Long-term anaerobiosis was studied in the priapulid Haljcryptus spinulosus and the bivalves Astarte borealis and Arctica island~ca, species with an extremely high resistance to lack of oxygen, from Kiel Bight. This shallow, brackish environment is frequently affected by extended periods of oxygen deficiency. Animals were experimentally subjected to anoxia for u p to 60 d. Glycogen was the most important substrate during long-term anaerobiosis, although it amounts to only 5 to 12 O/ O of soft tissue dry weight. Succinate, acetate and propionate were produced from glycogen degradation by anaerobic metabolic pathways. Succinate accumulation w t h i n tissues attained the highest concentrations found so far in marine invertebrates, and depended on duration of exposure to anoxia. Most H. spinulosus remained active during the first 3 or 4 wk of anoxia. They contained only low levels of aspartic acid, which seems to be of minor importance a s a substrate during long-term anaerobiosis. The activity of lactate dehydrogenase was low and lactate formation was also negligible. In A. borealis, activities of lactate, alanopine, and strombine dehydrogenases were low under anoxia and only small amounts of alanopine and strombine were produced. Volatile fatty acids were produced by all 3 species; in A borealis, acetate reached maximum concentrations during the transition phase from aerobic to anaerobic metabolism, whereas propionate was detected only after 2 d of anoxic incubation. Acetate and proplonate concentrations remained low, the excess being excreted. A. islandica, on the other hand, produced propionate immediately at the onset of anoxic conditions, and accumulated high concentrations of acetate and propionate. Propionate excretion was nearly negligible. Calorimetric measurements on the bivalves showed that they reduced their metabolism by u p to 40 % during the first day of oxygen lack when they turned to anaerobic metabolism. After prolonged anoxia, energy release decreased to less than 1 ",o of aerobic rates. These are the lowest rates found in marine invertebrates so far. A. lslandica is able to undergo periods of self-induced anaerobiosis, and can reduce its glycogen consumption drastically, as can b e calculated from calorimetric measurements. Thls behaviour may help it to withstand adverse environmental condltlons.

Toxic hydrogen peroxide (H202] is photochemically generated in hyperoxic and normoxlc intertidal pools. Surprisingly high amounts of H202 penetrate the redoxcline, and are measurable in pore water in spite of the anoxic conditions prevailing there. Bioturbate macrofauna inhabiting these sediments encounters oxic conditions with peroxide in surface pools and anoxic conditions with peroxide in the sediment. We studied antioxidative properties in the polychaete Nereis diversicolor. Antioxidant enzyme activities of catalase and superoxide dismutase (SOD) showed annual variations correlated to the environmental peroxide concentrations. Epitokous specimens from April exhibited very high enzyme activities. Catalase was predominantly localised in the mitochondria and is inducible by peroxide incubation under experimental conditions. SOD activities were not inducible during oxic peroxide incubation. In contrast, SOD had higher activities when the worms were kept anoxically in the presence of hydrogen sulfide (100 pM) or hydrogen peroxide (5 PM). The radical stress the worms encountered during anoxic peroxide incubation was reflected by a highly increased lactate formation.Under oxic conditions 500 nM H202 decreased oxygen uptake of N. diversicolor by 38%. In epitokous N. diversicolor, this was accompanied by a conversion of heme to the green pigment biliverdin, a potent antioxidant. Billverdin was accumulated by the worms dunng oogenesis under natural conditions. Therefore its formation during spawning seems to be a response to increasing hydrogen peroxide concentrations in sediment pore water during spring. Biliverdin was not formed under anoxic conditions, because the conversion requires oxygen. Atokous worms lacked the ability to form biliverdin when exposed to peroxide.

The priapulid worm Halicryptus spinulosus von Siebold, from a sulfide-rich braclush water habitat, is associated with various types of epibacteria. TEM observations reveal bacteria morphologically similar to Thiobacillus and Leucothrix/Thiothrix. The Thiobacillus-type is the dominant organism; it occurs in microcolonies throughout the bacterial layer and frequently contains carboxysomes, which are indicative of chemoautotrophic activity. Analyses of ribulose-1,5-bisphosphate carboxylase, X-ray microanalysis, and the ratio of stable 13C/"C and ' 5~/ ' 4~ isotopes do not indicate a participation of chemoautotrophic bacteria in the nutrition of the worm. It is concluded that the epibacteria might be able to detoxify the hydrogen sulflde to some degree, provided oxygen or nitrate is available, and that the worms must rely on their own metabolism of hydrogen sulfide detoxification whenever their environment becomes anoxic.

The priapulid Halicryptus spinulosus has an outstanding resistance to anoxia and hydrogen sulfide, which enables the animal to survive in deteriorating environments . Whole-body staining procedures, as well as light and scanning electron microscopy were used to study structures and mechanisms possibly involved in sulfide detoxification .The cuticle of the trunk is covered by a coat of mucus and bacteria . Within this coat considerable amounts of finely dispersed iron are precipitated, probably as a Fee +-compound . It is suggested that the iron functions as a rechargeable buffer against hydrogen sulfide, protecting both the bacteria and the priapulid host . Although this chemical shield may not alone account for long-term protection, it allows the animal to gain time for metabolic adaptations .

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