The Deepest Podasconidae (Cryptoniscoidea, Epicaridea) from the Japan Trench

Purpose The Podasconidae, which are parasitic on the Amphipoda, are poorly studied taxa within marine isopods. Consequently, information on occurrence has been limited to shallower waters and molecular sequence data are not available. Here we report first podasconid specimens from amphipods collected from abyssal and hadal depths. In this study, these podasconids were characterised through morphological and molecular analyses. Methods Podasconids were detected from amphipods sampled from abyssal and hadal depths of the Japan and Kuril-Kamchatka Trenches. Newly collected podasconids were observed under a stereoscopic microscope. Partial sequences of nuclear 18S and mitochondrial 16S rRNA genes were determined for all parasites, whilst those of mitochondrial COI gene were provided for the host amphipods. Results In total, seven podasconids were found from three species of benthic amphipods, Aristias sp. , Byblisoides arcillis and Epimeria abyssalis , collected at 4556–6539 m of the Japan Trench. Differences in body size and morphology of uropods and article 1 of antennule were observed among parasites. 18S phylogenetic tree, constructed with other cryptoniscoid sequences from the GenBank, agreed with morphological analyses. As a result, we have assigned three morphospecies for the present podasconid samples whilst the non-monophyletic nature of Podasconidae was also indicated. Conclusion This study identified three novel amphipod hosts of Podasconidae and extends the known geographical and bathymetrical distribution of Podasconidae to include the abyssal and hadal depths. Additionally, our results indicated a high diversity of Podasconidae in the deep water, with an inference to potentially complex diversification of cryptoniscoids.