Flexible herbivory of the euryhaline mysid Neomysis awatschensis in the microtidal Yura River estuary, central Japan

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.