Morphology and phylogenetic relationships of cryptoniscus larvae (Epicaridea, Isopoda, Cryptoniscoidea) parasitising amphipods from the abyssal and hadal depths of the Japan Trench

Abstract Cryptoniscoidea is a poorly studied parasitic superfamily of marine isopods, members of which are known to parasitise various crustaceans, including amphipods. However, our understanding of their phylogenetic relationships and evolutionary histories is limited by the scarcity of available molecular data. Herein, we report seven cryptoniscus larvae of Cryptoniscoidea parasitising three species of benthic amphipods, Aristias sp., Byblisoides arcillis Barnard, 1961, and Epimeria abyssalis Shimomura & Tomikawa, 2016, collected at the depths of 4556–6539 m in the Japan Trench. The collected specimens have greatly extended the deepest available records of cryptoniscoids parasitising amphipods (1618 m) and even for the entire superfamily (5741 m), while also identifying three novel amphipod hosts for epicaridean. Partial sequences of the nuclear 18S and mitochondrial 16S rDNAs were determined for all parasites. Phylogenetic analysis based on the 18S sequences of the parasites and other sequence data of cryptoniscoids from GenBank revealed three lineages in the present specimens. In addition, large variations were observed in the 16S sequences among the lineages. Morphological observations detected three morphotypes which were consistent with the results of the molecular analyses. Based on morphological and molecular analyses of present specimens, three species were recognised. Differences in the body size and morphologies of the antennae, mouthparts, coxa, and uropods were observed between the three species. The results obtained in the present study demonstrate the non-monophyletic nature of cryptoniscoids that parasitise amphipods. Additionally, our results indicate the diversity of cryptoniscoids parasitising amphipods in extreme deep water, suggesting a potentially high diversity in the superfamily.