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Partial rbcL sequences were obtained from the type specimens of Lithophyllum coarctatum and L. gardineri. Both species were variably assigned to either Hydrolithon or Porolithon during the last two centuries, with L. coarctatum, from the tropical eastern Indian Ocean currently considered a synonym of L. gardineri from the tropical western Indian Ocean. Phylogenetic analyses show that both belong in Porolithon and that they are distinct species despite their morpho-anatomical similarities, including a columnar morphology. Porolithon epiphyticum sp. nov., from the same locality as P. coarctatum, Cocos-Keeling Islands, is the first reported epiphytic species in the genus. Eight of the currently recognized 15 species of Porolithon have had their type specimens sequenced to correctly apply names, including the generitype specimen. DNA sequencing provides independent confirmation that the morpho-anatomical character of the presence of horizontal fields of trichocytes without intervening vegetative cells is diagnostic for Porolithon. The generitypes of the four genera classified in the subfamily Metagoniolithoideae, Dawsoniolithon, Floiophycus, Harveylithon, and Metagoniolithon as well as Hydrolithon in the subfamily Hydrolithoideae, have not been sequenced. The taxonomic and nomenclatural uncertainties in each of these genera are discussed.

The filamentous marine red alga Ceramium cimbricum was described from Denmark, but the name has since been applied to populations worldwide. To determine if C. cimbricum from the Pacific is conspecific with C. cimbricum from the Atlantic, we (1) deciphered an authentic rbcL sequence for the lectotype specimen of C. cimbricum, (2) performed phylogenetic analysis on Pacific and Atlantic C. cimbricum, and (3) assembled and analyzed the complete mitochondrial and plastid genomes of specimens of C. cimbricum from Denmark and California. The lectotype sequence of C. cimbricum was unique and fully resolved with other Atlantic Ceramium species, however, the specimens of C. cimbricum from Denmark, Norway, California, Oregon, and Korea formed a clade with Pacific species. The plastid and mitochondrial genomes of C. cimbricum from Denmark and California were similar in length and gene content, differing by only 67 SNPs and nine gaps for the plastid, and by 100 SNPs and 31 gaps for the mitochondrial genomes. These results support the continued application of the name C. cimbricum to specimens from Denmark, and warrant the naming of a new species, Ceramium sungminbooi sp. nov., native to Asia and introduced to the northeastern Pacific and Europe.