Experimental introduction of the endangered freshwater unionid Pronodularia japanensis to spring-fed ponds in Matsuyama, Ehime, Japan

Populations of two unionid mussel species, Pronodularia japanensis and Unio douglasiae nipponensis, have drastically decreased over the past 25 years in Matsuyama Plain, Ehime Prefecture, Japan. A native bitterling, Tanakia lanceolata, depends on these mussels as its spawning beds; thus, the conservation of unionid mussels is necessary to prevent an extinction cascade. In Matsuyama, the bitterling Tanakia limbata has been introduced by humans and its distribution now extends across the entire T. lanceolata habitat, leading to hybridization with T. lanceolata. In this study, we introduced P. japanensis to four sites within two nature restoration ponds (Matsubara Pond and Hirose-gasumi) and one site in a spring-fed pond (Yanahara Pond) and monitored them for 1 year. Our objective was to conserve this unionid and construct a refuge for endangered unionids and bitterlings, from which invasive bitterling species would be removed. We measured the density of the diatoms on which unionids feed, water temperature and depth, dissolved oxygen content, and the size of substrate particles at our study sites. We found that the survival rates of introduced P. japanensis were 37% over a period of 1 year at Hirose, and 75% over 6 months at the downstream Matsubara site. All individuals in the other three sites died within 1 year. These survival rates were affected by diatom density; the two sites with high survival rates were higher in diatom density than the three remaining sites. At Hirose and the downstream Matsubara site, dissolved oxygen content fell to 3-5 mg/L between October and November, 2015. During the monitoring period, we observed introduced mussels emerging from the substrate and exposing their entire shells to the bottom surface. This behaviour was significantly related to the probability of death within 2 weeks, and likely indicated an attempt at escape from an unsuitable environment. In Yanahara Pond, we observed that invading T. limbata spawned on introduced P. japanensis. These results suggest the following necessary conditions for constructing a refuge for P. japanensis and T. lanceolata: 1) a sufficient supply of diatoms, 2) dissolved oxygen content is maintained above 5 mg/L for years, and 3) prevention of invasion by non-native bitterlings. During monitoring, it is necessary to verify that introduced unionids are infaunal under the substrate during winter. In our study sites, the addition of silicate and/or the construction of a flow-retention structure to increase diatom density and remove invasive floating leaf plants (to maintain sufficient dissolved oxygen) will enhance the effectiveness of the downstream Matsubara site and Hirose as refuges.