Environmental DNA Reveals the Geographic Distributions of Two Eel Species, Anguilla japonica and A. marmorata, in Western Kyushu, Japan

Anguillid eels migrate thousands of kilometres from their spawning grounds, dispersing across a vast geographic area to fresh and brackish water habitats, where they settle and grow. Japanese eels (Anguilla japonica) and giant mottled eels (A. marmorata) are both found in Japan, although their distributions differ. However, details of these differences are unknown. We hypothesised that distribution patterns of Japanese and giant mottled eels must be different between and within rivers along the northwest coast of Kyushu, Japan. Environmental DNA (eDNA) analysis was conducted at 87 sites in 23 rivers. Japanese eel eDNA was detected in 19 rivers (82.6%) and that of giant mottled eels was detected in 8 (34.8%). eDNA for Japanese eels was detected at 6 of 9 sites in the North (66.7%), 13 of 23 sites in Omura (56.5%) and 37 of 55 sites in the South (67.3%). In contrast, giant mottled eel eDNA was detected at 1 of 9 sites in the North (11.1%), no sites in Omura and 15 of 55 sites in the South (27.3%). There was no correlation between eDNA concentrations of the two species at 10 sites in the five rivers where eDNA of both species was detected, implying that their habitat preference differ. This partially reveals dispersal and settlement mechanisms of these eel species.