Several species classified to the genus Ulva are primarily responsible for causing green tides all over the world. For almost two decades, green tides have been resulted in numerous ecological problems along the eastern coast of Jeju Island, Korea. In order to characterize the species of Ulva responsible for causing the massive blooms on Jeju Island, we conducted DNA barcoding of tufA and rbcL sequences on 183 specimens of Ulva from eight sites on Jeju Island. The concatenated analysis identified five bloom-forming species: U. australis, U. lactuca, U. laetevirens, U. ohnoi and a novel species, U. pseudo-ohnoi sp. nov. Among them, U. australis, U. lactuca, and U. laetevirens caused to the blooms coming mainly from the substratum. U. ohnoi and U. pseudo-ohnoi sp. nov. were causative the free-floating blooms. Four species, except U. australis, are characterized by marginal teeth. A novel species, U. pseudo-ohnoi sp. nov., is clearly diverged from the U. lactuca, U. laetevirens, and U. ohnoi clade in the concatenated maximum likelihood analysis. Accurate species delimitation will contribute to a management of massive Ulva blooms based on this more comprehensive knowledge.

Cosmopolitan Acrosorium species with hook-forming thalli have been merged under the name of Acrosorium ciliolatum (Harvey) Kylin through a long and complicated nomenclatural history. We examined the specimens of ‘A. ciliolatum’ and related taxa from the northwestern (NW) Pacific, the UK, southern Spain, Australia, New Zealand, and Chile, using morphological and molecular analyses. We confirmed that these specimens are separated into four clades based on rbcL phylogeny, and the absence or presence of terminal hook-like structures represent intraspecific variation. Our results indicated that Acrosorium flabellatum Yamada, Cryptopleura hayamensis Yamada, Cryptopleura membranacea Yamada and the entities known as ‘A. ciliolatum’ in the NW Pacific are conspecific; the name A. flabellatum is the oldest and has priority. This taxon exhibits extreme variations in external blade morphology. We also confirmed that the position of the tetrasporangial sori is a valuable diagnostic characteristic for distinguishing A. flabellatum in the NW Pacific. We also discussed the need for further study of European and southern hemisphere specimens from type localities, as well as the ambiguous position of California specimens.

The genus Phycodrys is characterized by having a Phycodrys-type apical organization, a well-developed central midrib with paired lateral veins, monostromatic areas between the veins, four-celled carpogonial branches with two groups of sterile cells, and a cystocarp with a large fusion cell and short-catenated carposporangia. In contrast, the monospecific genus Nienburgella has a faint midrib, a mostly polystromatic blade without lateral veins, bilaterally isometric second-order cell row length, and Phycodrys-like apical growth and cystocarp. Nothing is known about Nienburgella’s procarp. We collected several specimens of Nienburgella angusta from the East Sea of South Korea and noticed that they resemble Phycodrys radicosa. We compared these two species using morphological and molecular phylogenetic analyses. Our results revealed that both species are closely related to one another than to other taxa in the tribe Phycodryeae, and that Nienburgella angusta procarp developmental pattern is the same as the one observed in Phycodrys rubens, the generitype species of Phycodrys. Thallus structure (mono- or polystromatic) was species-delimiting but not genus-delimiting. Bilateral second-order cell row isometry occurs in other Phycodrys species, including the type species. Therefore, we propose transferring the species Nienburgella angusta as well as the monospecific genus Nienburgella to Phycodrys.