Thermal ecology of the fiddler crab Uca panacea: Thermal constraints and organismal responses.

Temperature is one of the primary environmental variables limiting organismal performance, fitness, and species distributions. Yet, understanding temperature effects requires thorough exploration of thermal constraints and organismal responses that can translate to fitness and non-lethal long-term consequences under both constant and changing thermal regimes. We examined the thermal ecology of the fiddler crab Uca panacea, including critical thermal limits, thermal sensitivity of locomotion, operative environmental temperatures, preferred body temperatures, and acclimation ability. Operative environmental temperatures frequently reached the critical thermal maximum (41.8±0.8°C, mean ± s.e.m.), especially in unvegetated microhabitats, indicating the need for behavioral thermoregulation to maintain diurnal activity patterns. Preferred body temperatures (21.1-28.6°C) were substantially below the thermal optimum (30-40°C), although further research is needed to determine the driver of this mismatch. Critical thermal limits shifted 2-4°C in response to exposure to low (20°C) or high (35°C) temperatures, with full acclimation occurring in approximately 9d. This capacity for rapid acclimation, combined with the capacity for behavioral thermoregulation, is a strong candidate mechanism that explains the broad habitat use and could help explain the successful pantropical distribution of fiddler crabs.