Baltic blue mussels can colonise and dominate habitats with far lower salinity (< 10 psu) than other Mytilus congeners. Pervasive gene flow was observed between Western Baltic Mytilus edulis living at high salinity conditions and Eastern Baltic M. trossulus living at lower salinites, with highest admixture proportions within a genetic transition zone located at intermediate salinities (Darss Sill area). Yet, we do not understand the impacts of low salinity on larval performance, and how salinity may act as an early selective pressure during passive larval drift across salinity gradients. This study tested, whether larvae originating from two different populations along the natural salinity cline in the Baltic Sea have highest fitness at their native salinities. Our results suggest that Eastern Baltic M. trossulus (Usedom, 7 psu) and Western Baltic M. edulis (Kiel, 16 psu) larvae display better performance (fitness components: growth, mortality, settlement success) when reared at their respective native salinities. This suggests that these populations are adapted to their local environment. Additionally, species diagnostic markers were used for genetic analyses of transition zone (Ahrenshoop, 11 psu) mussel larvae exposed to low salinity. This revealed that low salinity selection resulted in a shift towards allele frequencies more typical for Eastern Baltic M. trossulus. Thus, salinity acts as a selective pressure during the pre-settlement phase and can shape the genetic composition of Baltic mussel populations driving local adaptation to low salinity. Future climate change driven desalination, therefore, has the potential to shift the Baltic Sea hybrid gradient westward with consequences for benthic ecosystem structure.

中文翻译：

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波罗的海贻贝的盐度驱动选择和局部适应

波罗的海蓝贻贝可以在盐度（< 10 psu）远低于其他贻贝同类的栖息地定居并占据主导地位。在生活在高盐度条件下的西波罗的海贻贝和生活在低盐度条件下的波罗的海东部 M. trossulus 之间观察到普遍的基因流动，在位于中等盐度（达斯基尔地区）的遗传过渡带内具有最高的混合比例。然而，我们不了解低盐度对幼虫性能的影响，以及盐度如何在幼虫被动漂越盐度梯度期间充当早期选择压力。这项研究测试了来自波罗的海天然盐度线沿线的两个不同种群的幼虫是否在其原生盐度下具有最高的适应性。我们的结果表明，东波罗的海 M. trossulus (Usedom, 7 psu) 和 Western Baltic M. edulis (Kiel, 16 psu) 幼虫在它们各自的原生盐度下饲养时表现出更好的性能（健康成分：生长、死亡率、定居成功）。这表明这些种群适应了当地的环境。此外，物种诊断标记用于过渡区（Ahrenshoop，11 psu）暴露于低盐度的贻贝幼虫的遗传分析。这表明低盐度选择导致向东波罗的海 M. trossulus 更典型的等位基因频率转变。因此，盐度在预沉降相期间用作选择性压力，并且可以塑造驱动局部适应的波罗的贻贝群的遗传组成。未来气候变化驱动的海水淡化，因此，