Dasysiphonia japonica (=Heterosiphonia japonica) is an invasive red seaweed native to the Northwest Pacific. It was first reported from Galicia, Spain in 1988. and spread from Norway to the Mediterranean. It spread in Europe north to Norway, and south to the Western Mediterranean. In 2008, it was collected off Rhode Island, and is now found from Long Island to Nova Scotia. It grows as a deep rose-red, flattened, fan-shaped thallus, growing up to 200 mm high. In Great Bay, New Hampshire, it was found on eelgrass, shellfish farming gear, mud flats, oyster reefs. and inshore rock ledges. This seaweed is spreading aggressively in coastal waters. In Long Island bays, decaying blooms have been associated with fish die-offs. In experiments, toxins from the algae have been lethal to fishes and bivalve larvae.