Mitochondrial diversity and demographic history of Arctica islandica (Bivalvia: Venerida) in the German waters of the Baltic Sea

The semi-enclosed Baltic Sea is a comparatively young brackish ecosystem characterised by strong environmental gradients that can shape genetic diversity and demographic history in marine species. The amphi-Atlantic bivalve Arctica islandica (Linnaeus 1767) inhabits the southwestern Baltic Sea, where populations experience environmental conditions distinct from fully marine habitats. We analysed mitochondrial cytochrome c oxidase subunit I ( COI ) and cytochrome b ( CytB ) sequences to assess mitochondrial diversity and infer historical demographic patterns of A. islandica in the German waters of the Baltic Sea. Mitochondrial diversity was high, with haplotype diversity of 0.82–0.84 and nucleotide diversity of 0.69–0.74%. Widespread haplotype sharing and no evidence of regional structuring were observed. Most genetic variation occurred within rather than among regions (F ST = − 0.025, p = 0.99). Demographic analyses supported a history of population expansion, while comparison with previously published CytB haplotypes showed that specimens from the German waters of the Baltic Sea predominantly shared common haplotypes with samples from multiple regions across the species’ distribution. Overall, the mitochondrial data indicate that A. islandica populations in the German waters of the Baltic Sea retain substantial shared mitochondrial diversity despite inhabiting environmentally distinct brackish habitats. These patterns are consistent with relatively recent colonisation or expansion within the Baltic Sea and a shared demographic history and provide a baseline for future genome-wide studies of population connectivity and adaptive variation.