Attachment strength of Limnoperna fortunei on substrates, and their surface properties.

The attachment strength of the freshwater mussel Limnoperna fortunei was studied. The force and energy required for the mussel to detach from various substrata (coatings and materials) were measured by a load test method, and the results were compared and discussed with regard to the surface properties of the substrata (surface roughness and surface free energy). The detachment force and energy were measured from 0.02 to 2.56 N, and from 0.02 to 14.3 mJ, respectively, depending on the type of coating and material used. The mussel attached strongly to polar surfaces, such as glass, and attached weakly to non-polar surfaces, such as silicone. There was a fairly good correlation between the detachment force and the detachment energy, and the detachment force and energy correlated well with the number of secreted threads. The unit detachment energy (the detachment energy divided by the number of secreted threads) was low when the mussel detached through adhesive failure of byssal pads, whereas it was high when the mussel detached through breakage of byssal threads. Byssal pads attached to a substratum with a low hydrogen bonding component of the surface free energy (SITE) were mostly detached by adhesive failure at the pad-substratum interface. The byssal pad adhered firmly to a surface with high hydrogen bonding SFE, and the mussel detached through thread breakage. The energy required to detach one byssal pad increased in proportion to hydrogen bonding SFE, and reached a level equal to the energy required for breaking a byssal thread at a hydrogen bonding SFE of more than 10 mJ m(-2). For a substratum at this level of hydrogen bonding SFE, mussel detachment occurred with 80% of the threads in a byssal bundle broken and 20% detached at the pad-substratum interface. This finding indicates that low hydrogen bonding SFE at the substratum surface was a prerequisite in decreasing the detachment energy of L. fortunei.