Migration routes of individuals in the newly established <b><i>Macrophthalmus japonicus</i></b>(Brachyura: Macrophthalmidae)population in Otomo-ura, Iwate, northeastern Japan

A newly established population of the sentinel crab Macrophthalmus japonicus in Otomo-ura, Rikuzentakata, Iwate, exemplifies coastal organism range expansions in Japan. However, it remains unclear whether M. japonicus larvae migrated northward from southern Pacific coast populations or southward from the Mutsu Bay population in Aomori. This study analyzed mitochondrial COI gene sequences from 13 M. japonicus specimens collected from Otomo-ura to determine Otomo-ura population larval sources by comparing genetic diversity with published data on other Japanese populations. The Otomo-ura population exhibited high genetic diversity comparable to southern Pacific coast populations, contrasting with the low genetic diversity of the Asadokoro population in Mutsu Bay. These findings suggest that the Otomo-ura population primarily originated from southern Pacific coast larvae. Despite potential founder effects and genetic drift in new populations, the Otomo-ura population’s high genetic diversity suggests its establishment in recent years and/or by repeated larval influx. Thus, founder effects may not have occurred.