Occurrence of the benthic trachymedusa Ptychogastria polaris Allman, 1878 (Cnidaria: Hydrozoa) off Northeast Greenland and in the northern Barents Sea

Abstract

The trachymedusa Ptychogastria polaris Allman, 1878 has been identified in seabed photographs from high-Arctic shelves and upper continental slopes off Northeast Greenland and in the northern Barents Sea. It was found to be a common epifaunal element, being present at 34 of 57 stations in 40- to 495-m depth and at 7 of 11 stations in 70- to 330-m depth, respectively. All specimens recorded in the photographs either sat directly at, or hovered very closely (≤ about 1 cm) over, the sea bed, indicating a primarily epibenthic life style of this hydrozoan species. The small-scale (i.e. within-station) distribution of medusae was rather patchy, with frequencies along photographic transects – consisting of 35–73 pictures distributed over seabed strips of 150- to 300-m length – varying from 1 to 58% off Northeast Greenland and from 1 to 34% in the Barents Sea. Absolute maximum density was 6 ind m⁻², and station mean abundances ranged from 0.01 to 0.91 ind m⁻² and from 0.01 to 0.52 ind m⁻², respectively. Values tended to decrease with water depth, albeit significantly only off Northeast Greenland. Otherwise, no clear relationships to environmental conditions, such as geomorphology or near-bottom water hydrography, were detected. Circumstantial evidence suggests that seabed granulometry and potential food supply are important distribution determinants. However, further investigations are required to identify more stringently the key factors controlling the distribution of P. polaris.