Host mussel utilization of the Itasenpara bitterling (Acheilognathus longipinnis) in the Moo River in Himi, Japan

Bitterling are fishes that use freshwater mussels for oviposition. The pattern of mussel utilization of an endangered bitterling species, Acheilognathus longipinnis, which spawned in autumn, was investigated in a lowland river. Six Unio douglasiae nipponensis (1.5% of total 388 individuals) hosted A. longipinnis embryos consistently in their outer gills, and they inhabited areas along the river bank where mussel density was relatively high on a less compacted mud substrate. Unexpectedly low rates of host utilization and the rare occurrence of juvenile (shell length, < 40 mm) hosts (< 0.8% of total individuals) suggest the need for greater attention to the conservation of A. longipinnis.