Morphometry and mechanical design of tube foot stems in sea urchins: a comparative study

To withstand hydrodynamic forces, sea urchins rely on their adoral tube feet, which are specialized for attachment. Although it has been often suggested that the degree of development of these tube feet is intimately related to the maximum environmental energy a species can withstand, it has never been demonstrated by mechanical testing. To address this subject, we studied the mechanical properties of the stem of adoral tube feet from three species of sea urchins, Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis, which have distinct taxonomic, ecological and morphological characteristics. The tube feet of the three species have a very similar morphology. When a tensile force is applied to the tube foot stem, the connective tissue is the only tissue layer bearing the load. The mechanical properties of this tissue give the tube feet an ideal balance of extensibility (139-166%), strength (23-29 MPa) and stiffness (152-328 MPa), which together produce a material with adequate toughness (2.5-2.9 MJ/m3) to absorb the impact of waves and currents, and thus to resist the environmental challenges of the habitats in which sea urchins live. Extended stems of P. lividus were significantly stiffer (328 MPa) than those of the other two species (152 and 183 MPa, for A. lixula and S. granularis, respectively). No interspecific difference was found in terms of extensibility, strength, initial stiffness and toughness between the tube feet from the three species. The difference in local distribution between the species investigated is therefore not only explained by the mechanical properties of their tube feet, but may involve other factors such as tube foot number and arrangement, tube foot disc tenacity or sea urchin size.