The timings of spawning and glochidial release of Margaritifera togakushiensis were investigated in the Sakasa River, Nagano Prefecture, Japan, from 2002 to 2007. For the years in which the water temperature in spring rose earlier and the snowfall was lighter, females spawned eggs and discharged glochidia earlier. In general, however, females started to spawn eggs in April when the water temperature reached about 4°C and started to discharge glochidia in May with the water temperature reaching about 10°C. The accumulated temperature of the water during the incubation period (from spawning to glochidial release) was about 180 day-degree. These results suggest that water temperature inﬂuences the timing of reproductive behavior of M. togakushiensis. The maximum fertility rates ranged between 44% and 58%. Most gravid females which were marked also spawned eggs the next year. During a three-day continuous observation in 2006, glochidial release was found mainly at dawn (4:00 – 6:00) and in the daytime (10:00 – 15:00) but not at night (20:00 – 4:00).
We verified the host fish species for the freshwater pearl mussel Margaritifera laevis and compared the distributions of M. laevis and its hosts in the Abira River in central Hokkaido. Among the six species and a mutant form of rainbow trout (Oncorhynchus mykiss), mussel glochidia were found only from the gills of the masu salmon (Oncorhynchus masou masou) with an infection rate of 84.2%. Some of the attached glochidia on the ﬁsh were well grown. Although O. mykiss belongs to the same genus as the masu salmon, glochidia did not parasitize O. mykiss including the mutant form. The distribution of masu salmon was limited to a 2-km reach in the upper third of the stream where the mussel was distributed. The result suggested that in the Abira River, most of the free-living glochidia had been ﬂushed down to die without encountering their host ﬁsh.