Latitudinal differences in thermoregulatory color change in Uca pugilator

Animals can change color either rapidly in response to changes in their immediate environment or slowly as the seasons change. Such plasticity can permit local adaptation but it can also be constrained by physiological and behavioral mechanisms. Here, we explore how different temperature regimes along a latitudinal gradient spanning the natural range of a species of fiddler crab may affect the ability of crabs to change color rapidly in response to acute changes of temperature. Uca pugilator populations from New York (NY), North Carolina (NC) and Florida (FL) were exposed in air to 5 °C and 35 °C and carapace shade intensity (from dark to light color) and contrast was recorded. In general, individuals darkened when exposed to cold conditions, and lightened when exposed to warm conditions. Males from different populations differed in the magnitude of response to temperature, but had similar directional changes in shade (dark under cold conditions and light under warm conditions) suggesting differences among populations in the ability to change color. Females showed similar directional changes but there were no differences among populations. Differential responses of crabs to temperature suggest that crabs have local physiological adaptations and can acclimate quickly to withstand fluctuating or extreme temperatures.