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The euryhaline mysid Neomysis awatschensis occurs widely in estuaries and brackish lakes in Japan. Although N. awatschensis may serve as an essential energy channel to higher trophic levels, its feeding ecology has been studied only in sheltered and desalinated Lake Kasumigaura. To determine major food sources for N. awatschensis, monthly sampling was conducted during the day along the microtidal Yura River estuary from March 2014 to July 2015. Additional sampling was conducted in May 2019 to assess diel vertical migration in relation to feeding. Microscopic examinations revealed that benthic diatoms (e.g. Bacillaria paxillifer and Navicula spp.) dominated in the stomach irrespective of developmental stage, diel cycle, or season. Stable carbon and nitrogen isotope ratios in N. awatschensis and its potential food sources indicated great contributions to the diet of N. awatschensis from benthic microalgae (mainly benthic diatoms) in 2014 and from particulate organic matter (mainly phytoplankton) in 2015. Although N. awatschensis always concentrated close to the bottom in 2019, diel vertical migration might have happened in 2015, when fresh water occupied the whole estuary possibly due to river mouth clogging. Under the stagnant and oligohaline conditions, N. awatschensis could have swum up into the water column and fed on phytoplankton at night in 2015. This hypothesis may resolve the apparent contradiction between the microscopy and stable isotope analysis in 2015. In the Yura River estuary, N. awatschensis feeds primarily on benthic diatoms, although it can utilize phytoplankton flexibly in response to prevailing environmental conditions. [ABSTRACT FROM AUTHOR]

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 riversea 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. [ABSTRACT FROM AUTHOR]

Specimens of the intertidal snail, Batillaria multiformis were collected from 25 river mouths in the Kunisaki Peninsula and the Nakatsu and Usa districts in southwestern Japan to assess the relationships between their isotopic compositions (δ13C and δ15N) and watershed characteristics. Among 22 watershed parameters, which were classified into the land use, human population, topography, geology, and vegetation in the catchment area of each river, five parameters were significantly correlated with the δ15N of snails (Pearson's correlation coefficient r: all p<0.05). The δ15N values increased with a decrease in the inclination of the ground, volcanic rock rate, and forested land rate and with an increase in the cultivated land and paddy field rates. The δ13C and δ15N analyses showed minor contributions of terrestrial plants to the diet of snails and their dependence on estuarine and marine producers, mainly on benthic microalgae, for food. These findings suggest that the δ15N values of intertidal snails were determined by the consumption of estuarine and/or marine producers, which assimilated nutrients derived from the watersheds. [ABSTRACT FROM AUTHOR]