Bathymetric distribution of abyssal and hadal amphipod families and genera in the Japan Trench and the southernmost Kuril-Kamchatka trench

Amphipoda, belonging to superorder Peracarida, represents a conspicuous proportion of benthic fauna occurring across multiple depth zones. Extensive sampling has been conducted to understand the composition and distribution of benthic organisms, particularly in the Northwest Pacific. However, logistical challenges and limited opportunities have restricted our understanding of benthic fauna diversity and distribution. Using two types of trawls and an epibenthic sledge, abyssal and hadal benthic fauna were collected across 55 sampling sites from both the Japan Trench and the Kuril-Kamchatka Trench, at depths between 3689 m and 8010 m. A collection of 3126 deep-sea amphipods was obtained and the animals were assigned to at least 76 genera belonging to 37 families during the two expeditions. The most abundant family was Phoxocephalidae, followed by Ampeliscidae, Lysianassoidea and Oedicerotidae in descending order of abundance. In addition, high numbers of genera of the Pardaliscidae, Oedicerotidae, Stegocephalidae and Synopiidae families were identified. In the two trenches, the number of genera showed a weak inverse correlation with depth at both abyssal and hadal depths. Amphipod diversity and abundance were highest in the landward slopes, whereas those observed in the trench axes were the least diverse and abundant. High productivity, in addition to proximity to the Japanese archipelago, may explain the amphipod assemblage characteristics observed in our study areas. Our findings present novel data on the global depth records for eight identified and one unidentified amphipod genera, belonging to nine families.