Several species of marine organisms are regarded as cosmopolitan (i.e. with a broad geographical distribution for example across more than one Ocean). But is this really the case or are these broadly distributed species in fact more than one highly similar species? This project uses the flamboyant tropical species Micromelo undatus known from the Indo-West Pacific, Eastern and Western Atlantic oceans as a model organism to compare processes of morphological and genetic diversification in broadly distributed species complexes.