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

• We examined the thermal ecology of the fiddler crab Uca panacea.
• Operative environmental temperatures frequently exceeded the critical thermal maximum.
• Locomotor performance peaked between 30 and 40 °C.
• Preferred body temperatures were below the thermal optimum.
• Exposure to increased or decreased temperatures resulted in rapid acclimation.
• Plastic responses to temperature may explain pantropical distribution of fiddler crabs.

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 9 d. 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.