Polymorphism promotes edge utilization by marsh crabs.

Understanding how habitat edges affect ecological processes is crucial given widespread and increasing modifications to natural landscapes. Resource specialization is a key factor affecting among-species edge responses, but we know little about how intraspecific resource use variation mediates edge utilization. Here, we integrate stomach content analysis, geometric morphometrics and feeding experiments to explore the role of resource polymorphism in mediating marsh crab (Panopeus obesus) foraging within the marsh-oyster reef boundary. Stomachs of edge individuals contained a greater proportion of morphologically defended edge prey (bivalves) compared to core marsh individuals, and edge individuals possessed relatively tall and robust claw morphology for manipulating such prey. We further show experimentally that phenotypic changes of edge P. obesus are associated with enhanced feeding efficiency on small, but not large edge prey. Morphological and ecological traits of edge P. obesus overlapped with the edge-occurring congener, P. herbstii, suggesting some degree of functional convergence despite the potential for interspecific competition within edges. Though this polymorphism is likely plastic, the success of P. obesus along edges could subsidize predator production within marshes and alter top-down pressure across mosaic estuarine landscapes. More generally, our study reveals polymorphism as a driver of edge utilization, while yielding new insight into the processes that maintain or erode spatial niche differentiation within predator guilds.