Rediscovery of a native freshwater shrimp, Neocaridina denticulata, and expansion of an invasive species in Lake Biwa, Japan: Genetic and morphological approach

Invasive species prevention involves the prevention of two aspects: introduction and secondary spread. The latter is important in places that can become hubs for the spread of invasive species. Lake Biwa, the largest lake in Japan, is an important area for biodiversity and fisheries. However, several invasive species of fish and crustaceans became established in the lake during the last century. One of the conservation problems in Lake Biwa is the unresolved suspicion that the native freshwater shrimp Neocaridina denticulata have been replaced with alien Neocaridina species. To verify whether the native Neocaridina population in Lake Biwa has been replaced by alien species, we estimated the population structure of Neocaridina spp., collected from 19 sites in and around Lake Biwa, based on genome-wide SNPs and mitochondrial DNA. The three detected genetic clusters were characterized by quantitative analysis of multiple morphological traits. Two clusters were identified as non-native N. davidi and the other as native N. denticulata . However, species discrimination based solely on morphological analysis was difficult, highlighting the importance of genetic analysis. We rediscovered the native populations in the region for the first time in a century; however, in 11 sites the invasive species were dominant. These findings suggest that the native populations are in a critical situation. Furthermore, stocking in Lake Biwa as a source for fishery resources throughout Japan can cause the secondary spread of the invasive shrimps from the lake, acting as a hub, to other parts of the country.