SUMMARYReduction in global ocean pH due to the uptake of increased atmospheric CO 2 is expected to negatively affect calcifying organisms, including the planktonic larval stages of many marine invertebrates. Planktonic larvae play crucial roles in the benthic-pelagic life cycle of marine organisms by connecting and sustaining existing populations and colonizing new habitats. Calcified larvae are typically denser than seawater and rely on swimming to navigate vertically structured water columns. Larval sand dollars Dendraster excentricus have calcified skeletal rods supporting their bodies, and propel themselves with ciliated bands looped around projections called arms. Ciliated bands are also used in food capture, and filtration rate is correlated with band length. As a result, swimming and feeding performance are highly sensitive to morphological changes. When reared at an elevated P CO2 level (1000ppm), larval sand dollars developed significantly narrower bodies at four-and six-arm stages. Morphological changes also varied between four observed maternal lineages, suggesting within-population variation in sensitivity to changes in P CO2 level. Despite these morphological changes, P CO2 concentration alone had no significant effect on swimming speeds. However, acidified larvae had significantly smaller larval stomachs and bodies, suggesting reduced feeding performance. Adjustments to larval morphologies in response to ocean acidification may prioritize swimming over feeding, implying that negative consequences of ocean acidification are carried over to later developmental stages. Supplementary material available online at

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Predictions of ectotherms' responses to global warming depend on the relationship between environmental temperature and organismal performance. To date, most predictions of thermal responses are generated from experimentally derived measures of thermal tolerances of adults or estimates of thermal niches derived from geographic distributions of adults. It is unknown how measures derived from these different approaches compare, or, as thermal performance of ectotherms varies through ontogeny, how they compare across life stages. To understand the efficacy of the temperature–performance relationship of early developmental stages as predictors of geographic range, we empirically documented the upper and lower thermal tolerance limits of embryos of eight species of ectotherms with complex life cycles. We compared the embryonic thermal tolerances of each species to that of the adults (documented in a previous study), as well as to an estimate of the thermal niche derived from adult distributions. Our results show that in all eight species the upper thermal limit estimated from embryos is significantly lower than the upper thermal limit estimated from adults. The lower thermal limit for embryos and for adult performance are similar, but the lower thermal limit for adult survival is significantly lower. Warming tolerances, calculated as the difference between upper tolerance limit and the average ambient temperature during the warmest month, for each species are also smaller for embryos (~1°C) than for adults (~5°C). These results show that thermal tolerances of early developmental stages, which are often easier to obtain for large sample sizes over an array of temperatures than are tolerances of adults, are a powerful, and potentially more accurate tool for predicting future responses to climate change. A free Plain Language Summary can be found within the Supporting Information of this article.