Figure 1.
Schematic of the octopus suckers.
AR, acetabular roof; AW, acetabular wall; C, circular muscle (yellow sections); CC, cross connective tissue fibers (green crosses); CL, connective tissue layer; IN, infundibulum; M, meridional muscle (black lines); O, orifice; R, radial muscle (gray dotted line); RIM, rim around the infundibulum; RS, rough surface located on the surface of the infundibulum, orifice and acetabular protuberance; SP, primary sphincter muscle; SS, secondary sphincter muscle.
Figure 2.
Histological transversal sections of O. vulgaris sucker.
Picro-Ponceau staining. A) The scale bar equals 2.5 mm. In the image we can observe the protuberance at the central part of the acetabulum roof. B) Enlargement of the black box in A showing the protuberance surface roughness.
Figure 3.
Schematic of acetabular chamber.
A simplified scheme of the acetabulum where the envelope is depicted as an ellipsoid, the internal cavity as a spherical cap and the protuberance as a sort of paraboloid.
Figure 4.
3D reconstructions of O. vulgaris suckers using histological and MR images.
A) The scale bar equals 3 mm. B) The scale bar equals 10 mm. In both images the localization of the protuberance is evident. C) MR transversal section of octopus sucker highlighting the natural configuration of the octopus sucker structure.
Figure 5.
Morphological differences among O. vulgaris and the other octopus species.
A) Transversal histological section (10 µm thick) of O. vulgaris sucker stained with Picro-Ponceau. B-D) Transversal histological section (10 µm thick) of O. vulgaris sucker stained with Milligan trichrome, showing the observed morphological differences among O. vulgaris and the other octopus species. B) Rough surface (RS) of acetabular protuberance. The scale bar equals 200 µm; C) Arrangement of meridional muscles (MM) in infundibular portion. The scale bar equals 600 µm; D) Primary sphincter (SP) and secondary sphincters (SS). The scale bar equals 600 µm.
Figure 6.
Octopus vulgaris sucker during adhesion process.
A) Ultrasonography of middle transverse section of sucker attached to the ultrasonographic probe. B) Schematic of A: a, acetabulum; in, infundibulum; w, water; p, ultrasonographic probe. The acetabular protuberance is in contact with the upper part of the side walls of the orifice. The scale bar equals 1 cm.
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.
Figure 8.
Schematic of the surfaces of action involved in adhesion.
A) In Kier and Smith model [14], [15] the acetabular chamber is depicted as a hollow spherical cavity. Rin is the acetabular internal radius and h is the height of spherical cap that does not take part to suction. B) In the model proposed in this work the acetabular chamber presents a protuberance acting on the orifice and maintaining low pressure in the infundibular portion of the sucker. Ro is the radius of the orifice.