Adhesion of echinoderm tube feet to rough surfaces

Echinoderms attach strongly and temporarily to the substratum by means of specialized organs, the podia or tube feet. The latter consist of a basal extensible cylinder, the stem, which bears an apical flattened disc. The disc repeatedly attaches to and detaches from the substratum through adhesive and de-adhesive secretions. In their activities, echinoderms have to cope with substrata of varying degrees of roughness as well as with changing hydrodynamic conditions, and therefore their tube feet must adapt their attachment strength to these environmental constraints. This study is the first attempt to evaluate the influence of substratum roughness on the temporary adhesion of echinoderm tube feet and to investigate the material properties of their contact surface. It was demonstrated that tube foot discs are very soft (E-modulus of 6.0 and 8.1 kPa for sea stars and sea urchins, respectively), have viscoelastic properties and adapt their surface to the substratum profile. They also show increased adhesion on a rough substratum in comparison to its smooth counterpart, which is due mostly to an increase in the geometrical area of contact between the disc and the surface. Tenacity (force per unit area) increases with roughness [e.g. 0.18 and 0.34 MPa on smooth polymethyl-methacrylate (PMMA), 0.21 and 0.47 MPa on rough PMMA for sea stars and sea urchins, respectively] if only the projected surface area of the adhesive footprint is considered. However, if this tenacity is corrected to take into account the actual substratum 3-D profile, surface roughness no longer influences significantly the corrected adhesion strength (e.g. 0.18 and 0.34 MPa on smooth PMMA, 0.19 and 0.42 MPa on rough PMMA for sea stars and sea urchins, respectively). It can be hypothesized that, under slow self-imposed forces, disc material behaves viscously to adapt to substratum roughness while the adhesive fills out only very small surface irregularities (in the nanometer range). It is deposited as a thin film ideal for generation of strong adhesion. Under short pulses of wave-generated forces, attached discs probably behave elastically, distributing the stress along the entire contact area, in order to avoid crack generation and thus precluding disc peeling and tube foot detachment.