Plasticity and genetics shape thermal tolerance in non-native populations of the European green crab ( Carcinus maenas )

Some successful introduced species represent a genetic paradox: they are able to thrive in novel environments despite limited genetic diversity. The European green crab ( Carcinus maenas ) is a quintessential example of this paradox, having established across extensive introduced ranges in the northwest Atlantic and northeast Pacific in North America. Green crabs boast a broad thermal tolerance and exceptional thermal flexibility despite the loss of adaptive potential from repeated bottlenecks. Prior work has found a strong population-level association between an extended genomic region (supergene) and both sea surface temperature and cold tolerance. We conducted a series of three experiments to understand if this association translates to sublethal thermal tolerance and plasticity in righting response for individual adult crabs from multiple populations. While both crabs from Washington and Maine exposed to a 30°C heat shock showed signs of stress, only the Maine population demonstrated different righting responses by supergene genotype. A second experiment exposing Washington C. maenas to repeated heat stress showed that crabs with prior heat exposure do not exhibit a stress response to a subsequent heat stress, irrespective of genotype. The third experiment examined short- and medium-term cold acclimation in C. maenas from Washington and Massachusetts. At 5°C, crabs exhibited an acclimatory response where righting gradually sped up over the course of cold exposure. Some crabs in both populations failed to right at 1.5°C. Failure to right could be indicative of dormancy employed to reduce energy consumption in colder conditions, but allow for opportunistic feeding and predator avoidance. While a higher proportion of Massachusetts crabs with the putatively warm-adapted supergene allele failed to right at 1.5°C, this trend was not significant. We suggest that this subtle, context-specific impact of genotype on adult thermal tolerance is related to environmental temperature throughout life history shaping both supergene genotype distributions and thermal physiology without a direct causal link between supergene genotype and adult tolerance as measured in our experiments. Adult crabs maintain robust thermal plasticity despite a substantial reduction in genetic diversity, likely contributing to their global success. Linking population-level genetic associations with individual-level physiology is complex, and reflects the impact of environmental conditions such as temperature throughout life history in shaping adult phenotype. Our work illustrates the importance of investigating population-level associations at the individual level to better understand the full range of mechanisms by which organisms respond to their environment.