Metabolic and Blood Gas Transport Characteristics of the Hydrothermal Vent Bivalve Calyptogena magnifica

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Abstract
Individual Calyptogena magnifica are able to regulate their oxygen consumption rates down to low partial pressures of oxygen. Their rates of oxygen consumption are comparable to those of active shallow-living bivalves at comparable temperatures. Rates of carbon dioxide production relative to oxygen consumption in the absence of sulfide indicate a predominately heterotrophic metabolism. Calyptogena magnifica individuals do not show significant uptake of methane so this is unlikely to be an important external metabolite for these clams. Sulfide levels in the blood and gills of freshly recovered clams were high (up to 1.9 mM) and were generally in excess of estimated environmental sulfide levels (<0.2 mM). Sulfur-metabolizing ability (measured as ATP sulfurylase activity) of the gill tissue including endosymbiotic bacteria was substantial and variable, suggesting microhabitat variation. Blood serum separated from erythrocytic Hb showed marked sulfide accumulation and may function for sulfide transport as well as to protect the sulfide-sensitive hemoglobin by binding free sulfide. The hypothesis that C. magnifica may take up sulfide through the foot (which is extended into the vent water in the rock fissures) and transport it to the bacteria in the gills via the sulfide-binding component in the blood serum while simultaneously taking up oxygen and carbon dioxide through the siphon (which is extended upward into the ambient bottom water) is explored.