| Freshwater mussels (Super family Unionoidea) constitute one of the important biological components of freshwater ecosystems particularly because some fish species require them as spawning beds whereas mussels need other fish species to reproduce themselves. Furthermore, as their longevity and poor mobility render them exceedingly susceptible to environmental changes, they could serve as a useful indicator of habitat conditions. Unionoid mussels used to widely inhabit in both lentic and lotic environments and 18 species have been reported in Japan; approximately 1,000 species exist worldwide. However, decreases in species diversity, abundance, and habitat range of Unionoida are common throughout the world, and major causes include alterations of physical, chemical, and biological characteristics of habitat environments through human activities such as harvesting of individuals, river works, landuse change, and invasion of alien species. Comparisons of major habitat types reported for unionoid mussels between Japan and other countries show that one of the most common habitats for lotic species in Japan is agricultural ditches that are much smaller in size and more regulated compared to common habitat types (mostly large rivers) in other countries. Based on direct observation of ditch habitat over alteration works, habitat degradation resulted from reduced habitat heterogeneity in both longitudinal and lateral directions. An experimental case with placement of lateral structures and creation of artificial riffles may provide one possible approach in restoring degraded ditch habitat for Unionoida. There is a need for more studies on Unionoida in ditch habitat because various underlying processes may be different from those in ordinary river ecosystems. Most importantly, efficient and rapid dissemination of various type of information collected at the local level to those interested in the topic in other areas is crucial for conservation of unionoid habitat at the country level and beyond. |
