When is warmer better? Disentangling within- and between-generation effects of thermal history on survival

O_LIUnderstanding the fitness consequences of thermal history is necessary to predict organismal responses to global warming. This is especially challenging for ectotherms with complex life cycles, since distinct life stages can differ in thermal sensitivity, acclimate to different thermal environments, and accrue responses to acclimation within and between generations. C_LIO_LIAlthough acclimation is widely hypothesized to benefit organisms by helping them (or their offspring) to compensate for negative impacts of environmental change, equivocal support for this hypothesis highlights the need to assess alternatives. However, assessments that do so in ways that explicitly dissect responses across life stages and generations remain limited. C_LIO_LIWe assess alternative hypotheses for acclimation responses (none, beneficial, colder-is-better, and warmer-is-better) within and between generations of an externally-fertilizing marine tubeworm whose vulnerability to warming rests on survival at early planktonic stages (gametes, embryos, and larvae). We start by acclimating parents, gametes, and embryos to ambient and projected warmer temperatures (17 {degrees}C and 22 {degrees}C) factorially by life stage. We then rear individuals with differing acclimation histories to the end of larval development at test temperatures from 10 {degrees}C to 28 {degrees}C (upper and lower survival limits) to estimate thermal survival curves for development, and compare curves among acclimation histories. C_LIO_LIWe show that survival curves are most responsive to parental acclimation followed by acclimation at embryogenesis, but are buffered against acclimation at fertilization. Moreover, curves respond independently to acclimation within and between generations, and respond largely as predicted by the warmer-is-better hypothesis, despite the semblance of beneficial acclimation after successive acclimations to warmer temperature. C_LIO_LIOur study demonstrates the varied nature of thermal acclimation, and the importance of considering how acclimation responses aggregate across complex life cycles when predicting vulnerability to warming. C_LI