Hypotaurine and thiotaurine as indicators of sulfide exposure in bivalves and vestimentiferans from hydrothermal vents and cold seeps

Vesicomyid clams, vestimentiferans, and some bathymodiolin mussels from hydrothermal vents and cold seeps possess thiotrophic endosymbionts, high levels of hypotaurine and, in tissues with symbionts, thiotaurine. The latter, a product of hypotaurine and sulfide, may store and/or transport sulfide non‐toxically, and the ratio to hypotaurine plus thiotaurine (Th/H + Th]) may reflect an animal's sulfide exposure. To test this, we analyzed seep and vent animals with in situ sulfide measurements. Calyptogena kilmeri clams occur at high‐sulfide seeps in Monterey Canyon, while C. (Vesicomya) pacifica clams occur at seeps with lower levels but take up and metabolize sulfide more effectively. From one seep where they co‐occur, both had gill thiotaurine contents at 22–25 mmol kg^?1 wet mass, and while C. (V.) pacifica had a higher blood sulfide level, it had a lower Th/H + Th] (0.39) than C. kilmeri (0.63). However, these same species from different seeps with lower sulfide exposures had lower ratios. Bathymodiolus thermophilus East Pacific Rise (EPR 9°50′ N)] from high‐(84 μm ) and a low‐(7 μm ) sulfide vents had gill ratios of 0.40 and 0.12, respectively. Trophosomes of Riftia pachyptila (EPR 9°50′ N) from medium‐(33 μm ) and low‐(4 μm ) sulfide vents had ratios of 0.23 and 0.20, respectively (not significantly different). Ridgeia piscesae vestimentiferans (Juan de Fuca Ridge) have very different phenotypes at high‐ and low‐sulfide sites, and their trophosomes had the greatest differences: 0.81 and 0.04 ratios from high‐ and low‐sulfide sites, respectively. Thus Th/H + Th] may indicate sulfide exposure levels within species, but not in interspecies comparisons, possibly due to phylogenetic and metabolic differences. Total H + Th was constant within each species (except in R. piscesae); the sum may indicate the maximum potential sulfide load that a species faces.