| Summary: | Using molecular analyses based on COI, 16S and 28S rDNA sequences, lysianassoid species’ distributions were assessed in view of the paradigms emitted for Antarctic benthic invertebrates: circumpolarity and eurybathy. In several species of the genera Abyssorchomene and Pseudorchomene, a genetic homogeneity was found among specimens from remote sampling sites in the Southern Ocean, indicating a widespread or even a truly circum-Antarctic and/or eurybathic distribution. For example, in an undescribed Abyssorchomene species, low COI divergences were found between specimens from the Antarctic Peninsula, Weddell, Ross and Amundsen seas and the Argentine abyssal Basin and from depths ranging from 300 to more than 4500 m. In other lysianassoid species, genetically divergent lineages and (pseudo)cryptic taxa were revealed, of which some were restricted to certain geographic and bathymetric zones whilst others were characterized by a widespread distribution. For several species of the genus Orchomenella, genetically divergent lineages and possibly cryptic taxa were detected. The giant deep-sea lysianassoid Eurythenes gryllus appeared to be composed of several overlooked species, one of which being characterized by a bipolar distribution. This represents the first molecular evidence for a bipolar distribution in a macro-benthic organism. Moreover, within the Southern Ocean, three distinct species of Eurythenes gryllus sensu lato coexisted, of which two seemed restricted to abyssal waters whilst the bipolar species was found only in the bathyal zone. In this case, a clear genetic break occurred around 3000 m, rejecting the previously assumed eurybathy of Eurythenes gryllus. The observed variation in spatial genetic patterns of lysianassoid species was linked to both dispersal potential and historical events. Hence, species diversity is still largely underestimated for lysianassoid amphipods and undescribed species or additional species-level clades are likely to be uncovered with an increasing sampling effort. As polar ...
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