface. As such, the forces are in equilibrium, and the sucker can maintain the low pressure at the interface without further energy consumption.