Discrepancy between the Distributions of the Freshwater Pearl Mussel Margaritifera laevis and its Hosts in the Abira River, Central Hokkaido, Japan

Abstract

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.