Protist-dominated hard substrate faunas thrive at the deepest ocean depths.

Deep-sea hard substrates host faunal novelties and distinct evolutionary lineages. However, sessile organisms on rocks are difficult to sample and largely unknown at extreme hadal depths. Here, we report a deep hard-substrate fauna (9000 to 10,898 meters), comprising 32 species of six protist and metazoan phyla, most millimeter-sized and new to science, from the Kermadec and Mariana trenches, using the manned submersible Fendouzhe. We show that the filamentous organisms dominating these assemblages are heterotrophic foraminiferans, challenging the earlier chemolithoautotrophic hypothesis. Large-scale seafloor imaging and sampling suggest that similar protistan-dominated sessile communities thrive in seven hadal regions around Oceania. These faunas open new perspectives on biodiversity at the deepest ocean depths and unveil widespread, but previously unrecognized, carbon hotspots in global hadal trenches.