Not much is known on polychaete diversity in the deep Nordic Seas. Based on data from extensive
sampling over three decades, new data on polychaetes covering depths from 550 m to 3800 m are
presently reported. On a depth gradient, the number of polychaete families gradually declined from 35 at
the upper slope (500–750 m) to 11 at 3800 m. Species diversity in eight polychaete families subjected to
critical taxonomic studies showed a gradual decrease from a diverse group of slope species in the upper
and middle slope (500–1000 m) to a species poor fauna in the deepest areas (43000 m), with a distinct
break at about 2000 m. Faunal changes were documented both at alpha (sample species richness) and
gamma (large area species richness) scales, whereas the break at about 2000 m was clearly from beta
(turnover) species richness. Only six species (12%) in the examined families were found in the deepest
areas. The slope and deep-water polychaete fauna differs substantially from west Norwegian shelf and
costal fauna. Not more than 30% of the species in the examined families were recorded in coastal and
shelf waters, and several presumably refer to species complexes. The strong faunal change coincides with
the upper border of cold sub-zero temperature deep water at the shelf break and upper slope. Polychaete
taxonomy of the deep-water fauna is generally underdeveloped, illustrated by a high proportion of
recently described or undescribed species (about 40%) in the examined families. Based on the present
results, we hypothesise that the cold Nordic Sea deep-water polychaete fauna differs significantly from
the North Atlantic deep fauna and rather is more similar to the fauna of deep Arctic waters.