Ultrastructural evidence of detoxification in the alimentary canal of Urechis caupo

The marine worm Urechis caupo exhibits specialized secondary lysosomes term ed sulfide-oxidizing bodies throughout the epithelial tissues of the alimen tary canal which appear to concentrate environmental toxins such as aluminu m, silica and sulfur, as revealed by X-ray microanalysis. Ultrastructural e xamination of the alimentary tract of U. caupo reveals a digestive tube con sisting of an inner layer of epithelium Lining the central lumen, a layer o f connective tissue with embedded muscle fibers, and a peritoneal covering. The thickness, ciliation and morphology of the innermost epithelium show r egional variation that correlate with functional specialization, and the ma in cell types are ciliated, microvillate, and mucus- and enzyme-secreting c ells. In the anterior part of alimentary canal (esophagus, gizzard, and sto mach), the epithelium is almost entirely composed of ciliated and mucus cel ls, serving to move particulate matter and fluids, and the gizzard epitheli al morphology reflects comminution of food particles. Zn the midgut, the ep ithelium is composed of microvillate and enzyme-secreting cells, suggesting involvement in digestive and absorptive functions. The hindgut, which acts as a water lung and is devoid of any digestive function, is Lined by micro villate cells involved in gaseous exchange. The epithelium of the body wall and hindgut have been previously reported to contain numerous sulfide-oxid izing bodies, similar in ultrastructure and elemental composition to those of the alimentary canal described here (Menon & Arp 1992; 1993). These unus ual organelles, which form a peripheral defense on all exposed surfaces of the worm, are part of an extensive suite of sulfide detoxification and tole rance mechanisms which enable the worm to flourish in a sulfide-rich enviro nment.