Abstract
Fish fauna and host utilization of freshwater unionoid mussels were investigated in the riverside pools (called 'wando') along the Yodogawa River, Osaka, to assess the influence of the domination of exotic fishes (e.g. bluegill sunfish Lepomis macrochirus and largemouth bass Micropterus salmoides) on the reproduction of mussels. Total population ratios of bluegill and largemouth bass ranged from 10-64% at the study sites. Thirty-nine to 67% of Unio douglasiae nipponensis larvae and 18-96% those of Anodonta spp. parasitized both fishes. About 99% of U. douglasiae nipponensis larvae found on both were not encysted. About 97% of Anodonta spp. larvae on both were encysted or had metamorphosed successfully. Artificial infection experiments also certified the unsuitability of both fish to U. douglasiae nipponensis, as the ratio of successful metamorphosis was ca. 0.5% and more than ca. 80% of larvae were dead on both hosts. Innate rather than acquired immunity would induce these unsuitabilities. These results would indicate that the dominance of bluegill and largemouth bass negatively influence the reproduction of U. douglasiae nipponensis. We need to monitor the population of U. douglasiae nipponensis in the Yodogawa river, and pay attention to other inland waters where such a problem might arise.
