The threespine stickleback (Gasterosteus aculeatus) is a small, mesopredatory fish that is widespread in coastal regions of the northern hemisphere. Although this species does not directly support a commercial or recreational fishery, threespine stickleback often serve as important prey for larger fish that do support important fisheries, as well as many bird species. Although studied extensively as a model organism in evolutionary biology, behavioral ecology, genomics, and numerous related subfields, this research relies heavily on populations from the Pacific coastal regions of North America and Asia, and those of northern Europe. However, based on the morphology of some western Atlantic populations, the different ecological context, and the evolutionary history of the species, not all of the knowledge gained from Pacific and European lineages is likely to be fully transferrable to the populations of North America's East Coast. Nevertheless, work in eastern Canada does suggest high levels of intraspecific phenotypic diversity and local adaptation, though much of this diversity may be under threat from changing climate, altered land use patterns, and introduced species. These factors warrant a research program focused on broad sampling of previously identified populations, identifying previously undocumented populations, determining whether there are unique genetic mechanisms underlying the unusual trait combinations present in the region, and exploring novel community interactions. Such a research program would facilitate the documentation of phenotypic change and establish baselines for future work. Because the work on nearshore marine threespine stickleback populations is sparse in the western Atlantic, I focus here on freshwater populations-with the exception of a brief discussion of the "white" stickleback populations of Nova Scotia-but this is not to suggest that nearshore marine populations are not phenotypically diverse.
