Spatial dietary shift of the intertidal snail, Batillaria multiformis: stable isotope and gut content analyses

抄録

In order to identify the food sources of the intertidal snail, Batillaria multiformis, stable isotope (δ¹³C, δ¹⁵N) and gut content analyses were conducted. Snails and their possible food sources were collected from the river-sea transects at the mouths of the Iroha and Katsura rivers in northeastern Kyushu, Japan, which differ in land use parameters (e.g., the ratio of agricultural area in the catchment area). There was a shift in the δ¹³C of the snail along the transects, showing higher values at the upstream stations (−15.8 and −14.8‰) and lower values (−17.5 and −17.1‰) at the seaward stations. The δ¹⁵N showed no significant spatial gradient along the transects, but a significant difference was observed (t-test, p<0.001) between the Iroha (mean, 10.0‰) and Katsura (11.1‰) rivers. A Bayesian mixing model and the biomass of possible food sources on the tidal flats showed that (1) the major food sources were marine phytoplankton, seaweeds, and benthic microalgae, and that (2) the dependency on marine phytoplankton increased in the seaward stations. However, gut content analysis revealed that most of the identifiable dietary items across all stations were benthic diatoms, which is considered to be due to the short-term result of ingestion on the sediment surface at low tide. Increasing dependency on phytoplankton at seaward stations was considered to be due to consumption of suspended particles in the water column at high tide by filter feeding. Differences in the δ¹⁵N between the two rivers suggest the possibility of using the δ¹⁵N of snails as an indicator of watershed characteristics.