"The deep sea is the last great unexplored frontier on planet earth. In recent years, many hundreds of new species have been discovered in the deep sea. A key paradox in deep-sea biology is the degree to which deep-sea fauna are able to disperse. The prevailing paradigm has been one of cosmopolitanism, with animals having easy access to all ocean basins and cosmopolitan species traversing the world. However, this has been recently challenged and for many species there may be barriers to dispersal in the form of substrate specialisation, limited mobility or reproductive traits. This study will target one of the most abundant and species-rich groups, the polychaetes. To answer questions of dispersal and evolution in the deep sea we will study three contrasting groups of polychaetes. Firstly, polynoid (scale-worm) polychaetes are mobile with pelagic larvae that can drift some distances in ocean currents, although some polynoid species brood their offspring. Secondly, dorvilleid polychaetes which are mobile yet have direct developing larvae with a presumed limited dispersal ability. Finally, we will include the novel and recently-discovered diverse clade of ’bone-eating’ worms, Osedax, that are sessile and exist on the most specialised of habitats – whale bones on the sea floor. Using molecular data from material collected and planned to be collected in several ocean basins, we will construct phylogenies to evaluate the relationships within the three groups, and to determine levels of cryptic speciation and population connectivity in the deep sea. The results will have major consequences for our understanding of dispersal in the oceans, population connectivity, and the drivers of biodiversity in the deep sea."