Description: Thalli comprising narrow, uniseriate, unbranched, yellowish-green filaments growing on mangroves or in shaded brackish waters. Non-septate rhizoids develop frequently; thalli fragment at the rhizoidal cells. Thalli up to 5 mm long. Cells 20.7–29.3 μm in diameter, 16.5–58.0 μm long, with an L/D ratio of 0.7–2.4. Each cell contains 1–2 nuclei. Spindle- or band-shaped chloroplasts are densely arranged peripherally in the cytoplasm. Each chloroplast contains 3–12 pyrenoids. Pyrenoids polypyramidal, each covered with 2–8 starch plates; 1–4 thylakoid membranes traverse each pyrenoid. Nuclear-encoded SSU rRNA gene sequence: LC042103, nuclear-encoded LSU rRNA gene partial sequence: LC042096.

Holotype: SAP114678, collected by K. Miyaji on Oct 14, 2012, deposited in the Herbarium of the Faculty of Science at Hokkaido University, Sapporo.

Type locality: Tanotsu, Fukuoka, Fukuoka Pref., Japan (33°37′N, 130°26′E).

Etymology: From the Latin adjective “umbraticus,” meaning “in the shade,” in reference to shady growing environment.