Effects of anthropogenic seawater acidification on acid–base balance in the sea urchin Psammechinus miliaris

The purple-tipped sea urchin, Psammechinus miliaris, was exposed to artificially acidified seawater treatments (pHw 6.16, 6.63 or 7.44) over a period of 8 days. Urchin mortality of 100% was observed at pHw 6.16 after 7 days and coincided with a pronounced hypercapnia in the coelomic fluid producing an irrecoverable acidosis. Coelomic fluid acid–base measures showed that an accumulation of CO2 and a significant reduction in pH occurred in all treatments compared with controls. Bicarbonate buffering was employed in each case, reducing the resultant acidosis, but compensation was incomplete even under moderate environmental hypercapnia. Significant test dissolution was inferred from observable increases in the Mg2+ concentration of the coelomic fluid under all pH treatments. We show that a chronic reduction of surface water pH to below 7.5 would be severely detrimental to the acid–base balance of this predominantly intertidal species; despite its ability to tolerate fluctuations in pCO2 and pH in the rock pool environment. The absence of respiratory pigment (or any substantial protein in the coelomic fluid), a poor capacity for ionic regulation and dependency on a magnesium calcite test, make echinoids particularly vulnerable to anthropogenic acidification. Geological sequestration leaks may result in dramatic localised pH reductions, e.g. pH 5.8. P. miliaris is intolerant of pH 6.16 seawater and significant mortality is seen at pH 6.63.