Differences In Thermal Tolerance And Adaptive Physiological Performance In Two Congeneric Oyster Species

Understanding how and where environmental factors limit species'distributions can highlight key evolutionary processes and is important for predicting future biodiversity.As intertidal species,oysters face to the highly variable environment and have evolved various adaptations to cope with and even exploit these conditions.Worldwide,oysters are of great commercial significance and also serve as experimental models of studies on eco-physiological responses in relation to global change.Due to the effects of anthropogenic climate change on indigenous intertidal populations,it seems to be essential to specify relative vulnerabilities of oyster through the comparative physiological studies of oyster species from different geographic locations.In this study,two congeneric oyster species from north and south of China selected to define physiological performance differentiation under thermal stress.Particularly,we focus on the upper thermal limit(LT₅₀),physiological metrics(heart rate,standard metabolic rate,Arrhenius break temperature,key metabolic enzyme activities,and anaerobic metabolite production)and molecular responses(expression of thermal and metabolic responsive genes).1.Thermotolerance divergence and the physiological stress responses under ramping temperatureInvestigating the physiological mechanisms of closely related species that exhibit distinct geographic distributions and thermal niches is essential for understanding their thermal tolerance capacities and local adaptations in view of climate warming.The variations in upper thermal limits(LT₅₀)under acute heat shock and cardiac activity,standard metabolic rate(SMR),anaerobic metabolite production,enzymatic activities and gene expression under increasing temperatures in two oyster subspecies were studied.The populations of two oyster subspecies,Crassostrea gigas gigas and C.gigas angulata,exhibit different latitudinal distributions along the northern and southern coastlines of China,respectively,which experience different environmental conditions.The LT₅₀ was significantly higher,by 1.29°C,in the southern than in the northern oysters.In both subspecies,temperature increases had powerful effects on heart rate,SMR and gene expression.The southern oysters had the highest Arrhenius breakpoint temperatures for heart rate(31.42±0.17°C)and SMR(33.09°C),whereas the heart rate(28.86±0.30°C)and SMR(29.22°C)of the northern oysters were lower.The same patterns were observed for the Q₁₀coefficients.More thermal sensitivity was observed in the northern oysters than in their southern counterparts,as the heat-shock protein genes(HSPs)in the northern oysters were expressed first and had a higher induction at a lower temperature than those of southern oysters.Furthermore,different expression patterns of energetic metabolism genes were observed.Here,southern oysters displayed higher basal levels of metabolic gene expression,while northern oysters demonstrated lower basal expressions.In the northern oysters,increasing anaerobic glycolysis genes(PEPCK)and end products(succinate)were found at36-43°C,indicating a transition from aerobic to anaerobic metabolism and a lower aerobic scope compared with the southern oysters.These two subspecies experience different environmental conditions,and their physiological performances suggested species-specific thermal tolerance windows in which the southern oysters,with mild physiological flexibility,had a higher potential capability to withstand heat stress.Overall,our results indicate that comparing and unifying physiological and molecular mechanisms can provide a framework for understanding the likely effects of global warming on marine ectotherms in intertidal regions.2.Physiological and molecular stress responses during long-term acclimation at increasing ambient temperatureIn this study,we examined whether exposure to elevated temperatures that are possible in future climate outlines leads to express any specific mechanism to acclimation in the new environment.Survival,metabolic,enzymatic activities,and molecular stress responses(HSPs,antioxidants,and bioenergetic genes)to long-term thermal stress(26-30°C,in the periods of 15 and 30 days)were estimated in two oyster subspecies,Crassostrea gigas gigas and C.gigas angulata from the northern and southern coastlines of China,respectively.Although mortality did not exhibit any differences between subspecies,the northern oyster showed more capability of thermal tolerance in the face of long-term exposure at 30°C.Ability to withstand thermal stress in both subspecies related to the expression of heat shock proteins,especially at 26 and 30°C over periods of 15 days.Here,increasing in metabolic enzymes and expression of metabolism-related genes could indicate a higher metabolic compensatory response in the northern oyster subspecies than its southern counterpart.However,tend to accumulate oxidative lesion(MDA),decreasing activity and expression of antioxidant SOD in northern oysters revealed the increasing levels of oxidative stress during prolonged thermal stress.Further studies should define whether the thermal acclimation in oysters may facilitate evolution in the face of future global change.In conclusion,this experiment indicates that these oyster subspecies may have the potential to acclimatization in view of climate warming.