| Summary: | The super-family Eusiroidea is traditionally divided into 4 families, namely Calliopiidae, Eusiridae, Gammarellidae and Pontogeneiidae, but recent phylogenetic data suggests that the eusiroid clade forms a much broader assemblage. Representatives are found in all oceans, inhabiting every trophic niches and a wide bathymetric range. The eusirid concept is very inadequately defined morphologically. The super-family forms a diverse assemblage of taxa of gammaroid form, globally characterized by the loss or reduction of the accessory flagellum. The family assignment of genera is often challenging, since there is no consistent set of diagnostic characters defining most of them. With the aim of clarifying those major nomenclatural uncertainties and discussing the phylogenetic relationships, a deep phylogeny of the super-family Eusiroidea was reconstructed using two different gene fragments (28S and 18S rRNA). The analysis was performed at a global scale, mostly with Antarctic taxa, but also European and Arctic species. The study confirms that the Eusiroidea forms a must broader clade than claimed in classical literature. It revealed that most of the traditionally delimited families are not monophyletic and that a few taxa previously considered as eusiroids (Gammarellus, Cleippides) are in fact very distantly related, and therefore should be excluded from them. Eusiroids comprise several independent armoured and spiny lineages nested amongst taxa with plesiomorphic morphologies, suggesting convergent evolutions and rapid morphological specializations under intensive selection pressures. The limits of the super-family and its composing clades have to be completely reestablished in the light of this new genetic dataset. This study enables a clearer understanding of the taxonomy of one of the major amphipod assemblage in the Southern Ocean and gives us insights into the patterns of its phenotypic evolution.
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