Adaptations to Sulfide in Sulfide-System Meiofauna. Endproducts of Sulfide Detoxification in Three Turbellarians and a Gastrotrich

End-products of sulfide detoxification have been compared in three turbellarians and a gastrotrich typical of lenitic beaches with a well-developed sulfide system. Oxidation products were correlated with the oxygen concentration in the animal's habitat indicating that adaptation to life in the sulfide system involves reduction of dependency on molecular oxygen in metabolism. Archiloa wilsoni, a surface dwelling turbellarian, made sulfite and sulfate, which are rich in oxygen. Of the three sulfide-system species examined, the primary end-product in two (the turbellarian Pseudohaplogonaria sp. and the gastrotrich Dolichodasys carolinensis) was elemental sulfur which lacks oxygen The third species (the turbellarian Solenofilomorpha funilis) made thiosulfate, a compound with a high S : 0 ratio, plus sulfate and sulfite. The presence of an important pathway of sulfide oxidation with elemental sulfur as the major end-product was heretofore unknown in the animal kingdom. Its presence in two phyla, Platyhelminthes and Gastrotricha, however, indicates that it is widespread in the lower invertebrates. Similarly, the formation of thiosulfate as a major product of sulfide oxidation has not been reported previously in the invertebrates. The discoveries of two new pathways for sulfide detoxification and of major differences in sulfide metabolism within a single taxon, the turbellarian order Acoela, indicate that invertebrates have a variety of detoxification mechanisms based on sulfide oxidation and that a substantial degree of plasticity in the detoxification method can be expected even between closely related organisms. The possibility that the observed sulfide oxidation is by symbiotic prokaryotes, rather than the animals themselves, is discussed.