Antithamnion nipponicum (Ceramiaceae, Rhodophyta), incorrectly known as A. pectinatum in western Europe, is a recent introduction along the North Carolina and Pacific coasts of North America

Female and male reproductive structures in material from California known as Antithamnion hubbsii are illustrated and described for the first time. This partly prostrate species is characterized vegetatively by pinnae arranged in distichous opposite pairs, bearing adaxial and abaxial pinnules, with the distal-most pinnules restricted to the abaxial side of pinnae. Basal cells of the pinnae produce multicellular rhizoids with digitate holdfasts as well as indeterminate lateral axes, and gland cells originate adaxially alongside the lower pinnule cells. Antithamnion nipponicum has been placed in synonymy with Antithamnion pectinatum, a species described from Auckland I., New Zealand. The latter is here recognized as a separate southern hemisphere species bearing adaxial and abaxial distal-most pinnules and new indeterminate lateral axes in place of pinnae. The correct name for the invasive species known in western Europe as A. pectinatum is A. nipponicum. Our phylogenetic analyses of rbcL sequence data also indicate that Californian A. hubbsii and A. nipponicum are conspecific, but distinct from A. pectinatum and A. aglandum. The distribution of A. nipponicum includes the Pacific coast of California and the Atlantic coast of North Carolina, USA. Its presence is inferred in the Mediterranean Sea. Historical reports suggest that this species was recently introduced from Japan.