Analyses of sulfide, methane, oxygen, and CO, in pore-water sampdes from three cold seep sites in Monterey Bay indicate that fluid chemistry is a strong determinant of the distribution of chemolithoautotrophic vesicomyid clams. The distribution of Culyptogena pacijca and Culyptogenu kilmeri were aligned closely with sulfide concentrations at all cold seeps and reflected species-specific capabilities for sulfide binding. Live clams occurred only in sediment where sulfide was detectable. Sulfide was not detected in the absence of vesicomyid clams. The relative abundances of five vesicomyid species varied greatly among seeps. C. kilmeri accounted for 85-99% of all vesicomyids at seeps with high sulfide content, and C. puciJicu dominated (73%) seeps with low sulfide levels. These species were also partially segregated along sulfide gradients from the center to the margin of seeps, analogous to zonation of rocky intertidal communities. We hypothesize that the absence of thiotrophic or methanotrophic mytilid mussels from Monterey Bay cold seeps is related to the lack of physiological specializations for concentrating reduced sulfur compounds or methane and the absence of hypersaline brines that could extend the persistence of methane or sulfide-rich fluids very near the sea floor.

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Hydrothermal vent tubeworms, Riftia pachyptila Jones, were maintained alive and studied on board ship using flow-through pressure aquaria. Simultaneous measurements of O2, ΣCO2, ΣH2S fluxes showed that the intact symbioses reach maximum rates of uptake of ΣCO2 (>2 μmole g-1 h-1) at about 90 μM ΣH2S. Measurements were made of hemolymph and coelomic fluid ΣCO2, ΣH2S, thiosulfate, pH, and hemoglobin concentrations in worms kept under various conditions of O2 and ΣH2S. Normal hemolymph pH appears to be about 7.5 and is not affected by ΣH2S and ΣCO2 concentrations within the ranges observed. We conclude that Riftia is specialized to provide sulfide to its symbionts with minimal interaction of sulfide with the animal metabolism. The uptake of sulfide is apparently by diffusion into the hemolymph, facilitated by the sulfide-binding properties of the hemoglobins. Both ΣCO2 and PCO2 are elevated in the hemolymph above their levels in the medium, although they are reduced under autotrophic conditions. Thus inorganic carbon is apparently concentrated from the medium into the hemolymph by an unknown mechanism.

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