Ocean Acidification Reduces Spine Mechanical Strength in Euechinoid but Not in Cidaroid Sea Urchins

Echinoderms are considered particularly sensitive to ocean acidification (OA) as their skeleton is made of high-magnesium calcite, one of the most soluble forms of calcium carbonate. Recent studies have investigated effects of OA on the skeleton of "classical" sea urchins (euechinoids), but the impact of etching on skeleton mechanical properties is almost unknown. Furthermore, the integrity of the skeleton of cidaroids has never been assessed, although their extracellular fluid is under-saturated with respect to their skeleton, and the skeleton of their primary spines is in direct contact with seawater. In this study, we compared the dissolution of test plates and spines as well as the spine mechanical properties (two-points bending tests) in a cidaroid (Eucidaris tribuloides) and a euechinoid (Tripneustes ventricosus) submitted to a 5 week acidification experiment (pH_T of 8.1, 7.7, and 7.4). Test plates of both species were not affected by dissolution. The spines of E. tribuloides showed no mechanical effects at pH_SW-T 7.4 despite having traces of corrosion on secondary spines. On the contrary, spines of the T. ventricosus were significantly etched at both pH_SW-T 7.7 and 7.4 and their fracture force reduced by 16 to 35%, respectively. This increased brittleness is probably of little significance with regards to predation protection but has consequences in terms of energy allocation.