Thermogeographic variation in body size of Carcinus maenas , the European green crab

Populations that span a large geographic range often experience a thermal gradient that can differentially affect the phenotypic response of individuals across the population. Variation in temperature has been shown to affect the final adult size of ectotherms, which is referred to as the temperature–size rule for ectotherms. Body size is a fundamentally important trait, as it can impact physiological performance, fecundity, longevity, and macroecological patterns. Hence, temperature may affect body size across a range, which can in turn influence maintenance of populations and ecological interactions. Here, we test whether biogeographic differences in size (carapace width) exist for a recent invasion of the non-native European green crab, Carcinus maenas , along the west coast of North America. We assembled trapping and temperature data collected from 10 sites along the western North American coast over a 5-year period. We also conducted a literature review of C. maenas size across their native range. Our results indicate that adult body size shows negative correlation with environmental temperature in both the native and invaded ranges, conforming to the temperature–size rule for ectotherms. Given the short time since colonization and lack of evident genetic structure across the invasive range, it may be that phenotypic plasticity in response to environmental temperature is driving this relationship. Forces that shape the phenotypic trajectory of species may play an important role in both invasion dynamics and subsequent ecological impacts.