Temporal Changes in Infection by Myxobolus nagaraensis in Rhinogobius (Gobiidae) in the Nagara River Basin, Central Japan, from the 1960s to the 2020s.

Myxobolus nagaraensis, a parasitic cnidarian, was first isolated from a freshwater goby of the genus Rhinogobius collected from the Nagara River system in central Japan and was described as a new species in 2007. This study investigated the changes in parasitism of Rhinogobius species by M. nagaraensis in the Nagara River Basin. Rhinogobius specimens, collected between 1967 and 2020 and stored at the Lake Biwa Museum and Gifu Prefectural Museum, were visually examined for the presence or absence of swelling of the abdomen or caudal peduncle specific to M. nagaraensis infestation. No swelling was identified among the 45,018 specimens collected from the main stream; however, one R. flumineus specimen collected from the Itaya River in 2011 exhibited a swelling among 1274 specimens collected from the tributaries. The myxospores extracted from the abdominal swelling were identified as M. nagaraensis through microscopic observation. Interviews with 26 individuals yielded four sightings of parasitized specimens, all of which occurred after 2007. Thus, there have been almost no cases of Rhinogobius species parasitism in the main stream from 2007 to the present, suggesting that parasitism may have predominantly spread to tributaries after 2007. Reasons for the changes in parasitism rates over time were discussed, including the possibility that parasitism rates in the main stream may have been underestimated due to biases caused by the sampling methods.