* These authors contributed equally to this work.The increase in atmospheric CO 2 due to anthropic activities results in an acidification of the surface waters of the oceans. The impact of these chemical changes differs according to the considered organisms. The intertidal rocky shores may harbor organisms pre-adapted to the upcoming changes as they already face tidal pH and temperature fluctuations. In order to cope with the changes in seawater pH, these organisms possess different mechanisms involved in acid-base regulation. Some organisms present a higher buffer capacity than seawater, among which echinoderms. The properties of this buffer capacity and the factors influencing it were investigated in the sea urchin Paracentrotus lividus and in the starfish Asterias rubens, both species occurring in the intertidal zone of the North Atlantic and the North Sea, respectively. Buffer capacity is partly due to the coelomocytes present in the coelomic fluid and, in P. lividus, it is also due to a compound which contributes to a higher buffer capacity of the coelomic fluid of this species compared to that of the starfish. The effect of a decreased seawater pH (in the scope of predicted future ocean acidification) on this buffer capacity in P. lividus was investigated. A gradual increase of the buffer capacity was recorded when the seawater pH was decreased. Moreover, the comparison of different echinoderm species showed that Euechinoidea present a very high buffer capacity while Cidadroidea (other sea urchins), starfish and holothurians have a lower one. This can be explained either by the presence of the compound only in Euechinoidea, linked to differences in the respiratory machinery, or by metabolic differences between the various classes of echinoderms.

We examined the long-term effects of near-future changes in temperature and acidification on skeletal mineralogy, thickness, and strength in the sea urchin Tripneustes gratilla reared in all combinations of three pH (pH 8.1, 7.8, 7.6) and three temperatures (22 °C, 25 °C, 28 °C) from the early juvenile to adult, over 146 days. As the high-magnesium calcite of the echinoderm skeleton is a biomineral form highly sensitive to acidification, and influenced by temperature, we documented the MgCO3 content of the spines, test plates, and teeth. The percentage of MgCO3 varied systematically, with more Mg2+ in the test and spines. The percentage of MgCO3 in the test and teeth, but not the spines increased with temperature. Acidification did not change the percentage MgCO3. Test thickness increased with warming and decreased at pH 7.6, with no interaction between these factors. In crushing tests live urchins mostly ruptured at sutures between the plates. The force required to crush a live urchin was reduced in animals reared in low pH conditions but increased in those reared in warm conditions, a result driven by differences in urchin size. It appears that the interactive effects of warming and acidification on the Mg2+ content and protective function of the sea urchin skeleton will play out in a complex way as global climatic change unfolds.

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