Genetic Differentiation of the Jellyfish <i>Aurelia aurita</i> Along Steep Environmental Gradients

ABSTRACT Aim To quantify the connectivity and degree of population differentiation of a planktonic model species along an extreme environmental gradient in order to understand source–sink dynamics and processes leading to local adaptation. Location Baltic Sea/North Sea. Taxon Scyphozoan Jellyfish Aurelia aurita . Methods Levels of genetic connectivity and population differentiation were analyzed by integrating molecular data with 40 years of Lagrangian stepping‐stone drift‐route simulations. Drifters represented different A. aurita life‐stages that were traced over 40 consecutive life‐cycles from four empirically confirmed start locations. Suitable habitat for polyp recruitment was parameterized from high‐resolution bottom topography maps to allow for stepping‐stone range expansion across the Baltic Sea, with additional physiological experiments. Results Molecular and drift analyses revealed a large degree of isolation, identifying two major clusters in the Baltic Sea: (1) the low saline central‐eastern and (2) the intermediate saline south‐western regions, respectively. Additionally, molecular analyses confirmed the southern North Sea as an independent cluster with limited connectivity to the Baltic Sea. The Skagerrak/central North Sea was further confirmed as a transition zone between both regions. Lagrangian simulations suggested only one seeding event from this transition zone (Skagerrak) into the Baltic Sea over the 40‐year study period. Furthermore, a 2.5‐fold lower genetic diversity in the low saline Baltic relative to the high saline North Sea indicated limited current gene flow. A significant correlation was found between haplotype frequency and salinity in Baltic Sea samples, together with successful settlement of northern Baltic individuals at low salinities. Main Conclusions Limited connectivity leads to population differentiation of A. aurita in the Baltic Sea. Furthermore, active recruitment at low salinities suggests local adaptation within the low saline Baltic Sea. Local adaptation is likely widespread among Baltic species of marine origin, raising concerns for biodiversity conservation and ecosystem management of the Baltic Sea under continued freshening due to climate change.