Thermal Acclimation Of Septifer Virgatus To Fine-scale Temperature Heterogeneity

Temperature is among the main structuring agents on rocky intertidal shores.Extreme events associated with global warming have been more and more frequent,which would increase the risk of extreme body temperatures in intertidal organisms.The temporal and spatial patterns of thermal heterogeneity at small scales and its contributions to physiological variation remain poorly understood.Accurate predictions of species responses to extreme events in the context of global warming require a detailed understanding of the temperature that organisms experienced in the wild and inter-individual variations of thermal limits.This study examined the hypothesis below:First,thermal heterogeneity at small temporal and spatial scale exists in intertidal rocky shore habitats and this heterogeneity will provide thermal refugia for intertidal organisms.Second,thermal tolerance also displays temporal and spatial heterogeneity and physiological polymorphism is an important strategy for the maintenance of the local population of intertidal organisms.Finally,phenotypic plasticity plays a critical role in thermal acclimation response.By integrating in situ temperature measurements and physiological approaches,we tested our hypothesis in black ribbed mussel Septifer virgatus.We explicitly,quantified temporal and spatial patterns in four different microhabitats within an approximately 20m2 region of the intertidal rocky shore in Dongshan Swire Marine Station(D-SMART),Zhangzhou,Fujian.An autocorrelation analysis of temporal predictability showed that tidal signal is evident in the daily maximum temperature of sun-exposed microhabitats whereas completely absent in shaded microhabitats.Infrared thermograph showed between-site mussels' body temperature varied by up to 15? and within-site varied by up to 5?.Generalized linear model revealed high thermal heterogeneity at small scale.During the low tide,temperatures of sun-exposed microhabitats routinely reached much higher than their shaded counterparts.Sunny versus shaded differences were consistently larger than the variability associated with seasons.Therefore,shaded microhabitats provide thermal refugia and small scale thermal heterogeneity is likely to have profound effects on the way local populations respond to climate changes.We empirically determined the thermal performance curve of cardiac function,then the temperature at which heart rate dropped drastically with rising temperature(Arrhenius breakpoint temperature,ABT)and at which it fell to zero(flat line temperature,FLT).Thermal tolerance of field-acclimatized Septifer virgatus showed seasonal patterns and mussels in summer showed higher thermal tolerance.Mussels from sun-exposed microhabitats had higher thermal limits in summer and much lower thermal safety margin(TSM)than individuals from shaded microhabitats.Our results also revealed inter-individual variation of thermal limits,highlighting the importance of physiological polymorphism when encountered extreme events.Abolishing small scale environmental variation by common garden acclimation provide insights into the role of phenotypic plasticity in such thermal adaptation.Different pattern of thermal tolerance and phenotypic plasticity were observed in FLT and ABT,reflecting different mechanisms of ABT and FLT responding to thermal acclimation.Common garden reduced thermal limits of mussels collected in summer but it had weaker effect on mussels collected in other season.By contrast,basal thermal tolerance of mussels from different microhabitats often showed no difference,suggesting the short-term acclimation is reversible.These results suggest phenotypic plasticity played an important role in both short-term and seasonal thermal acclimation.In summary,thermal heterogeneity at small temporal and spatial scale is responsible for differences in thermal limits at the same scale.Moreover,polymorphism and plasticity of thermal tolerance played an important role in encountering high temperatures.This study shed light on the significance of small scale thermal heterogeneity when predicting effects of climate change.