Intraspecific Variation in Thermal Performance Suggests the Presence of Range‐Edge Divergence and Phenotypic Compensation of Northern Individuals in an Introduced Species

ABSTRACT Aim Intraspecific variation in functional traits may indicate adaptation to environmental gradients and is crucial to understanding species distributions and range dynamics. We tested the hypotheses that: (1) thermal performance of Littorina littorea follows a countergradient cline, reflecting compensation in northern individuals and (2) local differentiation occurs at the range limits of its non‐native Atlantic distribution. Location Atlantic coast of North America, 46 to 51°N. Time Period 2022. Major Taxa Studied Intertidal gastropods (Littorinidae). Methods Snails from 10 locations, spanning 10° of latitude and approximately 1700 km, were exposed to a gradient of 12 temperatures in the laboratory, and survival, growth, feeding, metabolic rate and heat tolerance were measured. Thermal performance curves obtained were compared and used to calculate performance parameters: that is, thermal optimum, maximum performance and thermal breadth. Results Snails from the northern and southern range edges exhibited a gradual divergence in growth performance parameters relative to central populations. Snails from colder northern locations exhibited greater survival, feeding rate and heat tolerance when exposed to higher temperatures. Main Conclusions The divergent local variation in the growth of individuals from range edges may reflect a stronger selective pressure occurring at the species range limits. In contrast, the clinal countergradient patterns observed for survival, feeding rate and heat tolerance suggest that individuals from northern locations have a greater acclimation capacity to compensate for stronger environmental fluctuations associated with higher latitudes. Our findings suggest that the greater acclimation capacity of northern populations could give them an advantage with the progression of climate changes, supporting the likelihood of a poleward range shift. By revealing trait‐specific trends of variation, our study demonstrates the importance of investigating multiple traits along a wide range of acclimation temperatures in individuals from locations across broad latitudinal gradients to uncover patterns of phenotypic change that would otherwise remain unexplored.