A taxonomic sudoku: Resolving the long-standing puzzle of Japanese Dictyoteae (Dictyotales, Phaeophyceae) with new records and biogeographic insights.

Japanese Dictyoteae have been documented since 1838, with a historical checklist of 21 names recorded prior to 1990. Persistent misidentifications-particularly of species originally described from Europe and the Americas-have complicated the taxonomy of the group in Japan. We reevaluated historical records and revised the national checklist for Canistrocarpus, Dictyota, Dilophus, and Rugulopteryx using integrated morpho-genetic analyses. Broad sampling from Hokkaido to Okinawa, including Japanese type localities, together with sequence data from type localities of species described elsewhere but reported in Japan, corrected persistent taxonomic ambiguities. The long-standing misapplication of the Asian D. dichotoma as D. spathulata was resolved. The revised checklist recognizes 19 species: 17 Dictyota, Canistrocarpus cervicornis, and Rugulopteryx okamurae. Species richness peaks in Kyushu with 15 species, whereas only D. spathulata extends to Hokkaido. Eight taxa appear native to the Northwest Pacific, including D. coriacea, D. haenyeosa, D. spathulata, D. spinulosa, D. sumbisoria, D. taewakia, Dictyota churaumia sp. nov., and R. okamurae. Twelve taxa exhibit broad, transoceanic distributions. We describe Dictyota churaumia sp. nov. and formally report nine species from Japan for the first time: Dictyota acutiloba, D. ceylanica, D. haenyeosa, D. humifusa, D. liturata, D. pfaffii, D. sandvicensis, D. sumbisoria, and D. taewakia. By integrating molecular data with morphological analyses, we resolved major taxonomic misidentifications and implemented the necessary taxonomic treatments (synonymies and typifications) to provide an updated taxonomic framework for the Dictyoteae flora of Japan. The presence of unique morphologies in historical herbarium specimens, coupled with unexplored regions of Japan, indicates a potential for additional, unidentified species.