| Summary: | Background: As the oceans simultaneously warm, acidify and increase in PCO2, prospects for marine biota are of concern. Calcifying species may find it difficult to produce their skeleton because ocean acidification decreases calcium carbonate saturation and accompanying hypercapnia suppresses metabolism. However, this may be buffered by enhanced growth and metabolism due to warming. Methodology/Principal Findings: We examined the interactive effects of near-future ocean warming and increased acidification/PCO2 on larval development in the tropical sea urchin Tripneustes gratilla. Larvae were reared in multifactorial experiments in flow-through conditions in all combinations of three temperature and three pH/PCO2 treatments. Experiments were placed in the setting of projected near future conditions for SE Australia, a global change hot spot. Increased acidity/PCO2 and decreased carbonate mineral saturation significantly reduced larval growth resulting in decreased skeletal length. Increased temperature (+3uC) stimulated growth, producing significantly bigger larvae across all pH/PCO2 treatments up to a thermal threshold (+6uC). Increased acidity (-0.3-0.5 pH units) and hypercapnia significantly reduced larval calcification. A +3uC warming diminished the negative effects of acidification and hypercapnia on larval growth. Conclusions and Significance: This study of the effects of ocean warming and CO2 driven acidification on development and
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