Widespread decoupling between abundance and genetic diversity, and strong local genetic structuring in marine unicellular eukaryotes

How abundance, connectivity, and genetic diversity covary across space remains a central question in evolutionary biology. Using a global environmental DNA metabarcoding dataset targeting mitochondrial cytochrome c oxidase subunit I (COI), we examined relationships among metabarcoding-derived abundance, nucleotide diversity, and spatial genetic connectivity across marine eukaryotes. We found a widespread but incomplete decoupling between abundance and genetic diversity, particularly in unicellular lineages. By contrast, distance-decay patterns in abundance, diversity, and connectivity were broadly similar across eukaryotes and more strongly associated with cellularity. Focusing on the Iberian Peninsula, we reconstructed locality-centred haplotype networks to quantify the spatial organization of intraspecific genetic variation. Unicellular lineages showed stronger local genetic structuring, whereas multicellular organisms exhibited clearer large-scale biogeographic partitioning. Overall, our results reveal shared biogeographic organization across eukaryotes while demonstrating that abundance-based biodiversity metrics alone cannot fully capture evolutionary and demographic processes, especially in microbial organisms.