Community composition and distribution of epi- and suprabenthic macrofauna in the bathyal, abyssal, and hadal zones of the northern North Pacific

The deep sea, Earth’s largest biome, harbors numerous unknown species. Prior to the AleutBio (Aleutian Trench Biodiversity Studies) expedition from July to September 2022, the Northeast (NE) Pacific at abyssal and hadal depths was virtually unexplored. Our study presents new findings from the AleutBio project on the macrofaunal composition of the Bering Sea (BS) and Aleutian Trench (AT) collected by means of an epibenthic sledge (EBS), comparing these results with data from the Kuril-Kamchatka Trench (KKT) and the Northwest (NW) Pacific. Additionally, we examine variations in macrofaunal composition and abundance across different regions and depths. A biogeographic gap analysis using data from the Ocean Biodiversity Information System (OBIS) and the Global Biodiversity Information Facility (GBIF) found that, out of 170,627 occurrence records from the North Pacific and Bering Sea, only 153 were from depths below 3,500 m. The AleutBio project addressed this gap by significantly expanding the dataset with 36,499 new records collected during the expedition using an EBS. Nearly 98% of the specimens were from five phyla: Arthropoda, Annelida, Mollusca, Echinodermata, and Nematoda, with Polychaeta, Copepoda, and Nematoda being the most abundant taxa. Although the number of individuals varied between stations, there was no significant decrease in abundance with increasing depth, and some hadal stations had similar numbers of invertebrates as abyssal stations. Regional differences were observed, with Polychaeta and Nematoda being dominant in the BS, and Copepoda more prevalent at western abyssal stations. Depth emerged as the key factor influencing macrofaunal distribution, with distinct patterns across bathyal, abyssal, and hadal depths. Comparisons with other NW Pacific regions, like the Sea of Japan and the Sea of Okhotsk, show that depth and water body isolation play crucial roles in shaping faunal communities. AleutBio’s extensive sampling below 3,500 m has vastly increased available data, aiding in the understanding and conservation of deep-sea biodiversity. While certain taxa showed patchy distributions, no significant differences in faunal composition were found between geographic areas or depth zones. These findings underscore the dynamic nature of deep-sea ecosystems and highlight the importance of depth in shaping macrofaunal communities, emphasizing the need for continued research in these fascinating environments.