O-2 plays a key role in early sedimentary diagenetic processes, but the effect of most macrofaunal species on the pathways and rates of supply of O-2 into the seabed are not well known. We investigated the effect of the ophiuroid Amphiura filiformis, one of the dominant macrobenthic species on soft bottoms in the northeast Atlantic, at depths of similar to15-100 m, in a laboratory environment. We determined how the presence of the ophiuroid changed the total O-2 uptake of macrofauna-free sediment by combining measurements from a microcosm approach and an approach that uses microelectrodes and a flushed aquarium. We suggest that natural populations of A. filiformis can account for 80% of the total flux of O-2 into the soft bottom. At least 67% of this portion is due to the diffusion of O-2 across additional sediment-water interfaces excavated by the brittle star.