High thermal plasticity, and vulnerability, in extreme environments at the warm distributional edge: The case of a tidepool shrimp

Climate change threatens the resilience of species, especially at their warm distributional edge in extreme environments. We investigated the thermal vulnerability of the tidepool shrimp, Palaemon elegans in the fastwarming southeastern Mediterranean Sea, its warm distributional edge. Tidepool organisms experience strong and fast thermal fluctuations. This might make them more resilient to change, but also bring them closer to their thermal limits during extreme conditions. To examine the shrimp's resilience, we tested three hypotheses: (1) P. elegance in the southeast Mediterranean has higher critical thermal maximum (CTMax) than in cooler regions, (2) the shrimp possess seasonal acclimatization, but (3) long exposure to extreme summer temperatures might erode its thermal performance making it vulnerable to future climate change. In a series of field observations and laboratory experiments we characterized the shrimp's thermal environment and population dynamics, determined its CTMax and tested diverse physiological performance attributes (respiration, digestion, activity, growth) under a wide range of temperatures during winter and summer. The thermal performance curves show that peak respiration rates occur at higher temperatures in the summer compared to winter, indicating seasonal acclimatization. The shrimp's CTMax was 38.1 degrees C; higher than its Atlantic counterparts, and an indication of biogeographic adaptation. However, its warming tolerance (the thermal safety margin: distance between maximum pool temperatures and CTMax) was only 0.3 degrees C, indicating low capacity for dealing with further warming in mostly isolated pools, compared to northeast Atlantic populations that have a wider safety margin. Performance also eroded with increase in exposure duration to high temperatures, and the optimum temperature reduces with time. We predict that prolonged exposure to current (similar to 32 degrees C) and future mean summer values in open water would significantly reduce its performance and increase its mortality. Further, juvenile P. elegans were found to be more vulnerable to extreme (cold and hot) temperatures than adults. Together, these findings indicate that the population viability of this robust coastal water shrimp would eventually be reduced under continuous regional warming and the intensification of extreme events.