Phylogenetic relationships among hadal amphipods of the Superfamily Lysianassoidea: Implications for taxonomy and biogeography

• Phylogenetics showed an incongruence between molecular data and morphological classification.
• Some Lysianassoidea taxa do not form monophyletic clades at the family, genus and species levels.
• Cryptic species-level diversity is shown in two genera (Eurythenes and Paralicella).
• Hirondellea dubia has a greater geographical range than previously considered.
• Various species show both cosmopolitan distributions and bathymetric partitioning in trenches.

Amphipods of the superfamily Lysianassoidea are ubiquitous at hadal depths (>6000 m) and therefore are an ideal model group for investigating levels of endemism and the drivers of speciation in deep ocean trenches. The taxonomic classification of hadal amphipods is typically based on conventional morphological traits but it has been suggested that convergent evolution, phenotypic plasticity, intra-specific variability and ontogenetic variation may obscure the ability to robustly diagnose taxa and define species. Here we use phylogenetic analysis of DNA sequence variation at two mitochondrial (COI and 16S rDNA) and one nuclear (18S rDNA) regions at to examine the evolutionary relationships among 25 putative amphipod species representing 14 genera and 11 families that were sampled from across seven hadal trenches. We identify several instances where species, genera and families do not resolve monophyletic clades, highlighting incongruence between the current taxonomic classification and the molecular phylogeny for this group. Our data also help extend and resolve the known biogeographic distributions for the different species, such as identifying the co-occurrence of Hirondellea dubia and Hirondellea gigas in the Mariana trench.