Brittle star fauna (Echinodermata: Ophiuroidea) of the arctic northwestern Barents sea: composition, abundance, biomass and spatial distribution

Abstract

Epibenthic brittle star assemblages were investigated on the northwestern Barents Sea shelf between 81° and 77°N in July 1991. At 9 drift stations in water depths between 80 and 360 m, series of 35–71 photographs, each depicting about 1 m² of the seabed, were taken along transects of about 150- to 300-m length to assess abundances and spatial distribution patterns of adult brittle stars (disc diameter ≥1 mm). Biomass values were derived by combining abundances with size-weight relationships and size frequencies established using specimens from trawl catches. Six brittle star species were identified on the seabed images.Ophiocten sericeum was the most abundant species on shallow shelf banks (≤100 m). Up to 2,800 individuals were counted on a single photograph; median abundances per station ranged from 32 to 524 ind.m⁻² and biomass from 0.3 to 5.0 g ash-free dry weight (AFDW) m⁻². The spatial distribution along the transects (i.e. on the 100-m scale) was, however, extremely patchy. Disc diameters ofO. sericeum ranged between 1.6 mm and 15.4 mm. In deeper shelf habitats (>150 m),O. sericeum was rare or absent, andOphiacantha bidentata dominated the brittle star fauna with median densities and biomasses of 2–49 ind.m⁻² and 0.07–1.9 g AFDW m⁻², respectively. Its disc diameters ranged from 2.9 to 14.4 mm. The other species (Ophiura sarsi, Ophiopholis aculeata, Ophioscolex glacialis, Ophiopleura borealis) occurred in distinctly lower numbers. Our findings provide further evidence that brittle stars dominate epibenthic communities on Arctic shelves and locally reach very high abundances. Dense beds ofOphiocten sericeum seem to be a general phenomenon on high-Arctic shallow shelf banks.