Habitat types and megabenthos composition from three sponge-dominated high-Arctic seamounts

Seamounts are isolated underwater mountains often stretching >1,000 m above the seafloor. They are usually identified as biodiversity hotspots of marine life, and host benthos assemblages that may vary on regional (among seamounts) and local (within seamounts) scales. Here, we collected seafloor imagery of three seamounts at the Langseth Ridge in the central Arctic Ocean to assess habitats and megabenthos community composition at the Central Mount (CM), the Karasik Seamount (KS), and the Northern Mount (NM). The majority of seafloor across these seamounts comprised bare rock, covered with a mixed layer consisting of sponge spicule mat intermixed with detrital debris composed primarily of polychaete tubes as well as sand, gravel, and/ or rocks. The megabenthos assemblages consisted of in total 15 invertebrate epibenthos taxa and four fish taxa, contributing to mean megabenthos densities of 55,745 ind. ha-1 at CM, 110,442 ind. ha-1 at KS, and 65,849 ind. ha-1 at NM. The faunal assemblages at all three seamounts were dominated by demosponges of the order Tetractinellida that contributed between 66% (KS) and 85% (CM) to all megabenthos. Megabenthos assemblages living on bare rock or on mixed substrate differed among seamounts and across individual seamounts.