Abstract
The estuarine planktonic calanoid copepod Eurytemora americana is described for the first time from Japan based on specimens from Dokai Bay, a small inlet of northern Kyushu in 1975, when the species was exclusively dominant in the innermost bay. This is the southernmost record of the species in the Northern Hemisphere. Morphological descriptions are provided, especially on characters different from or overlooked in previous descriptions. Compared with the recent description of the species from Korea, a distinct morphology is found in the spinule number on the male fifth leg, suggesting inter-population variation and therefore no gene flow between the Japanese and Korean populations. Eurytemora americana in Dokai Bay is regarded as an invasive alien species introduced via ship ballast water. Despite the dominance in 1975, the copepod was not collected from brackish waters in the innermost part of the bay in 2008. Global warming in spring and/or winter periods after the 1970s is a possible reason for the disappearance of this cold water species.
