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

Understanding 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, where distinct life
stages can differ in thermal sensitivity, acclimate to different thermal
environments, and accrue responses to acclimation within and between
generations. Although acclimation is often hypothesized to benefit
organisms by helping them (or their offspring) to compensate for negative
impacts of environmental change, mixed support for this hypothesis
highlights the need to assess alternatives. Assessments that explicitly
dissect responses across life stages and generations, however, remain
limited.  We assess alternative hypotheses of acclimation (none,
beneficial, colder-is-better, and warmer-is-better) within and between
generations of a marine tubeworm whose vulnerability to warming rests on
survival at early planktonic stages (gametes, embryos, and larvae). First,
we acclimate parents, gametes, and embryos to ambient and projected warmer
temperatures (17°C and 22°C) factorially by life stage. Next, we rear
offspring with differing acclimation histories to the end of larval
development at test temperatures from 10°C to 28°C (lower and upper
survival limits, respectively). Last, we estimate thermal survival curves
for development and compare them among thermal histories.   We
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 doses of warmer
temperature.  Our study demonstrates the varied nature of thermal
acclimation and the importance of considering how responses aggregate
across complex life cycles when predicting vulnerability to warming.