Abyssal and hadal macroinbenthic assemblages in the eastern Aleutian Trench (NE Pacific)

• First insights into eastern Aleutian trench macroinfauna since 1976. • Changes in densities and composition linked to depth and sediment characteristics. • Most isopod species undescribed. • Aleutian trench as potential barrier for organisms of low dispersal ability. The Aleutian Trench area is characterized by topographic and hydrographic discontinuities that may act as dispersal barriers, as well as strong environmental gradients that are likely to limit species distribution and promote diversification. The western part of the Aleutian Trench is comparably well studied, but there has been a paucity of benthic samples from its eastern part. This study aims to shed light on the benthic fauna of the eastern Aleutian Trench by analysing patterns in density and community structure of macrobenthic infauna, particularly isopod crustaceans and polychaetes, in relation to various environmental variables. The samples for this study were collected during the Aleutian Trench Biodiversity Studies (AleutBio) expedition (SO293) in 2022, which involved transects from north to south (abyssal North, hadal, abyssal South) along the Aleutian Trench and two sampling areas in the adjacent Bering Sea. In total, 14 stations at depths of 3,511–7,287 m were sampled using a box corer. Our findings indicated polychaetes to dominate macrofaunal communities followed by peracarids and molluscs. We observed a significant decrease in the densities of total macrofauna, polychaetes, and isopods with depth. There were no significant differences in macrofaunal composition at higher taxonomic levels, nor in the family-level composition of polychaetes and isopods between regions. Yet, at higher taxonomic resolution, i.e. genus and species level of Isopoda, the composition of isopods exhibited significant differences between the north and south abyssal as well as hadal region of the Aleutian Trench. Most isopod species identified (∼77%) were new to science and only a few were shared between regions or depth zonation. Additionally, the abyssal north Aleutian Trench showed a higher species diversity than the hadal and abyssal south. Overall, depth and sediment characteristics appeared to be the main drivers of differences in macrofaunal densities, community composition and isopod diversity. The study of deep-sea organisms, particularly isopods — one of the most abundant and diverse groups of macrofauna in the abyssal and hadal zones — is crucial for understanding, monitoring, and assessing the integrity of deep-sea benthic ecosystems. This includes assessing their biodiversity, distribution patterns, and the potential impacts of global change on species with limited dispersal capacity. However, more samples, higher taxonomic resolution and genetic analyses would help to elucidate ecological and environmental drivers of macrofaunal and especially isopod composition and connectivity across the trench.