Benthic marine animals come in two general flavors. In some species, females produce large numbers of small eggs, and these develop into morphologically complex pelagic larvae that feed in the plankton before they metamorphose into benthic juveniles. In other species, females produce small numbers of large eggs, and these develop directly into juveniles, never having to feed as larvae. Transitions between these life-history strategies have occurred hundreds of times across the phylogeny of animals. In only one species, the polychaete annelid Streblospio benedicti, do both strategies coexist as a heritable dimorphism. I’ll describe our genetic dissection of developmental variation in this species, and I’ll show how the genetic details of developmental variation influence life-history evolution.