MULTIPLE MECHANISMS PROVIDE TOLERANCE TO ENVIRONMENTAL SULFIDE IN URECHIS-CAUPO

During tidal exposure the echiuran worm Urechis caupo encounters toxic sulfide in its burrow microhabitat on the marine mudflat. Although sulfide freely penetrates the two respiratory epithelia (the body wall and hindgut) and tissue cytochrome oxidases are extremely sulfide sensitive, the worm tolerates sulfide concentrations which exceed environmental exposures. Sulfide tolerance results from a suite of sulfide detoxification mechanisms that extend from the coelomic fluid to the epithelial surfaces. The coelomic fluid is rich in hemoglobin and hematin, both of which bind sulfide and catalyze the oxidation of sulfide to thiosulfate in vitro. Peripheral defense mechanisms responsible for protection of aerobic epithelia from sulfide poisoning may include mucus secretion, sulfide oxidation by symbiotic bacteria and/or specialized organelles termed SOBs, and sloughing of sulfide damaged cells. Thiosulfate, the principal detoxification endproduct produced in vivo, is eliminated primarily by diffusion across the hindgut, and sulfide metabolites may be sequestered and eliminated by the anal sacs. Based on a hypothetical model, we conclude that sulfide tolerance in U. caupo is due primarily to the sulfide oxidation activity of the coelomic fluid and that the specialized integument and hindgut protect the metabolically active, sulfide-exposed epithelial cells. These sulfide detoxification mechanisms may allow populations of this filter-feeding worm to exploit nutrient-rich, sulfidic environments.