131 J. exp. Biol. 179, 131–158 (1993) Printed in Great Britain © The Company of Biologists Limited 1993 THE ROLE OF A ZINC-BASED, SERUM-BORNE SULPHIDE- BINDING COMPONENT IN THE UPTAKE AND TRANSPORT OF DISSOLVED SULPHIDE BY THE CHEMOAUTOTROPHIC SYMBIONT-CONTAINING CLAM CALYPTOGENA ELONGATA J. J. CHILDRESS, C. R. FISHER*, J. A. FAVUZZI, A. J. ARP† and D. R. OROS Oceanic Biology Group, Marine Science Institute and Department of Biological Sciences, University of California, Santa Barbara, CA 93106, USA Accepted 8 February 1993 Summary Calyptogena elongata is a small (about 7cm maximum length) species of vesicomyid clam which lives at depths of 494–503m, 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 H 2 S 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 mol l -1 H 2 S, gill and blood total H 2 S concentrations in individual C. elongata exceed the external concentration; within 2 h they reach maximum concentrations of about 2mmol l -1 . When such experiments are carried out under anoxic conditions, the blood and gill total H 2 S concentrations approach saturation (10–20mmol 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 *Present address: Department of Biology, Pennsylvania State University, 208 Mueller Laboratory, University Park, PA 16802, USA. †Present address: Department of Biology, San Francisco State University, San Francisco, CA 94132, USA. Key words: Vesicomyidae, Calyptogena elongata, Solemya reidi, sulphide uptake, carbon fixation, sulphide concentration, sulphide loss, chemoautotrophic symbiosis, hydrogen sulphide, zinc, endosymbiosis, thiosulphate, ribulose-bisphosphate carboxylase.