Figure 7. Schematic view, in four phases, of the adhesion mechanism proposed for the O. vulgaris sucker.

A) Forming of a tight seal that prevents water from leaking at the rim. Infundibular radial muscles begin to contract (black arrows) to increase the contact area between (flattened) infundibulum and substrate; B) Contraction (black arrow) of acetabular radial muscles creates suction and moves water from infundibulum-substrate interface toward the acetabulum (blue arrows), as well, enhancing attachment; C) Meridional muscles of acetabulum contract (black arrows), allowing the protuberance makes contact with the upper part of the side walls of the orifice; meanwhile, the acetabular radial muscles are still contracted (gray arrow). Rough surfaces of both orifice and acetabular roof (coming into contact) contribute to adhesion. A torus of water is created in the acetabular cavity around the protuberance itself; D) Acetabular radial and meridional muscles stop to contract. The protuberance is passively kept in contact with the upper part of the side walls of the orifice, due to the cohesive force of water in the infundibular compartment and the friction of the two roughness surfaces that are in contact (acetabular protuberance and upper part of side walls of orifice). These two forces are balanced by the elastic restoring force of acetabular protuberance.