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The polymitarcyid mayfl y Ephoron shigae is widely distributed in Japan, and inhabits burrows in gravel riverbeds in the mid and lower regions of rivers. This mayfl y is renowned for synchronized mass emergences at dusk in early autumn, which have been frequently observed in various Japanese rivers since the 1970s. Although much geographic documentation of synchronized mass emergence has accumulated for major populations, there is not much information for smaller populations. There are a number of reasons the full distribution of this mayfl y is not well known, including; 1) the comparatively short period of the adult stage (shorter than two hours), 2) the short emergence season (one or several weeks per year), 3) the short nymphal season (E. shigae remains as diapause stage eggs from autumn to spring), and 4) the typical habitats are below deep water riverbeds in the mid to lower reaches of comparatively large rivers. In this study, we investigated the current distribution range (i.e., the geographic distribution range along the river’s course) of the mayfl y E. shigae in the Chikuma-gawa River (Nagano Prefecture), where it is considered to be one of the largest continuously extant meta-populations of this mayfl y. The fi rst record of mass emergence of this mayfl y in the Chikuma-gawa River was reported in the novel entitled “Hounen-mushi” written in 1928 by Shiga Naoya, and has since been observed for more than 80 years. In the results of our research, it was revealed that E. shigae inhabits an extended distribution range from the city of Ueda to the town of Obuse. Further, we observed high mayfl y density and high biomass in a region around the Heiwabashi and Awasa-bashi bridges. From these areas, the density, body size and the biomass of the mayfl ies decreased gradually in both the upstream and downstream zones. This result is consistent with a previous study investigating the distribution of the adult stage of this species. However, contrary to the previous study, we found many more nymphs at the downstream sites (the Yashima-bashi and Obuse-bashi bridges) than at the Sai-gawa tributary merge point. However, the collected individuals at these two downstream sites were markedly smaller in size. The

Current distribution of the polymitarcyid mayfly Ephoron shigae in the Chikuma-gawa River