Biomechanics of adhesion in sea cucumber cuvierian tubules (echinodermata, holothuroidea).
Several species of sea cucumbers, all belonging to a single family, possess a peculiar and specialized defense system, the Cuvierian tubules. It is mobilized when the animal is mechanically stimulated, resulting in the discharge of a few white filaments, the tubules. In seawater, the expelled tubules lengthen considerably and become sticky upon contact with any object. The adhesiveness of their outer epithelium combined with the tensile strength of their collagenous core make Cuvierian tubules very efficient at entangling and immobilizing most potential predators. We have designed a method to measure the adhesion of holothuroid Cuvierian tubules. Tubule adhesive strength was measured in seven species of sea cucumbers belonging to the genera Bohadschia, Holothuria and Pearsonothuria. The tenacities (force per unit area) varied from 30 to 135 kPa, falling within the range reported for marine organisms using non-permanent adhesion. Two species, H. forskali and H. leucospilota, were selected as model species to study the influence of various factors on Cuvierian tubule adhesive strength. Tubule tenacity varied with substratum, temperature and salinity of the seawater, and time following expulsion. These differences give insight into the molecular mechanisms underlying Cuvierian tubule adhesion. Tenacity differences between substrata of varying surface free energy indicate the importance of polar interactions in adhesion. Variation due to temperature and time after expulsion suggests that an increase of tubule rigidity, presumably under enzymatic control, takes place after tubule elongation and reinforces adhesion by minimizing peeling effects.