Biogeography of luminous marine ostracod driven irreversibly by the Japan current.

The biogeography of the luminous marine ostracod Vargula hilgendorfii, also called "Umihotaru," shows that this organism may have arrived relatively recently on the Japanese islands during the final glacier period approximately 10,000 years ago. Phylogenetic relationships also strongly indicate that the Japan Current drove the Umihotaru ostracod northward. It is evident that the Umihotaru ostracod spread rapidly to the major Japanese islands 3,000 km north, whereas its spread was slow in the southwest of the Japanese islands, covering a distance of 400 km. The meandering of the Japan Current, where it passes by the Tokara Gap at 28 degrees N latitude, may be a barrier to Umihotaru ostracod extension.