Biogeographic variation in reproductive strategy in a range-expanding marine gastropod

Although species geographic distributions are increasingly shifting poleward with climate change, the processes that facilitate or impede range shifts are poorly understood. More extreme environmental conditions near poleward range boundaries often impose reproductive constraints, which may limit fitness and future range expansion. However, few studies evaluate the extent to which adaptive evolution and/or phenotypic plasticity might mitigate the effects of reproductive constraints and contribute to successful range shifts. We examined reproductive trait variation across the geographic range of Acanthinucella spirata, an intertidal snail that has recently expanded poleward along the coast of California, USA. We found that, compared to the range core, A. spirata from cooler range-edge populations allocated approximately 16x more nurse eggs to each developing embryo and had larger offspring at hatching, which may lead to greater juvenile growth rates and survival. Furthermore, range-edge A. spirata populations had lower fecundity, highlighting a potential tradeoff between offspring size and number. Some trait differences between range-core and range-edge populations persisted in an 18-month common-garden experiment, consistent with the hypothesis that such traits may be under selection. This study suggests that life-history adaptations in range-edge populations are an important and understudied mechanism that may contribute to species range expansions.