Ulva meridionalis R.Horimoto & S.Shimada in Horimoto et al. (2011: 161)

Material examined:— SINGAPORE. Pulau Ubin: Chek Jawa, 17 July 2022, NYF220716, NYF220717, NYF220718, NYF220719, NYF229720, NYF220722, NYF220725 (SING); East Coast: East Coast Park Carpark G, 18 July 2022, NYF220742 (SING); Sentosa: Tanjong Rimau, 23 August 2022, NYF220805, NYF220807 (SING); Pulau Hantu, 14 August 2022, NYF220832 (SING).

Description:—Thallus in the shape of tubular filaments, smooth or wrinkled (Figs. 4C–F, 4H). Thallus width decreases towards the base (Fig. 4G). Second-order branching restricted to the thallus base. Some thalli have one to multiple transverse constrictions (Fig. 4I). Thallus filaments are 0.6–19.5 cm in length; apex/median of filament are 3–100 mm in width (n = 6); base of filament are 60–472 μm in width (n = 6). Surface cells are mostly cuboidal, polygonal, or irregular. Cells in surface view are 6–15 μm in length and 5–11 μm in width (n = 20). In surface view, chloroplast covers the periphery of cell with 1 to 3 pyrenoids (Fig. 4J). In transverse sections, thallus 28–33 μm in width (n = 5), cell shape is rectangular, 20–23 μm in length and 13–17 μm in width (n = 6), with parietal chloroplast (Fig. 4L).

Distribution in Singapore:—Chek Jawa, East Coast Park Carpark G, Tanjong Rimau, and Pulau Hantu.

Habitat:—Anchors onto sand, in between pebbles, or on rocks (Figs. 4A–B).

Remarks:—New species record for Singapore. Ulva meridionalis has similar gross morphology to the previously recorded U. compressa, U. flexuosa and U. intestinalis due to its filamentous thalli. There was generally strong support for the Ulva meridionalis clade in the tuf A, ITS and rbc L trees (Fig. 2; Supplementary Files 3, 4). The Vietnamese Ulva spp. 3 and 4 (Tran et al. 2023) are closely related to U. meridionalis. However, they can be distinguished from U. meridionalis because there are no pyrenoids in Ulva sp. 3 and 3–6 pyrenoids in Ulva sp. 4 (Tran et al. 2023).