Effects of season, size, and sex on tolerance of a simulated cold snap in latitudinally separated populations of the poleward-invading green porcelain crab (Petrolisthes armatus)

Poleward range expansion in the green porcelain crab Petrolisthes armatus is thought to be associated with rising coastal water temperatures along the southern Atlantic Bight. Periods of anomalous cold temperature (“cold snaps”), however, could delay or limit further expansion, especially if northern populations are unable to adjust cold tolerance through adaptation or acclimation. We tested cold tolerance in crabs from four latitudes, from Savannah, GA to North Inlet, SC, along the species' poleward invasion front. Laboratory incubation trials were conducted in multiple seasons using adult, non-ovigerous P. armatus to compare cold temperature thresholds between populations, sexes, and sizes. These trials recorded temperature thresholds for induction of both sublethal (loss of righting response) and lethal effects. Following acclimation, crabs were exposed to a rapid (1 °C d⁻¹) temperature drop, modeled after water temperatures recorded during a severe winter cold snap in Charleston Harbor, SC. Crabs generally experienced mobility loss below 8 °C and mortality below 4 °C. In trials at the end of fall, crabs from the two more northern populations survived significantly longer, consistent with latitudinal differences in temperature as crabs prepared physiologically for winter. At the end of summer, crabs from the northernmost population showed greater cold resilience in both lethal and sublethal measures. At the end of spring, when animals are reproductive, there were mixed results with regard to a latitudinal pattern. When differences between sexes and sizes were significant, females were less susceptible than males and, in all but one case, smaller individuals were less susceptible than larger. In general, crabs at the start of winter maintained mobility and survived longer in the cold snap than they did when tested in the other two seasons. Although cold snaps may reduce population sizes of P. armatus, increased cold tolerance of more poleward populations could help to facilitate range expansion, especially if increasingly mild winter conditions move their thermal barrier toward higher latitudes.