Intra- and interspecific variation of attachment strength in sea urchins

To withstand hydrodynamic forces, sea urchins rely on their oral tube feet, which are specialised for attachment. It has been proposed that the degree of development of these tube feet is intimately related to the maximum wave force a species can withstand. To address this, the variation of scaled attachment force and tenacity among and within echinoid species, and with environmental conditions, was investigated. Three populations of Paracentrotus lividus from different habitats and geographical regions were compared. There were few significant intraspecific variations in tenacity, but those that were detected were found to be positively correlated with the seawater temperature. For one P. lividus population, the influence of environmental parameters on the temporal variation of the attachment strength measured under laboratory and field conditions was analyzed. Strong significant correlations were found with wave height at the time of collection, but only when sea urchins were tested directly in their natural habitat, where they appear to respond to increased wave height by using more tube feet, thereby increasing their attachment force. Among species, P. lividus attached with a significantly higher tenacity (adhesion force per unit adhesive surface area) (0.37 MPa) than Sphaerechinus granularis (0.19 MPa) and Arbacia lixula (0.12 MPa). However, when the safety factor (which accounts for animal shape, size and number of adoral tube feet) was calculated, the larger S. granularis from calm deep subtidal habitats was predicted to be the first species to be dislodged at water speeds above 4.6 m s-1, whereas the smaller A. lixula and P. lividus, which typically occur in shallow areas subjected to stronger hydrodynamic forces, were able to remain attached up to water velocities of 5.5 and 8.2 m s-1, respectively