Calyptogena elongata is a small (about 7 cm maximum length) species of vesicomyid clam which lives at depths of 494–503 m, near the sill depth, in the Santa Barbara Channel in mildly reducing muds at low ambient oxygen concentrations. This species has abundant autotrophic sulphur- oxidizing bacteria in bacteriocytes in its gills. The stable carbon isotope composition values of its gills and other tissues range from −36 to −38 “, supporting the suggestion that the primary carbon source for this symbiosis is inorganic carbon fixed by the endosymbionts. This species of clam concentrates sulphide into its blood serum by using a sulphide-binding component and into the gills by using an unknown sulphide-binding activity. In both tissues, total H2S concentrations within the clam can greatly exceed those outside. This apparently enables the clam to concentrate sufficient sulphide from the mildly reducing muds to support the needs of its endosymbionts. Both of these binding activities are reversible in vivo as shown by the rapid declines in blood and gill sulphide levels when the clams are deprived of sulphide and the rapid concentration of sulphide into the blood and gills when it is provided. For example, within minutes of exposure to 65 micromolar H2S, gill and blood total H2S concentrations in individual C. elongata exceed the external concentration; within 2 h they reach maximum concentrations of about 2 mmol l-1. When such experiments are carried out under anoxic conditions, the blood and gill total H2S concentrations approach saturation (10–20 mmol l-1), indicating that under oxic conditions the oxidation of sulphide by the clam and its endosymbionts holds the binding components below saturation and enables them to protect the animal tissues and endosymbionts from toxic concentrations of sulphide. In contrast to these results for C. elongata, our experiments show that the host of another chemoautotrophic symbiosis, Solemya reidi, does not concentrate sulphide from the medium into either its blood or its gills. Data are presented which indicate that the serum sulphide-binding component is a large molecular mass molecule with Zn2+ at the active site. This study strongly supports the model of vesicomyid functioning in which the blood- borne sulphide-binding component concentrates sulphide from the reducing environment around the clam's foot and transports this sulphide to the symbionts in the gills. Data are also presented which indicate that the clam oxidizes some sulphide to thiosulphate and transports this to the gills as well. Thus, individual C. elongata, like the previously studied C. magnifica, appear to bridge the reducing and oxidizing zones of their habitats to provide needed substrates to their endosymbionts. Examination of these two species and the anatomy of other vesicomyid species suggest that vesicomyid clams are functionally quite conservative.

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