The coypu (Myocastor coypus) is a semi-aquatic herbivorous rodent that has been suspected to prey on freshwater unionid mussels in Japan. Fecal DNA metabarcoding that targeted bivalves and terrestrial plants was performed to examine the diet of exotic coypu in the Kiso River, central Japan. Thirty-two fecal samples from four floodplain waterbodies were collected seasonally. In one of the waterbodies, live and dead mussels were investigated to examine the possible effects of coypu predation on mussel population structures. Common coypu predation on unionid mussels was confirmed in two waterbodies. Midden surveys showed that medium- to large-sized mussels tended to be consumed by coypu; possibly resulting in population structures with few large-sized individuals. Seasonal diet selection of terrestrial plants was also detected: two perennial plants were consumed throughout the year, whereas some perennial plants were consumed seasonally. The number of terrestrial plant species/taxa as primary food was higher in summer than in winter, which was attributed to the high consumption of annual plants in the summer. Future studies need to examine the exhaustive diet of coypu including other animals and plants, as well as its effects on freshwater ecosystems in invaded regions.

The terrestrialization of floodplains has become a concern to river managers and ecologists because it has degraded habitats for floodplain-dependent organisms. We examined the temporal distributions of the endangered deepbody bitterling (Acheilognathus longipinnis) throughout its life history, which is an autumn-spawning annual fish spending its egg and larval stages in unionid mussels and emerging in spring, to understand its population decline in the terrestrialized floodplains of the Kiso River, central Japan. We first validated our A. longipinnis environmental DNA (eDNA) sampling method and observed an 89.3% probability of consistency between the eDNA and the direct capture surveys of 56 floodplain waterbodies (FWBs). Subsequently, the temporal distributions with autumn dispersal (9 of 14 FWBs) were found using time-series eDNA samples collected from 14 FWBs on a floodplain with a length and width of 1.4 and 0.2 km, respectively. In the following spring, juveniles were only detected in the two FWBs connected to the river channel. Moreover, the direct capture data revealed that juveniles occurred in 52.9% (9/17) of the connected FWBs, but only in 5.1% (2/39) of the FWBs isolated from the river channel. Autumn dispersal of A. longipinnis would be disadvantageous for reproduction in terrestrialized floodplains with numerous isolated FWBs.