MORPHOLOGICAL AND ECOPHYSIOLOGICAL ADAPTATIONS OF THE MARINE OLIGOCHAETE TUBIFICOIDES BENEDII TO SULFIDIC SEDIMENTS

The marine oligochaete worm Tubificoides benedii inhabits coastal tida l sediments in which sulfide can reach toxic concentrations. The role of external ironsulfide deposition in sulfide detoxification is discus sed together with a review of morphological and ecophysiological adapt ations of T. benedii to sulfide. The body wall of T. benedii turns bla ck in the presence of sulfide. Histochemical studies and micro-X-ray a nalyses provide evidence for the reaction of iron in the mucus layer a bove the cuticle of the worm with environmental sulfide to produce iro nsulfide. The deposited ironsulfides are either reoxidized or shed off through moulting, a process otherwise unknown in oligochaetes. Howeve r, calculations on the diffusion rate of sulfide into T. benedii show that the deposition of ironsulfides do not play an important role in s ulfide detoxification. The first and last few segments of T. benedii a re not blackened by sulfide and do not appear to precipitate sulfide. The diffusion rate of sulfide through these segments is so rapid that internal sulfide concentrations reach levels inhibitory to cytochrome c oxidase, the key enzyme of aerobic respiration, within minutes. When internal sulfide concentrations increase to toxic levels, reliance on an anaerobic metabolism represents a successful mechanism of sulfide tolerance in T. benedii. Metabolic adaptations to hypoxia and sulfide include the maintenance of aerobic pathways despite low oxygen or high sulfide concentrations and the ability to gain energy through anaerob ic pathways when oxygen and/or sulfide concentrations become limiting.