| Summary: | We describe the occurrence of basal (‘primitive’) foraminifera and gromiids (a distinct taxon related to the foraminifera) in different bathyal habitats of the Håkon Mosby Mud Volcano (HMMV, Barents Sea). The foraminifera include two distinct groupings: (1) soft-shelled monothalamous foraminifera (‘allogromiids’, ‘saccamminids’ and ‘psammosphaerids’) and (2) brown organic-walled foraminifera (genera Conicotheca, Nodellum, Placopsilinella and Resigella). Samples were analysed from (1) the ‘hot centre’ of the volcano where fluid upflow was maximal; (2) smooth and structured muds in a ‘warm’ area of the centre where fluid upflow was reduced; (3) areas covered by Beggiatoa mats; (4) an area colonised by siboglinid polychaetes (‘pogonophores’); and (5) areas of ‘normal’ seafloor outside the volcano. Together, the studied organisms represented 57.8 % (‘normal’ seafloor; n?=?418), 28.5 % (‘warm centre’; n?=?122), and 26.4 % (Beggiatoa mats; n?=?5) of all ‘live’ (stained) foraminifera in three samples (0–1 cm layer) that were sorted for all stained foraminifera. In total, the 17 samples examined yielded 21 ‘allogromiid’ (organic-walled), 25 ‘saccamminid’ (agglutinated) and 5 other morphospecies among the monothalamous foraminifera, as well as 5 ‘brown-walled’ and 5 gromiid morphospecies. Four taxa were identified to species (Conicotheca nigrans, Resigella moniliforme, R. polaris, Micrometula hyalinosphaera); another seven were assigned to the genera Bathyallogromia, Conqueria, Nodellum, Placopsilinella, Resigella, Tinogullmia and Vanhoeffenella. All others were undescibed at the genus level. Some species, notably ‘Saccamminid sp. 5’, C. nigrans and Nodellum sp., were distributed in sediment layers down to 5 cm depth, and sausage-shaped gromiids were also present in the deeper layers of some samples. The number of basal foraminiferal and gromiid morphospecies varied considerably from site to site. It was relatively high north of the volcano (42 morphospecies in 4 samples) and in the ‘warm centre’ (28 morphospecies in 2 ...
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