| Summary: | The continued and unprecedented emissions of anthropogenic carbon dioxide (CO 2 ) are causing progressive ocean acidification (OA). While deleterious effects of OA on biological systems are well documented in the growth of calcifying organisms, lesser studied impacts of OA include potential effects on gamete interactions that determine fertilisation, which are likely to influence the many marine species that spawn gametes externally. Here, we explore the effects of OA on the signalling mechanisms that enable sperm to track egg-derived chemicals (sperm chemotaxis). We focus on the mussel Mytilus galloprovincialis , where sperm chemotaxis enables eggs to selectively bias fertilisation in favour of genetically compatible males. Using a factorial experimental design, we test whether the experimental manipulation of seawater pH (comparing ambient conditions to predicted end-of-century scenarios) alters these patterns of differential sperm chemotaxis. While we find no evidence that patterns of male-female gametic compatibility are impacted by OA, we do find that individual males exhibit consistent variation in how their sperm perform in lowered pH levels. This finding of individual variability in the capacity of ejaculates to respond to chemoattractants under acidified conditions suggests that climate change will exert considerable pressure on male genotypes that can withstand an increasingly hostile fertilisation environment. The readme file contains an explanation of each of the variables in the dataset and (if applicable) their measurement units. Funding provided by: Australian Research Council Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100000923 Award Number: DP170103290
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