Abstract
Since their 1st description in 1956, Gnathostomulida have been regarded as one of the rarest taxa in marine benthos. On tidal flats in the North Sea they live alongside permanent polychaete burrows with up to 29 individuals and 5 spp, in 1 cm3 of sediment. Near burrows of Arenicola marina and Nereis virens, all species show a high degree of spatial overlap, indicating a narrow niche for the entire taxon. As a key factor to gnathostomulid distribution the activity of sulfur bacteria is proposed.