Reproductive strategies, whether sexual or asexual, are critical aspects of introduction success and spread for non-indigenous species. The Western Pacific Diadumene lineata (Verrill, 1869), the world’s most widely distributed sea anemone due to numerous introductions, is believed to reproduce only by asexual means outside of its home range. Over the past 100 years, no populations with both males and females have been reported to co-occur outside of its native Japan. We report the first discovery of sympatric reproductive male (sperm-bearing) and female (egg-bearing) D. lineata in Coos Bay, Oregon, USA, confirmed by histological analysis. Given that only single gender introduced populations have been reported elsewhere, the presence of both genders in this US Pacific Northwest bay may be linked to high and continuous propagule pressure resulting from a history of intensive lumber and timber shipping directly between Japan and Coos Bay. Novel modern-day introductions of this species, in which reproductive traits previously only associated with native populations are manifested, could influence the future invasion success and spread of this species.
