Abstract
Extensive research has shown that the early life stages of marine organisms are sensitive to ocean acidification (OA). Less is known, however, on whether larval settlement and metamorphosis may be affected, or by which mechanisms. These are key processes in the life cycle of most marine benthic organisms, since they mark the transition between the free swimming larval stage to benthic life. We investigated whether OA could affect the larval settlement success of the sea urchin Evechinus chloroticus, a key coastal species with ecological, economic and cultural importance in New Zealand. We performed four settlement experiments to test whether reduced seawater pH (ranging from 8.1 to 7.0, at an interval of ~0.2 pH units) alters larval settlement and metamorphosis success. Our results show that settlement success was not significantly reduced when the larvae were exposed to a range of reduced seawater pH treatments (8.1–7.0) at time of settlement (on direct effects). Similarly, when presented with crustose coralline algae (CCA) pre-conditioned in seawater pH of either pH 8.1 or 7.7 for 28 days, larval settlement success remained unaltered (on indirect effects). We conclude that competent larvae in this species are resilient to OA at time of settlement. Further research on a range of taxa that vary in settlement selectivity and behaviour is needed in order to fully understand the effects of OA on the life cycle of marine invertebrates and the consequences it might have for future coastal marine ecosystems.
•Settlement success of E. chloroticus was unaffected when left to settle in a range of reduced seawater pH (7.0 to 8.1).•Larval settlement success remained unaltered in presence of CCA pre-conditioned for 28 days at either ambient or reduced pH (7.7).•Larvae did not lose their ability to recognize their preferred settlement substrate in presence of reduced seawater pH.•No interactive effects were observed between seawater pH and preconditioned substrates on the larval settlement success.