Pronounced megafaunal community transition driven by polymetallic nodule cover in the Northwest Pacific abyss

Polymetallic nodules provide essential hard substrata for benthic fauna and are widely assumed to enhance biodiversity on abyssal plains. Yet, our understanding of how nodule cover shapes megafaunal communities remains limited and largely restricted to low nodule cover conditions. Using a large image dataset from a nodule-rich abyssal basin in the northwest Pacific (NWP), we characterized megafaunal communities and quantitatively assessed their responses across a broad gradient of nodule cover. A total of 169 morphospecies from 14 phyla were recorded, with holothurians and sponges comprising the dominant taxa. Surprisingly, megafaunal density exhibited a U-shaped response to nodule cover, declining from low to intermediate nodule cover, remaining relatively stable at intermediate levels, and increasing sharply at high nodule cover. Community composition shifted markedly from mobile deposit-feeder dominance at low cover to sessile filter-feeder dominance at high cover. This transition was primarily driven by changes in the relative density of dominant taxa rather than species turnover. Compared with the Clarion–Clipperton Zone (CCZ), the NWP showed lower megafaunal density and distinct assemblage composition but comparable diversity, consistent with large-scale influences of particulate organic carbon flux and carbonate saturation. Our findings reveal a previously unrecognized relationship between megafauna and nodule cover in the NWP nodule province and underscore the need for region-specific management of deep-sea mining. Abyssal benthic megafaunal density exhibit a unique U-shaped response to polymetallic nodule cover in the Northwest Pacific that is distinct from other well-studied areas, highlighting the need for region specific management of deep-sea mining, according to photographic surveys.