Intraspecific variation in thermotolerance and morphology of the invasive European green crab, Carcinus maenas, on the west coast of North America

Abstract

For marine ectotherms, environmental temperature plays an important role in helping to determine biogeographical distribu tions. Understanding the impacts that temperature can have on organismal function can lead to insights into the nature of adaptive variation in thermotolerance. This approach has been widely used to understand the spatial distribution of related species within the context of evolutionary time scales. Here, we measured the upper lethal thermal threshold of the invasive European green crab, Carcinus maenas (Linnaeus, 1758), sampled from two populations from the southern and northern limits of its recipient range on the west coast of North America: Sea Drift Lagoon, Stinson Beach, California (CA), and Pipestem Inlet, Vancouver Island, British Columbia (BC). These two populations differ in their natural thermal regimes; sea surface temperatures near the southern, CA population range from ~ 23 °C to ~ 11 °C while temperatures near the northern population in BC range from ~ 14 °C to ~ 6 °C during summer and winter, respectively. We demonstrate that after laboratory acclimation to 6 °C or 23 °C, the CA group had a significantly higher mean critical thermal maximum (CTmax) than its northern counterpart, in both acclimation groups. After acclimation to 6 °C, the CA group displayed a mean CTmax of 34.7 °C whereas the mean CTmax of the BC group was 31.7 °C. In the 23 °C acclimation groups, mean CTmax was 36.2 °C in the CA population and 35.5 °C in the BC one. To establish whether these differences in whole-organism thermotolerance were manifested at the molecular scale, levels of the inducible stress protein, heat shock protein 70 (Hsp70), were quantified in the chelae of the crabs used in the CTmax assay, at each individual's time-of-death. In the 6 °C acclimation comparison, the CA population had significantly greater Hsp70 levels than the BC population. No difference in Hsp70 levels was observed between the 23 °C acclimation groups. In a separate experiment, collection site-specific differences were found in the thermal threshold for induction of Hsp70 in isolated chelae tissue. The two populations also differed morphologically. Significant differences in carapace widths (CW) were measured for adult male and female green crabs from the two populations, sampled during 2008–2009. Observable divergences in both morphology and in physiological responses to heat stress have occurred within the relatively recent time span (~ 20 years) of the invasion of the European green crab on the west coast of North America and it is possible that a large, northern cold-water phenotype may have already arisen.