Studies On The Effects Of Ocean Acidification On Physical And Behavioral Defense Of Thick Shell Mussel Mytilus Coruscus

Since the Industrial Revolution,the atmospheric carbon dioxide?CO₂?concentration has increased by nearly 40%,which is mainly caused by anthropogenic activities?e.g.burning of fossil fuel?.Approximately one third of the CO₂ released into the atmosphere is eventually absorbed by the ocean,lowering the pH of the surface seawater and causing wholesale shifts in seawater carbonate chemistry,a process known as "Ocean Acidification"?OA?.Over the past 250 years,the average surface seawater pH has decreased by approximately 0.1 units,from approximately 8.21 to 8.10.According to IPCC's prediction,the average surface seawater pH will decline to 7.7-7.8 by the end of the 21st century and 7.3-7.4 around the year 2300.Mussels are ecologically and economically important species living on rocky shores in the intertidal zone.However,to date,little is known about the effects of OA on physical and behavior defense of mussel.Therefore,three independent studies were performed to show the respective effects of OA on byssal attachment defense,shell and shell-closure defense,and spatial.self-organization?aggregation behavior?defense of thick shell mussel Mytilus coruscus.The study on byssal attachment defense found that both mechanical properties?i.e.thread strength,breaking stress,breaking strain,and toughness?and the numbers of byssal threads produced by M.coruscus were significantly reduced in OA treatments.OA also shifted the most probable failure location from the proximal region to the adhesive plaque.These may lead to a 61%,60%,and 65%decrease in mussel byssal attachment strength at pH 7.8,7.6,and 7.4,respectively.However,OA elicited little and even no effects on byssal thread morphology?i.e.diameter and length?.In addition,real-time PCR suggested that OA altered the gene expression levels of byssal proteins.The data obtained suggest that it was the increased seawater acidity and down-regulation of some specific byssal proteins?e.g.mfp-4 and preCOL-NG?that resulted in the weakened mechanical properties of byssal thread.In contrast,the up-regulation of some other specific byssal proteins?e.g.PTMP,preCOL-P and preCOL-D?suggested that mussels adopted adaptive responses to minimize the adverse effects of OA on byssal attachment.The study on shell defense found that both internal and external shell surface were significantly corroded by OA.SEM analysis of corroded part confirmed that external surface corrosion was the dissolution of periostracum and/or prismatic layer,internal surface corrosion was the dissolution of nacreous layer and the impairment of aragonite deposition.These results suggested that OA hindered the calcification process and shell integrity.Compared to that at pH 8.1,shell strength decreased to 65%and 58%at pH 7.8 and 7.4,respectively.Meanwhile,adductor muscle diameter was reduced to 95%and 86%at pH 7.8 and 7.4,respectively.These suggested that OA decreased both shell and shell-closure strength,and would subsequently weaken mussel defense capacity of shell and shell-closure.OA also led to extracellular acidosis and decrease of haemolymph Ca2+ level.Further differently expressed genes analysis through mantle transcriptome sequencing suggested that proteins related to aragonite deposition?e.g.nacre protein,perlwapin,and perlucin?and carbonic anhydrase were significantly down-regulated by OA.However,proteins related to ion transportation and regulation?e.g.calmodulin?,adductor muscle-shell linkage?e.g.filamin-A?,and periostracum formation were up-regulated significantly by OA.The results obtained suggest that internal shell surface corrosion was resulted from the extracellular acidosis and decrease of haemolymph Ca2+ level,and down-regulation of proteins related to aragonite deposition and carbonic anhydrase.In contrast,the up-regulation of proteins related to ion transportation and regulation,adductor muscle-shell linkage,and periostracum formation were adaptive responses of mussels to minimize the adverse effects of OA on shell defense.The study on spatial self-organization?aggregation behavior?defense found that mussels self-organized into patchy distribution?i.e.small scale:aggregation,large scale:random and/or even distribution?in both control and OA treatments.However,mild OA?pH 7.80?advanced the emergence and increased pattern degree of mussel patchy distribution.These suggested that mild OA?pH 7.80?increased mussel spatial self-organization efficiency and aggregation level.In contrast,severe OA?pH 7.40?showed no significant effects on mussel spatial self-organization efficiency,but decreased aggregation level:Therefore,mild OA?pH 7.80?would enhance,but severe OA?pH 7.40?would weaken the defense capacity of spatial self-organization?aggregation behavior?.In this study,an individual-based model?IBM?was established to simulate mussel spatial self-organization?aggregation behavior?.Computer simulation with IBM showed that mussel spatial self-organization efficiency changed with the alteration of mussel movement strategy?i.e.the scaling exponent ? of Levy walk?.Interestingly,with seawater pH declining from 8.1 to 7.8 and 7.4,the scaling exponent ? of Levy walk changed from 1.64 to 1.71 and 1.68,respectively.Computer simulation with these three scaling exponent ? suggested that mild OA?pH 7.80?significantly increased,but severe OA?pH 7.40?had no significant effects on mussel spatial self-organization efficiency.These results obtained suggest that OA influenced mussel spatial self-organization?aggregation behavior?and ultimate spatial pattern through altering mussel movement strategy.In summary,the present study found that OA would weaken mussel defense capacity of byssal attachment,shell and shell-closure,but mussel would also adopt adaptive responses to minimize the adverse effects.However,OA showed mixing effects on the defense capacity of spatial self-organization?aggregation behavior?.Mild OA?pH 7.80?would enhance,but severe OA?pH 7.40?would weaken the defense capacity of spatial self-organization?aggregation behavior?.