Fig. 2: Sponge density map and representative seafloor images.

(Map) Sponge abundances estimated for different habitat categories: category “a”, on flat shelves on the steep flanks of the Langseth Ridge (red, occasional concentrations of <100 individuals); category “b”, on shallow sloping upper flanks, with abundant blackened siboglinid tube detritus exposed (orange, average ± SD: 0.8 ± 0.7 ind. m⁻², 202 ± 195 g C_org m⁻²); category “c”, on the flat summits predominantly covered by sponge spicule-mat with numerous sponges, also present (yellow, average ± SD: 2.8 ± 1.1 ind. m⁻², 725 ± 217 g C_org m⁻²); and category “d”, on the center of flat seamount peaks (blue, average ± SD: 5.9 ± 1.7 ind. m⁻², 1702 ± 904 g C_org m⁻²). OFOBS dive tracks are shown in green. (Seafloor images a–d) white scale bars 50 cm length: a Category “a”: at the edge of the flat summit, a rapid increase in slope correlates with a sharp decrease in sponge abundance. Sand and rock outcrops are covered with the siboglinid tube mat. b Category “b”: on the more gently shallowing slopes of the mount peaks, dense sponge cover is replaced by occasional individuals on top of the siboglinid tube mat. c Category “c”: dense agglomerations of sponges cover the flat summits, with occasional gaps in sponge spicule cover visible and with 100% tube cover within many of these. d Category “d”: in the shallow center of flat summits, where sponge abundances exceed >50% of seafloor coverage. (Photo credits PS101 AWI OFOS system).