| Summary: | <p>Abstract</p> <p>Background</p> <p>The cosmopolitan moon jelly <it>Aurelia</it> is characterized by high degrees of morphological and ecological plasticity, and subsequently by an unclear taxonomic status. The latter has been revised repeatedly over the last century, dividing the genus <it>Aurelia</it> in as many as 12 or as little as two species. We used molecular data and phenotypic traits to unravel speciation processes and phylogeographic patterns in <it>Aurelia</it>.</p> <p>Results</p> <p>Mitochondrial and nuclear DNA data (16S and ITS-1/5.8S rDNA) from 66 world-wide sampled specimens reveal star-like tree topologies, unambiguously differentiating 7 (mtDNA) and 8 (ncDNA) genetic entities with sequence divergences ranging from 7.8 to 14% (mtDNA) and 5 to 32% (ncDNA), respectively. Phylogenetic patterns strongly suggest historic speciation events and the reconstruction of at least 7 different species within <it>Aurelia</it>. Both genetic divergences and life history traits showed associations to environmental factors, suggesting ecological differentiation forced by divergent selection. Hybridization and introgression between <it>Aurelia</it> lineages likely occurred due to secondary contacts, which, however, did not disrupt the unambiguousness of genetic separation.</p> <p>Conclusions</p> <p>Our findings recommend <it>Aurelia</it> as a model system for using the combined power of organismic, ecological, and molecular data to unravel speciation processes in cosmopolitan marine organisms.</p>
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