Sargassum siliquosum J. Agardh (1848: 316) (Fig. 6A–F) Thallus up to 60 cm high, attached by discoidal holdfast, 1 cm in diameter. Main axes terete, smooth, 0.15 cm wide, producing 3–5 primary branches. Primary branches cylindrical, smooth, terete 2–3 cm at base. Lateral branches numerous, alternate at an interval of 3–4 cm. Basal leaves thick, large, oval, 2.5–5.0 cm long, 0.3–1.0 cm wide; upper leaves 1.5– 3 cm long and up to 0.6 cm wide, with shallowly dentate margin. Midrip conspicuous but fading towards the apex. Cryptostomata irregularly scattered. Vesicles oval or lanceolate, about 0.3–0.5 cm long and up to 0.2 cm wide, with very short flat petioles, mucronate. Plant monoecious. Receptacles cylindrical, smooth, simple to once furcate, about 2 cm long or more. Maturation in spring in the southwestern Vietnam. Habitat:— Sargassum siliquosum grows on rock and dead coral in the lower intertidal zone. Specimens examined:— Duong Dong, Phu Quoc, lower intertidal, 01 Mar. 2018 V. T. Nguyen, S.M. Boo (NVT2043, NVT2044, NVT2045), Duong Dong, lower intertidal, 16 Feb. 2019, V. T. Nguyen (NVT2510, NVT2511). Taxonomic remarks: — Sargassum siliquosum is distinguished from other species by having numerous small vesicles, secondary axis somewhat compressed and smooth at lower part. The syntype localities of this species are Jakarta in Indonesia and Singapore (Silva et al. 1996). The cylindrical and warty axes described by Nguyen (2007) were not observed in specimens from the Phu Quoc Island. Primary branches are terete, 2–3 cm at base, basal leaves are longer and bigger than those positioned at the lateral branches. Leaves of Vietnamese specimens are shorter than those from China (Tseng & Lu 1992). Vesicles are oval to lanceolate, sharply pointed or rounded, 0.2 cm in length, as Chinese plants (Tseng & Lu 1992). Identification of species from southwestern Vietnam: In the present study, eleven species of Sargassum including five species described above were collected and examined (Table S1). Six species commonly occur along the east coast of Vietnam, and their morphologies and descriptions have been introduced by my thesis and other reports (Pham 1969, Nguyen 2007, Nguyen 2014, Nguyen 2020, Nguyen & Boo 2020). Based on morphological descriptions and illustrations, a key for identification of species from southwestern Vietnam is given below.

Published as part of Tu, Nguyen Van, 2023, Morpho-anatomical study of Sargassum (Fucales, Phaeophyceae) diversity reveals noteworthy collections and range extensions from southwestern Vietnam, pp. 281-290 in Phytotaxa 600 (5) on pages 286-287, DOI: 10.11646/phytotaxa.600.5.3, http://zenodo.org/record/8129374