Structure and environmental drivers of an abyssal-hadal scavenging amphipod assemblage

Understanding the drivers of assemblage structure across broad depth gradients remains a central challenge in abyssal and hadal ecology (3000 – 11000 m), particularly in disentangling the relative roles of depth and environmental heterogeneity. Scavenging amphipods are a model group for investigating drivers of deep-sea species distributions, owing to their high abundance and ubiquity. Here, we resolve amphipod assemblage structure and its drivers across a 5000 m depth gradient within the Nova Canton Trough (central Pacific Ocean) using a high-resolution dataset composed of 102 baited lander deployments spaced at ∼ 100 m depth intervals and integrated with multiscale environmental descriptors. A total of 139,271 amphipods were recovered, comprising 26 species identified using integrative taxonomy. Hierarchical clustering organised species into seven distinct assemblages spanning lower bathyal to hadal depths. Assemblages in the abyssal-hadal transition zone exhibited particularly high turnover, consistent with an ecotone nested within a broader depth-related environmental ecocline. Lower-hadal assemblages (>7000 m) were comparably cohesive, dominated by three species ( Bathycallisoma schellenbergi, Hirondellea dubia, and Profundum solomoni (species in review)). Multivariate analyses identified depth as the dominant gradient structuring assemblage composition, with additional contributions from in-situ temperature, regional topography, local seafloor composition, and spatial gradients. Together, these predictors explained 53.5% of total variance. Variation partitioning revealed that a substantial proportion of this variation was shared between depth and environmental predictors, reflecting covariance along the depth gradient. Despite this, environmental predictors explained an independent fraction of variation. This study represents one of the most detailed assessments of scavenging amphipod assemblages within a single hadal feature. By integrating integrative taxonomy, high-resolution sampling across a broad bathymetric gradient, and multiscale environmental variables within a seascape ecology framework, it provides new insights into the relative roles of depth, regional context, and macro- to microscale environmental heterogeneity.