A new locality record of the ectoparasitic copepod Salmincola californiensis in Japan: A potential glacial relict population?

Abstract

Salmincola californiensis, an ectoparasitic copepod, infects salmonid fishes’ branchial cavities and fins, mainly of Oncorhynchus species. While this species is widely distributed along the Northern Pacific Ocean rim, only three populations have been documented in Japan, and these records are primarily from highly fragmented local areas at high altitudes or latitudes. Here, we report S. californiensis parasitizing masu salmon Oncorhynchus masou masou captured in upstream reaches of the Naka River, Tochigi Prefecture, central Honshu. This is the fourth locality record in Japan and for the first time in 40 years. Copepods were identified as S. californiensis based on partial 28S rDNA sequences. Infection levels were relatively high (Prevalence 66.7%, mean intensity 5.0), and some copepod individuals had egg sacs, suggesting that this population is naturally sustained. Given their fragmented distributional records in high-altitude areas and considering that Japan represents the southernmost margin of their distributional range, these populations are assumed to be glacial relics.