Attachment capacity of the sea urchin Paracentrotus lividus in a range of seawater velocities in relation to test morphology and tube foot mechanical properties

Intertidal rocky shores are stressful environments where benthic invertebrates experience large wave-induced hydrodynamic forces that can detach them from the substratum. The tube feet of echinoids counteract these forces and help them remain securely affixed. Sea urchins display a high degree of phenotypic plasticity which may help them cope with hydrodynamic stress. We evaluated whether habitats presenting different seawater velocities induce plastic responses in the attachment capacity of the sea urchin Paracentrotus lividus by quantifying their morphology and the adhesive and mechanical properties of their tube feet. Intertidal adult sea urchins from three subpopulations were collected around the Crozon peninsula (France). Localities differed according to measured water velocities. Size was significantly lower in the two most exposed sites where sea urchins also presented a higher density of tube feet. Tube foot adhesive properties were not significantly different between sites, but their extensibility and toughness were significantly higher in individuals from the most exposed site. Using this information, we calculated a safety factor to predict the flow velocity that would cause detachment from the substratum. It showed individuals from the most exposed habitat would resist higher flow velocities (up to 7.59 ± 0.90 m s). Both morphometry and tube foot mechanical properties vary among subpopulations and show an intraspecific plasticity in P. lividus. Although, differences in sea water velocity may be one cause of this intraspecific variation, it likely results from a combination of biotic and abiotic factors.