Roseolithon littorale Min-Khant-Kyaw, A.Kato & M.Baba sp. nov.

Figures 4–12

DESCRIPTION: Thalli were non-geniculate, warty and epilithic on pebbles or forming entirely free-living rhodoliths with sub-spheroidal to sub-elliptical shapes, c. 2 cm wide. Colour of living thalli was rose pink (Fig. 4). Thallus construction was monomerous with a multistratose non-coaxial (plumose) hypothallus composed of filaments running parallel to the substratum. Cells of perithallial filaments arose from hypothallial filaments and were composed of approximately square to elongate cells (Fig. 5). Cells of contiguous hypothallial and perithallial filaments were frequently joined by cell fusions; secondary pit connections were not observed. Subepithallial initials (intercalary meristematic cells) were composed of square to elongate cells, shorter than or approximately equal to their immediate inward derivatives (Fig. 6). A single layer of epithallial cells was flattened with flared outer cell walls (trapezoidal; Fig. 6, inset). Trichocytes were not observed. Tetra/bisporangial conceptacles were multiporate, raised above the surrounding thallus surface without differentiation into a peripheral rim and pore plate (Figs 7–9). Each pore opening was surrounded by 5–6 rosette cells in depressions giving a pitted appearance in surface view (Fig. 8). Conceptacle roofs were composed of 3–6 cell layers including epithallial cells. Pore canals were blocked by apical plugs before releasing spores and lined by 2–5 celled filaments (Fig. 10). Pore canal filaments include rosette cells with slightly sunken roofs and the underlying squarish, elongate and/or wedge-shaped cells (Fig. 11). Pore canal cells were generally longer than and/or approximately equal to other roof cells (Figs 10, 11). Tetrasporangia were zonately divided. Conceptacles appeared to be buried (Fig. 12), but also appeared to be shed. Gametangial thalli were not observed. Data on vegetative and reproductive features are summarized in Table 1.

HOLOTYPE: SAP 115684 (tetrasporophyte), collected on 20 April 2023 by Min-Khant-Kyaw & A. Kato, deposited in the Herbarium of the Graduate School of Science (SAP), Hokkaido University, Sapporo, Japan (Fig. 4).

HOLOTYPE GENBANK ACCESSION NUMBERS: psb A, LC822410; rbc L-3P, LC822433; COI-5P, LC822444.

TYPE LOCALITY: 32°33.08'N, 130°06.38'E; intertidal; Tsujishima Island, Amakusa City, Kumamoto Prefecture, Japan.

ETYMOLOGY: ‘ littorale ’ in reference to the shallow water habitat of the species in the intertidal zone.

ADDITIONAL SPECIMEN EXAMINED: In addition to the holotype sequences, six psb A sequences, two rbc L-3P sequences and two COI-5P sequences were generated from Japanese specimens (Table S1).

DISTRIBUTION: Based on DNA sequences, R. littorale is currently known only from the type locality ( Tsujishima Island) and Shirasu in Japan.

HABITAT: Occurring as free-living rhodoliths or as attached epilithic plants on pebbles in the intertidal zone.