.Melanothamnus japonicus (Harvey) Díaz-Tapia & Maggs 2017(Harvey) Díaz-Tapia & Maggs 2017 (=Neosiphonia japonica) is a member of the Polysiphonia group, a globally wide-ranging red algal seaweed group, which has undergone extensive taxonomic revision, and been split into several similar genera an and many similar species. Melanothamnus japonicus was described from Japan by W. H. Harvey in 1853. J. W. Bailey described a similar species, M. harveyi. from Connecticut in 1848. Melanothamnus japonicus had been synonymized with M. harveyi as Polysiphonia harveyi, and considered a possible introduction to the Atlantic Ocean from the Pacific. Savoie and Saunders in 2015 genetically identified M. japonicus from the Northwest Atlantic as a a distinct species from M. harveyi. Currently, M. harveyi is considered cryptogenic or native in the east Pacific (as Polysiphonia acuminata), and in North Atlantic. Genetic analysis indicates that there has been extensive hybridization between M. japonicus and M. harveyi in the Northwest Atlantic, 'Purebred' M. japonicus have been found in Narragansett Bay, and hybrids from Nova Scotia to North Carolina, Melanothamnus japonicus has also been identified from France, Italy, Australia, New Zealand and Peru. Likely vectors include hull fouling, ballast water, and Pacific Oysters. Much more genetic sampling and analysis will be needed to clarify the status of these two species in the Atlantic.