Spatial sorting and accelerated larval development in a range expanding fiddler crab

Although climate change has driven the widespread occurrence of species range shifts, the ecological and evolutionary processes that generate these patterns remain poorly understood. Here, we investigate spatial sorting and countergradient selection as evolutionary mechanisms contributing to range expansion. We focus on the rapid range expansion of a fiddler crab (Minuca pugnax) in the Gulf of Maine, an ocean warming hotspot. We use common garden experiments to quantify change in pelagic larval duration (PLD), a key determinant of dispersal ability, across edge and interior core populations, and between timepoints separated by 19 years. Consistent with spatial sorting, larvae from poleward, edge populations had shorter PLDs relative to equatorward, core populations, which is more dispersive in this system because it is characterized by equatorward flows. Compared to 19 years prior, PLDs were significantly shorter, consistent with genetic change over time. Analysis of sea surface temperatures reveal that the larval development window in the edge populations has doubled over the past twenty years, suggesting the combined influence of ocean warming, trait differentiation over time, and spatial sorting in promoting range expansion. While spatial sorting is known to aid in terrestrial range expansions, our results are the first to suggest that this process might be important in the coastal oceans.