Unraveling the population structure of blooming Cyanea capillata jellyfish: insights into connectivity between the Baltic Sea and North Sea

Tracing population structure and dispersal of planktonic organisms is a challenging yet crucial task, as it helps identify how populations are connected across regions, detect isolated or vulnerable populations and anticipate how human-driven changes in ocean may alter these dynamics. Here, we used the top predator Cyanea capillata, a bloom-forming jellyfish, as a target organism since its population structure, dispersal and reproductive patterns in the Baltic Sea-North Sea ecosystem remain poorly understood. Using a multi-faceted approach combining population genetics with Lagrangian dispersal simulations, the connectivity and population differentiation in the species along a salinity gradient was assessed. A division of C. capillata into two distinct clades (based on 32 haplotypes from 187 mtCOI sequences) was identified: one clade inhabiting the Baltic Sea and one in the North Sea/Norwegian Sea. Based on a dispersal model, our results suggest potential differentiation within the Baltic Sea metapopulation with eastern locations tending to form one sub-population and western locations another. Despite the lack of documented C. capillata polyps in the Baltic, local reproduction and the presence of a self-sustaining population in the Baltic Sea is suggested thus acting as a source rather than sink for a broader metapopulation. A Baltic population distinct from that in the North Sea is suprising, as it challenges the long-standing paradigm on the origin of jellyfish blooms in the Baltic Sea and highlights the potential for local population adaptations. Importantly this study underscores the relevance of genetic diversity, dispersal, and connectivity in gelatinous zooplankton, providing insights that can guide ecosystem management strategies. In particular, understanding jellyfish population dynamics can improve forecasts of bloom formation, and support adaptive management in the Baltic Sea–North Sea ecosystems.