Population decline in unionid mussels in the Matsuyama Plain, Ehime Prefecture

Unionid mussel is a key taxon for conservation of freshwater biodiversity. Four species of unionids, Nodularia douglasiae nipponensis, Pronodularia japanensis, Sinanodonta lauta, and S. japonica, inhabit the Matsuyama Plain of Ehime Prefecture, Japan. The distributions of three taxa (N. douglasiae nipponensis, P. japanensis, and Sinanodonta spp.) were investigated in 2013-2014 and compared with those recorded in 1988-1991. The habitat conditions and reproductive status (abundance of egg-incubating females and host fish infected by glochidia) of P. japanensis were also investigated. Nodularia d. nipponensis and Sinanodonta spp. were found at only two and one of the 96 sites, respectively, and a comparison with the previous record showed drastic decreases in their distributions. Although P. japanensis was found at several sites, its distribution area had also decreased to less than half of that previously recorded, and its maximum density decreasing from 58 to 2.7 individuals/m2 over 25 years. Furthermore, no P. japanensis with shell lengths below 51.5 mm were found, suggesting that no recruitment had occurred during the past decade. These results reveal that the unionids in the Matsuyama Plain are under high risk for extinction. A habitat analysis of P. japanensis indicated that its present distribution was limited to downstream of weirs, which could prevent upstream dispersal of their larvae, and P. japanensis preferred fine sediments containing sand with the rate of <38.8%. Egg-incubating P. japanens is females and their host fish infected with glochidia were found at high frequencies, suggesting that their recruitment is inhibited at the settlement and/or early survival stages of their juveniles.