Infestation of Rhinogobius flumineus (Family Gobiidae) by the Myxozoan Myxobolus nagaraensis in the Nagara River System, Central Japan.

Myxobolus nagaraensis is a species of myxozoa, a parasitic cnidarian that moves between fish and annelids, and was described in 2007 as a parasite of the genus Rhinogobius. The purpose of the present study was to clarify the actual status of parasitism of Rhinogobius flumineus by M. nagaraensis. Collection surveys conducted in the fall/winter season confirmed infestation at 18 of 38 locations of tributaries of the Nagara river. At all eight sites with high infection rates (> 9%) during fall/winter, the number of parasitized individuals decreased dramatically in spring/summer. The body length of the infected individuals was 20-45 mm, corresponding to yearlings or fish that had just turned 1 year old; no infestation was observed in older fish. In fall/winter, sites with high parasitism had significantly lower flow velocities and significantly higher electrical conductivity and total organic carbon compared to sites with no parasitism. In conclusion, infection with M. nagaraensis appears to occur locally through the migration of young R. flumineus into small, eutrophic tributaries with slow currents that are suitable for Oligochaeta habitat, and most of the infected fish subsequently die.