Data from: Historical baselines and the future of shell calcification for a foundation species in a changing ocean

Seawater pH and the availability of carbonate ions is decreasing due to anthropogenic carbon dioxide emissions, posing challenges for calcifying marine species. Marine mussels are of particular concern given their role as foundation species worldwide. Here, we document shell growth and calcification patterns in Mytilus californianus, the California mussel, over millennial and decadal scales. By comparing shell thickness across the largest modern shells, the largest mussels collected in the 1960s-1970s and shells from two Native American midden sites (1000-2420 years BP), we found that modern shells are thinner overall, thinner per age category, and thinner per unit length. Thus, the largest individuals of this species are calcifying less now than in the past. Comparisons of shell thickness in smaller individuals over the past 10-40 years, however, do not show significant shell thinning. Given our sampling strategy, these results are unlikely to simply reflect within-site variability or preservation effects. Review of environmental and biotic drivers known to affect shell calcification suggest declining ocean pH as a likely explanation for the observed shell thinning. Further future decreases in shell thickness could have significant negative impacts on M. californianus survival and, in turn, negatively impact the species rich complex that occupies mussel beds.受人为二氧化碳排放影响，海水pH值与碳酸盐离子可用性持续下降，给钙化海洋生物带来了生存挑战。海洋贻贝作为全球分布的建群种，尤其受到学界关注。本研究针对加州贻贝（Mytilus californianus），在千年至十年际的时间尺度上，记录其贝壳生长与钙化模式。通过对比三类样本的贝壳厚度：现存最大个体的贝壳、1960至1970年代采集的最大型贻贝贝壳，以及两处北美原住民贝冢遗址出土的贝壳（年代为距今1000至2420年），本研究发现现代贝壳整体更薄，且在各年龄层级、单位长度下均更纤薄。因此，该物种现存的最大个体，其钙化程度较历史时期有所下降。不过，对过去10至40年间采集的小型个体贝壳厚度的对比分析，并未发现显著的贝壳变薄现象。结合本研究的采样设计，上述结果不太可能仅由采样位点内的个体差异或保存效应导致。对已知影响贝壳钙化的环境与生物驱动因子的梳理显示，海水pH值持续下降，或为观测到的贝壳变薄现象的合理解释。若未来贝壳厚度进一步下降，将对加州贻贝的存活造成显著负面影响，并进而波及依附于贻贝床的物种丰富群落。