Contrast in larval sensitivity to low pH in sea urchins from neighbouring seagrass meadows at Inhaca Island, Mozambique

Abstract

Ocean acidification presents a major threat to marine life, and a large body of evidence has documented its negative effects on various  marine species and ecosystems. Recent meta-analyses highlight the key role of local adaptation and have linked species’ sensitivity to  extremes of the present range of pH variability. More specifically, ‘physiological tipping points’ can be correlated with the lowest  experienced pH where the organisms live. We verified this hypothesis by comparing the responses of larvae of the collector urchin  Tripneustes gratilla originating from two neighbouring seagrass meadows at Inhaca Island in southern Mozambique. The two seagrass  meadows experience different conditions: the site at the Inhaca Marine Biology Research Station (Estação de Biologia Marítima de  Inhaca, EBMI) is subtidal, and the site at Banguá is intertidal. Larvae of adults collected from the two sites were cultured in the laboratory  under four different target pH treatments (8.04, 7.67, 7.46 and 7.29) for 7 days. The results showed an overall negative effect of lower pH  on survival and growth as well as on an index of symmetry. Larvae originating from the intertidal seagrass habitat at Banguá which  experiences large pH fluctuations were more sensitive to a lower pH than those from the subtidal seagrass habitat at EBMI. This finding  suggests that different mechanisms for pH tolerance and local adaptation may apply in subtidal versus intertidal environments.  Consequently, the unifying principle that physiological tipping points are correlated with the extreme low pH in the present range of  variability, may reach its limit in increasingly extreme environments under natural conditions.