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

Figures 13–30

DESCRIPTION: Thalli were non-geniculate, warty to fruticose and forming entirely free-living rhodoliths with spheroidal to sub-spheroidal shapes, c. 2 cm wide. In fruticose specimens, branches (protuberances) were up to 7 mm long, apically enlarged (1.5–7.8 mm wide), and dichotomously branched or anastomosing. Colours of living thalli were purplish red to rose pink (Fig. 13). Thallus construction was monomerous with a multistratose non-coaxial (plumose) hypothallus composed of filaments running parallel to the substratum (Fig. 14). Cells of perithallial filaments arose from hypothallial filaments and were composed of square to mostly elongate cells (Fig. 14). Cells of contiguous hypothallial and perithallial filaments were frequently joined by cell fusions (Fig. 15); secondary pit connections were not observed. Subepithallial initials (intercalary meristematic cells) were composed of square to elongate cells, shorter than, approximately equal to or longer than their immediate inward derivatives. A single layer of epithallial cells was flattened with flared outer cell walls (trapezoidal; Fig. 15). Trichocytes were not observed. Tetra/bisporangial conceptacles were multiporate, raised above the thallus surface without differentiation into a peripheral rim and pore plate (Figs 16–18). Tetra/bisporangial conceptacles at early developmental stages were flush with or raised above the surrounding thallus surface (Fig. 18) and became overgrown by marginal growth of surrounding vegetative cells when older (Figs 19–22). Each pore opening was surrounded by 4–7 rosette cells in depressions giving a pitted appearance in surface view (Figs 17, 23). 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 2–4 celled filaments (Fig. 24). Pore canal filaments include rosette cells with disintegrated roofs and the underlying elongate and/or wedge-shaped cells. Pore canal cells were generally longer than other roof cells (Fig. 24). Tetrasporangia were zonately divided and developed across the entire chamber floor (Fig. 25). Conceptacles appeared to be buried with infilled vegetative cells (Figs 26, 27), but also appeared to be shed. Carpogonial (female) conceptacles were uniporate (Fig. 28) with elliptical chambers (Fig. 29). Carpogonial branches developed across the entire chamber floor of mature conceptacles and were composed of a single supporting cell, a hypogynous cell, and a carpogonium and elongated trichogyne (Fig. 30). Spermatangial (male) and carposporangial thalli were not observed. Data on vegetative and reproductive features are summarized in Table 1.

HOLOTYPE: SAP 115691 (tetrasporophyte), collected on 21 April 2023 by Min-Khant-Kyaw & A. Kato, deposited in the Herbarium of the Graduate School of Science (SAP), Hokkaido University, Sapporo, Japan (Fig. 13).

HOLOTYPE GENBANK ACCESSION NUMBERS: psb A, LC822418; rbc L-3P, LC822439; COI-5P, LC822449.

TYPE LOCALITY: 32°37.46'N, 130°15.06'E; intertidal; Shirasu, Minami-Arima, Minami-Shimabara City, Nagasaki Prefecture, Japan.

ETYMOLOGY: ‘ sabulosum ’ in reference to the sandy habitat of the type locality.

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

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

HABITAT: Occurring as free-living rhodoliths in the intertidal zone.