Physiological Adaptation Of Typical Intertidal Molluscs To Thermal Stress

Intertidal ecosystem is one of the harshest ecosystems on the earth,and has been significantly affected by global warming.Some intertidal organisms have shown especially pronounced and rapid biogeographic changes or local extinction.The capacity of physiological and evolutionary adaptations to temperature increase may determine the population dynamics of intertidal organisms in the scenario of global warming.In the present study,I address the following questions:What are the implications of anaerobic metabolism for thermal adaptation?Which localities are more vulnerable in response to temperature increase?And what are the roles of physiological and evolutionary adaptation in coping with global warming?Ecophysiological and genetic methods were used to investigate these questions in three typical intertidal molluscs(Cellana toreuma,Septifer virgatus and Mytilus galloprovincialis).The high-efficiency anaerobic metabolism and extensive upregulation of heat shock proteins have important implications for coping with high temperature in C.toreuma.Results of real-time PCR and enzyme activity showed that one key enzyme of anaerobic metabolism,alanopine dehydrogenase(AIDH),significantly up-regulated in gene expression and increased in enzyme activity.Real-time PCR results also showed that 15 hsp genes significantly increased in response to thermal stress.Under acute temperature stress,high-efficiency anaerobic metabolism can provide energy for the synthesis of various heat shock proteins to maintain intracellular protein stability.The thermal tolerance limits of C.toreuma diverge among populations,and have no correlation with latitude.Measurements of lethal temperature,cardiac performance and gene expression were carried out in 3 geographical populations(Dagong Island,Nanding Island and Sichang Island)of the limpet C.toreuma.Although the populations lacked differences in upper thermal limit,variations were noted among populations in transcriptional responses in genes linked to energy metabolism,indicating distinct physiological responses to thermal stress among populations.Limpets on the subtropical shore in Nanding Island live closer to their upper thermal limits and thus will be very sensitive to future temperature increases.Balancing selection is an important strategy for S.virgatus to cope with extreme high temperature stress in summer.Integrated with in situ temperature measurement,individual thermal tolerance determination and ddRADseq,it was found that individuals with heterozygous genotype will survive in response to extreme thermal stress in S.virgatus.Since the recombination of heterozygote will yield all three genotypes,the balanced polymorphism allows S.virgatus to maintain genetic diversity and increase thermal tolerance at the population level.The thermal tolerance of all individuals exceed acute thermal stress in shaded habitat,making the shaded microhabitat to serve as a "refuge" for mussels.Balancing selection and environmental heterogeneity are both important factors for intertidal organisms in resistance to global warming.Physiological evolution can occur on ecological time-scales in M.galloprovincialis.Using cardiac performance curve and survival time as indicators,the comparison of thermal tolerance for blue mussels was carried out in three populations(Qinhuangdao,Dalian and Qingdao).The genetic differentiation between populations was also calculated.The results showed that both thermal tolerance and genetic composition were different between populations,indicating physiological evolution in M.galloprovincials along China coast.Temperature drives the adaptive genetic variation in M.galloprovincialis at continental scale.The blue mussel M.galloprovincialis were collected in native area and(Europen coast)invasive area(Western American and China coast).Genome-wide SNPs calling was carried out among 8 populations.The results showed that genetic composition were significantly different among populations and adaptive genetic variations were strongly associated with habitat temperature,indicating M.galloprovincialis has adapted to local thermal condition at continental scale,and this evolution occurred within 100 years since its introduction from Europe.In summary,intertidal molluscs have evolved physiological and evolutionary adaptations to cope with extremely and highly variable thermal environments.The high-efficiency anaerobic metabolisms improve the thermal tolerance of intertidal molluscs.Thermal tolerance limits of mulluscs diverge among populations.Balancing selection and microhabitat allow molluscs to maintain genetic diversity.These results will help to understand the impact of global warming on population dynamics and biogeographic distributions of intertidal molluscs.