Does grass puffer migrate upstream in rivers to eliminate ectoparasitic copepods?: likely natural freshwater bathing

Grass puffer Takifugu alboplumbeus swims upstream in rivers to eliminate certain pathogens. In the Seto Inland Sea, Japan, and its neighboring brackish areas, the parasitism of ectoparasitic copepods infecting T. alboplumbeus was compared between the sea and brackish waters to clarify this hypothesis from April to November 2022. A marine station was set up off Takehara City, Hiroshima, and two stations for surveys in brackish waters were located at the mouths of two rivers flowing into the sea. The salinity of the bottom water at the two brackish stations was nearly 30, the same as the surface water at the marine station, whereas that of the surface water at the brackish stations was lower than 7. The host harbored four species of ectoparasitic copepods: Caligus fugu (adults and copepodids), Brachiochondria pinguis (adult females), Parabrachiella hugu (adult females), and Taeniacanthus yamaguchii (adults and copepodids). The adult and copepodid stages of C. fugu were found on the body surface and fins of the host, respectively, whereas those of the other species were found in the mouth cavities or gills. The mean abundance of all species and stages was significantly lower at the river mouths than that at the marine station. Parabrachiella hugu and B. pinguis firmly adhering to the mouth showed low prevalences at the brackish stations. In laboratory experiments, low salinity caused C. fugu to detach from their hosts and die. The present study demonstrated that freshwater bathing can efficiently eliminate freely moving ectoparasitic copepods.