Impact of elevated pCO2 on acid–base regulation of the sea urchin Echinometra mathaei and its relation to resistance to ocean acidification: A study in mesocosms

• The effects of seawater pHT 7.7 on adult sea urchin Echinometra mathaei was studied.
• The study was conducted in mesocosms during 7 weeks.
• Sea urchins fed on naturally growing algae (no added food).
• E. mathaei maintained the pH of the coelomic fluid through a bicarbonate increase.
• Growth and respiration were not affected.

Due to their low metabolism and apparent poor ion regulation ability, sea urchins could be particularly sensitive to ocean acidification resulting from increased dissolution of atmospheric carbon dioxide. Therefore, we evaluated the acid–base regulation ability of the coral reef sea urchin Echinometra mathaei and the impact of decreased pH on its growth and respiration activity. The study was conducted in two identical artificial reef mesocosms during seven weeks. Experimental tanks were maintained respectively at mean pHT 7.7 and 8.05 (with field-like night and day variations). The major physico-chemical parameters were identical, only pCO2 and pHT differed. Results indicate that E. mathaei can regulate the pH of its coelomic fluid in the considered range of pH, allowing a sustainable growth and ensuring an unaffected respiratory metabolism, at least at short term.