Response of sea urchin pluteus larvae (Echinodermata: Echinoidea) to reduced seawater pH: a comparison among a tropical, temperate, and a polar species
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Ocean acidification, as a result of increased atmospheric CO2, is predicted to lower the pH of seawater to between pH 7.6 and 7.8 over the next 100 years. The greatest changes are expected in polar waters. Our research aimed to examine how echinoid larvae are affected by lower pH, and if effects are more pronounced in polar species. We examined the effects of lowered pH on larvae from tropical (Tripneustes gratilla), temperate (Pseudechinus huttoni, Evechinus chloroticus), and a polar species (Sterechinus neumayeri) in a series of laboratory experiments. Larvae were reared in a range of lower pH seawater (pH 6.0, 6.5, 7.0, 7.5, 7.7, 7.8 and ambient), adjusted by bubbling CO2 gas. The effect of pH on somatic and skeletal growth, calcification index, development and survival were quantified, while SEM examination of the larval skeleton provided information on the effects of seawater pH on the fine-scale skeletal morphology. Lowering pH resulted in a decrease in survival in all species, but only below pH 7.0. The size of larvae were reduced at lowered pH, but the external morphology (shape) was unaffected. Calcification of the larval skeleton was significantly reduced (13.8–36.9% lower) under lowered pH, with the exception of the Antarctic species, which showed no significant difference. SEM examination revealed a degradation of the larval skeletons of Pseudechinus and Evechinus when grown in reduced pH. Sterechinus and Tripneustes showed no apparent difference in the skeletal fine structure under lowered pH. The study confirms the need to look beyond mortality as a single endpoint when considering the effects of ocean acidification that may occur through the 21st century, and instead, look for a suite of more subtle changes, which may indirectly affect the functioning of larval stages.
KeywordsOcean Acidification Ambient Seawater Calcification Index Seawater Carbonate Chemistry Larval Skeleton
We thank Antarctica New Zealand for their logistical support in Antarctica, and Ian Bertram and The Cook Islands Ministry of Marine Resources for assisting in research at Aitutaki. Thanks to Richard Story, manager of the Cook Island Marine Research Centre, and staff at the Portobello Marine Laboratory, University of Otago. Professor Keith Hunter provided expert information on carbonate chemistry. Professor Hunter also developed the software we used for determining the carbonate chemistry of seawater. Liz Girvan provided expertise during SEM preparation and imaging. This research was generously supported by a University of Otago Research Grant (MDL, MFB). Dana Clark was awarded a Brenda Shore Award from the Federation of Graduate Women, Otago Branch, which supported her Post-graduate research activities.
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