ABSTRACT
Echinoids being exclusively benthic animals, their activities and adaptations are correlated with an existence on the ocean bottom. To withstand hydrodynamic forces, sea urchins rely on their oral podia specialized in attachment and anchoring. The degree of development of these podia seems to be intimately in relation with the maximum environmental energy a species can withstand. To address this subject we studied the oral podia from three species of regular sea urchins Sphaerechinus granularis, Paracentrotus lividus and Arbacia lixula, which have distinct taxonomic, ecological and morphological characteristics. Intraspecific variability was also investigated by studying four populations of P. lividus from different habitats and geographical origins. The tenacity (adhesion force per unit area) with which an individual can attach depends on the strength of its podia which, in turns, depends on the tensile strength of the stem, and the adhesive power of the disc. Therefore, we measured the material properties of the stems of oral podia, the tenacity of their discs as well as the tenacity of individual sea urchins. The measurements of tensile strength of the stem indicated that S. granularis, which lives on muddy and sand bottoms, had the weakest stems while the individuals of other species which prefer rocky substratum and shallower depths thus being more exposed to environmental forces, had similarly strong stems. In terms of the adhesiveness of the podial disc and the tenacity of individual sea urchins the relationship between the mechanical properties and the habitat of the sea urchins was more complex. This might be due to behavioral artifacts given that these two measurements depend on the animals’ will to attach to the substratum, a factor which varied appreciably within and between species.
