Deep phylogeny of the amphipod super-family Eusiroidea

The super-family Eusiroidea is traditionally divided into 4 families, namely Calliopiidae, Eusiridae, Gammarellidae and Pontogeneiidae (De Broyer et al., 2007), but recent phylogenetic data suggests that the eusiroid clade forms a much broader assemblage (Englisch, 2001). Representatives are found in all oceans, inhabiting every trophic niches and a wide bathymetric range. With 216 described species in the Southern Ocean (i.e. about 24% o f its known amphipod fauna), eusiroids constitute a significant fraction of the Antarctic and sub-Antarctic biodiversity (De Broyer et al., 2007). The eusirid concept is very inadequately defined morphologically. The super-family form s a diverse assemblage of taxa o f gammaroid form , globally characterized by the loss or reduction of the accessory flagellum (Bousfield & Hendrycks 1995). The fam ily assignment of genera is often challenging, since there is no consistent set o f diagnostic characters defining most o f them. The 6 families form erly comprised in the non-validated taxon Iphimedioidea (Lowry & Myers 2000), as well as the Astyridae and Stilipedidae were recently considered as eusiroids. Moreover, previous 18S phylogenetic reconstructions revealed that the Astyridae, Iphimediidae, Epimeriidae and Pleustidae are nested w ithin the eusiroid clade (Englisch 2001). With the aim o f clarifying those major nomenclatural uncertainties and discussing the phylogenetic relationships, a deep phylogeny of the super-family Eusiroidea, including representatives o f all the aforementioned families, was reconstructed using two different gene fragm ents (28S rRNA and 1 8S 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 form s a must broader clade than claimed in classical literature. It revealed that most o f the traditionally delimited families are not m onophyletic 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 m orphological specializations under intensive selection pressures. The limits of the super-family and its composing clades have to be completely re-established in the light of this new genetic dataset. This study enables a clearer understanding o f the taxonom y of one of the major amphipod assemblage in the Southern Ocean and gives us insights into the patterns o f its phenotypic evolution. It proposes a solid frame for further analysis of the systematics of various Antarctic eusiroid lineages, especially the epimeriid and iphimediid clades, which will be studied in more depth.