In contrast to the generally bare aspect of the deep-sea bottom, dense and rich faunistic communities have flourished in the vicinity of hydrothermal vents (Hessler and Kaharl, 1995). The root of these unique ecosystems are chemoautotrophic bacteria, which oxidise the geothermal sulphide released in hydrothermal fluids (Jannasch, 1995). Many of the metazoan species that dominate these extreme habitats live in association with intracellular sulphur-oxidising symbionts. Such endosymbioses are found in mytilid and vesicomyid bivalves, provannid gastropods and vestimentiferan tubeworms (for a list of references, see Fisher, 1995). The host is provided with a regular input of organic matter produced by the symbionts, but also has to transport in its tissues the poisonous compound used by the bacteria as energetic substrate (i.e. sulphide). These symbioses therefore require structural and functional adaptations by the host for the uptake, transport and storage of sulphide (Powell and Somero, 1986). For example, in the vestimentiferan tubeworm Riftia pachyptila, sulphide is taken up from the surrounding seawater across the plume; the sulphide is then reversibly bound in the blood to extracellular haemoglobins and transported to the trophosome, where the bacteria are located. The high affinity of these haemoglobins ensures that internal sulphide can reach very high levels in the body fluids of vestimentiferans (e.g. up Symbiotic associations between marine invertebrates and sulphur-oxidising bacteria are a common feature in communities from sulphide-rich environments, such as those flourishing in the vicinity of hydrothermal vents. While the bacterial endosymbionts provide the host with an undoubted nutritional advantage, their presence also requires specific adaptations for the transport and storage of sulphide, which is a potent toxin of aerobic respiration. Although different mechanisms such as the reversible binding of sulphide to serum binding proteins or its oxidation to less toxic forms have been described, many questions still remained unanswered. In the last decade, large amounts of thiotaurine, an unusual sulphur-amino acid, have been reported in sulphur-based symbioses from hydrothermal vents and cold seeps. Compounds such as thiotaurine are known to take part in trans-sulphuration reactions, so the involvement of thiotaurine in sulphide metabolism has been suggested. We present here an experimental study on thiotaurine biosynthesis in three sulphur-oxidising symbiont-bearing species from the East Pacific Rise: the vesicomyid Calyptogena magnifica, the mytilid Bathymodiolus thermophilus and the vestimentiferan Riftia pachyptila. In all three species, thiotaurine synthesis is stimulated in vitro by an input of sulphide, as well as by thiosulphate in B. thermophilus.Several distinct metabolic pathways seem to occur, however, since hypotaurine is the only precursor in the bivalves C. magnifica and B. thermophilus, whereas thiotaurine is also produced from taurine in R. pachyptila. Hypotaurine (NH 2-CH2-CH2-SO2H)...