ABSTRACT: In the present study environmental sulfide concentrations in the vicinity of and within burrows of the lugworm Arenicola marina during tidal exposure are presented. Sulfide concentrations in the pore water of the sediment ranged from 0.4 to 252 uM.During 4 h of tidal exposure no significant changes of pore water sulfide concentrations were observed. Up to 32 uM sulfide were measured in the water of lugworm burrows. During 4 h of low tide the percentage of burrows containing sulfide increased from20 to 50% in July and from 36 to 77% in October. A significant increase of median sulfide concentrations from 0 to 14.5 uM was observed after 5 h of emersion. Sulfide and thiosulfate concentrations in the coelomic fluid and succinate, alanopine andstrombine levels in the body wall musculature of freshly caught A. marina were measured. During 4 h of tidal exposure in July the percentage of lugworms containing sulfide and maximal sulfide concentrations increased from 17% and 5.4 uM to 62% and150 uM, respectively. A significant increase of median sulfide concentrations was observed after 2 and 3 h of emersion. In October, changes of sulfide concentrations were less pronounced. Median thiosulfate concentrations were 18 to 32 uM in July and 7 to12 uM in October. No significant changes were observed during tidal exposure. Succinate accumulated in the tissue of A. marina, indicating the onset of an environmental and sulfide dependent anaerobiosis. Recovery experiments after 4 h of tidalexposure showed that sulfide is completely removed from the coelomic fluid after 30 min. Succinate levels began to decrease immediately after the disappearance of sulfide, reaching control levels after 60 min of recovery. Thiosulfate concentration showeda significant increase after 30 min, indicating that internal sulfide is removed by its oxidation to thiosulfate. The present study shows that A. marina exhibits the same mechanisms of sulfide tolerance under habitat conditions as underexperimental conditions. In addition, it is demonstrated that the lugworm is able to recover from tidal sulfide stress within 1 h.