Pelagic occurrence of the sympagic amphipod Gammarus wilkitzkii in ice-free waters of the Greenland Sea – dead end or part of life-cycle?

Abstract

The sympagic (=ice-associated) amphipod Gammarus wilkitzkii usually lives attached to the underside of Arctic sea ice. During an expedition to the Greenland Sea in May/June 1997, high numbers of this species were found in pelagic Rectangular Midwater Trawl catches (0–500 m water depth) in an ice-free area, 35–42 km away from the ice edge. The amphipods seemed to have maintained position in the water column for at least 4 days. Mean biomass data (length: 2.9 cm, organic content: 73% dry mass), gut fullness (>50% in 85% of specimens) and sex ratio (females:males = 1:1.5) of these amphipods were very similar to values for under-ice populations. Due to their relatively high body density (mean: 1.134 g cm⁻³), the energy demand for swimming was assumed to be high. Measurements of oxygen consumption of swimming and resting amphipods (8.8 and 4.0 J g wet mass⁻¹ day⁻¹, respectively) suggested that, from an energetic point of view, G. wilkitzkii would maintain position in an ice-free water column for the time period.