Tolerance measured at one developmental stage mispredicts climate resilience in an animal with a complex life cycle

Forecasts of whether marine species can evolve fast enough to survive climate change rely largely on single-stage tolerance assays, tacitly assuming that tolerance measured at one developmental stage reflects adaptive capacity across the organism's life cycle. We test this assumption in a common marine bivalve (Mercenaria mercenaria) and find that it fails in sign, not degree: larval and post-metamorphic tolerance to end-century ocean acidification are strongly negatively correlated (r = -0.53, 95% CI [-0.69, -0.33]) across family-level, pedigree-conditioned and animal model estimates, with pervasive rank reversals among the top performers of each stage. A multivariate analysis of the among-family covariance matrix G shows why: under acidification, 57% of the among-family variance collapses onto a single dominant axis that loads positively on every larval trait and negatively on every post-metamorphic trait, whereas under ambient conditions the same families show no cross-stage covariance at all. Acidification therefore does not merely depress performance-it reorganizes the genetic architecture into a cross-stage conflict. Under ramping RCP 8.5 forcing this conflict geometry dictates the direction-not merely the magnitude-of adaptation: only balanced multi-stage selection preserves family diversity, whereas single-stage selection systematically erodes the opposite axis. In complex life cycles, resilience is a geometric property of ontogeny that forecasts cannot compress into a single-stage without inverting the sign of the answer.