The co-occurrence of two endangered freshwater pearl mussels (Margaritifera laevis & Margaritifera togakushiensis) in rivers in Hokkaido, northern Japan

We investigated the co-occurrence of two endangered freshwater pearl mussel species (Margaritifera laevis and M. togakushiensis) within 100-m reaches of rivers in Hokkaido, northern Japan, using DNA-based identification techniques to compare differences in PCR-amplified fragments of mtDNA. Mussel sampling was conducted in 53 rivers across 31 watersheds between May 2016 and October 2017, and co-occurring distributions were charted on a 10×10-km mesh map. We detected M. laevis in 39 rivers in 23 watersheds and M. togakushiensis in 40 rivers in 25 watersheds. The two species co-occurred in 26 rivers in 17 watersheds. The ratio of the number of study rivers where both species were found to the total number of study rivers in the region was highest in eastern Hokkaido (68%). In addition, the two species co-occurred in 19 out of a total of 38 study meshes, and the ratio of the number of study meshes where the species co-occurred to the total number of study meshes in the region was also highest in eastern Hokkaido (67%). Genetic analysis of tissue samples revealed areas with higher probabilities of co-occurrence of the two target species within the same reaches. We believe that these findings could provide important information for future studies and conservation planning for pearl mussels in Hokkaido.