Sulfur isotopic variations in soft tissues of five benthic animals from the reductive, tidal-flat sediments in northern Kyushu, Japan

The sulfur nutrition of five benthic animals living in or on reductive, tidal-flat sediments has been studied by using the sulfur isotopic signatures of their soft tissues. The mean δ³⁴S values (+6.1‰ to +13.8‰, relative to the Cañyon Diablo troilite reference) of these animals' tissues are lower than those of many common marine animals, which have values close to those of seawater sulfate-sulfur (+21‰). This indicates that these animals use a ³⁴S-depleted sulfur source, which may be derived from bacterial sulfide in the sediments (less than -20‰). The animals that inhabit such sediments are adapted to an anoxic environment, where toxic hydrogen sulfide prevails. Due to the sulfide-rich habitat, benthic animals are expected to assimilate hydrogen sulfide directly during detoxification, or to prefer a diet which has a low δ³⁴S signature. Variations in δ³⁴S values of the sampled molluscs were fairly small, whereas migratory shrimp and fish tended to have larger variations, implying that migratory animals feed on diverse prey, while the molluscs feed on a common food, or their assimilative capacity for sulfide-sulfur does not vary among individuals. A gastropod inhabiting the surface of the reductive sediment showed a distinctively low δ³⁴S value (+5.8‰ to +6.4‰), indicating that the animal fed on detritus from reeds and halophytes with low δ³⁴S values (-19.1‰ to -2.1‰). This study suggests that some benthic animals incorporate sulfide-sulfur for their nutrition and that sulfur isotope composition can be a useful indicator in the analysis of the food habitat of animals.