Reassessment of the taxonomic status of Gelidium subfastigiatum (Gelidiales, Rhodophyta)

SUMMARY The taxonomic relationship between Gelidium elegans Kützing and Gelidium subfastigiatum Okamura, two morphologically similar species of the red algal genus Gelidium (Gelidiaceae) growing in the north‐western Pacific, was critically re‐examined. Gelidium subfastigiatum has been distinguished from G. elegans by its more robust thalli, which have antrorse tooth‐like branches, although their distinction has been said to be often difficult or impossible. We determined the nuclear encoded internal transcribed spacer 1 (ITS1) for 14 samples from eight populations of this G. elegans/ G. subfastigiatum complex, and two types of ITS1 sequences were found. Analysis of seasonal variations of subterminal portions of major branches revealed that this complex includes two groups: one possessing the type 1 ITS1 sequence and antrorse tooth‐like branches that are subterminally thickened and widened during only colder months, and another possessing the type 2 ITS1 sequence and thin and narrow branches throughout the year. These groups should be recognized as separate species; the former is assigned to G. subfastigiatum and the latter to G. elegans .