LAUSR

Abstract Understanding the extent of the hadal ecosystem (habitats exceeding 6000 metres water depth) is convoluted due to the complexity of seafloor geomorphology that accounts for 45% of the total ocean depth range. Furthermore, at such great depths, features such as fracture zones and basins, although numerous, are less prominent and therefore have drawn less focus compared to the conspicuous subduction trenches that are typically associated with hadal science. Here we focus on the Northwest Pacific Ocean, where the majority of hadal features are located, to evaluate the true extent of the deepest marine ecosystem. This analysis has highlighted that the Mariana Trench, in terms of continuous hadal habitat, is in fact five isolated areas, with the most northern being what Russian scientists used to call the Volcano Trench. Conversely, we identified that there are no physical partitions either north or south of the Japan Trench to isolate it from the neighbouring Kuril-Kamchatka or Izu-Bonin trenches respectively, thus it forms one continuous hadal habitat. By evaluating the frequency and distribution of smaller features, such as basins and fracture zones, we conclude that in the northwest Pacific, the total area occupied by depths >6000 m is 2,793,011 km2, which is considerably larger than the 686,114 km2 accounted for by subduction trenches alone. These results demonstrate not only that the hadal ecosystem may be far larger than previously anticipated but that the geomorphology is crucial in understanding the distribution and genetic connectivity of endemic hadal species that inhabit these great depths.