Abstract
Morphological taxonomy suggests marine faunas are species poor compared to terrestrial and freshwater faunas. This dichotomy has been attributed to the unique potential of marine plankters for distant dispersal across homogenous oceans with few barriers to gene flow. Relatively few opportunities for allopatric divergence have resulted in depauperate marine faunas characterized by a high proportion of widespread or cosmopolitan species. Aurelia aurita (Linnaeus) has been considered a good example of such a cosmopolite. However, recent molecular studies have revealed cryptic species in many marine taxa suggesting that marine biodiversity is higher and opportunities for speciation have been more frequent than generally recognized. Here, we present nuclear and mitochondrial DNA sequence evidence of seven sibling species of Aurelia aurita and two additional species, A. limbata Brandt and A. labiata Chamisso & Eysenhardt. These sequence data indicate speciation events as early as the Late-Cretaceous or Early-Tertiary consistent with the formation of well-recognized biogeographic barriers to gene flow in the seas.