Does thermal physiology shape the local and geographical distribution of intertidal invertebrates? : a case study with brachyuran crabs

Abstract

Climate change and global warming are a matter of concern for species conservation. With global temperature changes, species may experience shifts in their latitudinal distribution or even go extinct. Many authors argue that these changes will be more intense for tropical species, which live at temperatures close to their thermal limits and have limited acclimatory capacity. Due to these aspects, tropical ectotherms are often considered more vulnerable to climate change than species at higher latitudes. However, the validity of this assumption has been put under scrutiny, as regions at higher latitudes are predicted to have higher increases in temperature and higher frequency of extreme climate events. Tropical and subtropical intertidal species provide a perfect model to investigate the responses and vulnerability of ectotherms to temperature. These organisms live under unique environmental conditions and must cope with periods of submersion and emersion. There is a body of literature on intertidal organism responses to temperature and climate change, but these studies mainly focus on rocky shore organisms, with limited studies focusing on soft shore species. Crabs inhabiting intertidal soft shores are diverse and abundant and provide a range of ecosystem functions. Despite their importance, these organisms are perpetually overlooked in studies on thermal biology and vulnerability to climate change. In this thesis, I assessed different aspects of the thermal physiology of intertidal crabs, as well as the adaptations of these organisms to live in intertidal zones. My findings show that intertidal crabs inhabiting soft shores and with tropical latitudinal ranges are tolerance to heat and have broad latitudinal distribution. The results in this thesis show that upper thermal limits of intertidal crabs are an evolutive conserved trait strongly related to maximum experienced temperatures rather than to latitudinal distribution, contradicting what is expected by the latitudinal trends of temperature variability, and therefore contrary to what is expected from the Climate Variability Hypothesis. Additionally, lower thermal limits and more restricted distribution of species at higher latitudinal ranges indicate that these species may be under a greater threat from climate change. In this thesis, I also show that the metabolic rate of crabs is related to their breathing strategy. Species that are dependent on water to obtain oxygen have a narrow aerobic scope when exposed in air, what reflects in lower upper thermal limits. Contrastingly, air breathing species have broader aerobic scope when breathing outside the water, allowing for higher upper thermal limits. Furthermore, I show that male and female intertidal crabs can differ in their metabolism and aerobic scope, likely due to different energetic investment in reproduction between sexes. These differences also seem to be related to lower upper thermal limits in female crabs, which may represent the bottlenecks for species survival in the warming world. The findings in this thesis show the differences in the thermal physiology and vulnerability in different taxa of intertidal crabs. My results show the importance of considering the different biological traits and phylogenetic relationships when discussing the vulnerability of ectotherms to climate change.