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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, {"references":["Agardh, J. G. (1848) Species genera et ordines algarum. Seu descriptiones succinctae specierum, generum et ordinum, quibus algarum regnum cnstituitur. Volumen Primum. Algas fucoideas complectense. C. W. K. Gleerup, Lund, 363 pp.","Silva, P. C., Basson, P. W. & Moe, R. L. (1996) Catalogue of the benthic marine algae of the Indian Ocean. University of California Publications in Botany 79: 1 - 1259.","Nguyen, H. D. (2007) Flora of Vietnam. Science and Technology Publishing House, Hanoi, 117 pp.","Tseng, C. K. & Lu Baoren (1992). Studies on the malacocarpic Sargassum of China II. Racemosae J. Agardh. In: Abbott, I. A. (Ed.) Taxonomy of economic seaweeds with reference to some Pacific species. Vol. 3. California Sea Grant College Program, La Jolla, CA. pp. 11 - 34.","Pham, H. H. (1969) Marine algae of south Vietnam. Sai Gon study Center. Ministry of Education and Youth, 558 pp.","Nguyen, V. T. (2014) Seaweed diversity in Vietnam, with an emphasis of the brown algal genus Sargassum. Ph. D. thesis. Ghent University, Ghent, 191 pp.","Nguyen, V. T. & Boo, S. M. (2020) Distribution patterns and biogeography of Sargassum (Fucales, Phaeophyceae) along the coast of Vietnam. Botanica Marina 63: 463 - 468. https: // doi. org / 10.1515 / bot- 2019 - 0082"]}

Key for Sargassum from southwestern Vietnam 1a. Axes with spines.................................................................................................................................................................................2 1b. Axes smooth.......................................................................................................................................................................................3 2a. Holdfasts lobed.................................................................................................................................................................. S. confusum 2b. Holdfasts conical, giving rise to stolon-like branches.................................................................................................... S. polycystum 3a. Axes compressed................................................................................................................................................................................4 3b. Axes cylindrical..................................................................................................................................................................................6 4a. Leaves with entire margin................................................................................................................................................... S. swartzii 4b Leaves with serrate margin.................................................................................................................................................................5 5a. Vesicles with terminal spines; petioles compressed or leaf-shaped; leaves thin........................................................... S. oligocystum 5b. Vesicles on leaves; petioles compressed; leaves thick curved inside branches..................................................................... S. feldmannii 6a. Leaves with entire or sparse spines, undulate....................................................................................................................................7 6b. Leaves with sharply serrated margin..................................................................................................................................................8 7a. Basal leaves blunt at the apex; vesicles ovate................................................................................................................... S. cinereum 7b. Basal leaves thick, larger and oval; vesicles oval or lanceolate...................................................................................... S. siliquosum 8a. Vesicles spherical to obovate..............................................................................................................................................................9 8b. Vesicles leaf-shaped..........................................................................................................................................................................10 9a. Vesicles numerous; leaves lanceolate........................................................................................................................... S. microcystum 9b. Vesicles few; leaves obovate to spathulate................................................................................................................................... S. ilicifolium 10a. Leaves serrated; midribs conspicuous up to ¾ of leaves................................................................................................. S. aquifolium 10b. Leaves spathulate to obovate; midribs inconspicuous..................................................................................................................... S. mcclurei, 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 287-288, DOI: 10.11646/phytotaxa.600.5.3, http://zenodo.org/record/8129374

Sargassum cinereum J. Agardh (1848: 305) (Fig. 2A–F) Thallus 30–50 cm high, attached by a conical holdfast that is slightly discoidal lacking lobes, and 1 cm wide. Main axes cylindrical, up to 1 cm long and 0.2 cm wide. Primary branches cylindrical, smooth, 0.1 cm wide; lateral branches sparsely alternate at intervals of 3–5 cm, 10–20 cm long. Basal leaves 2–4 cm long and 0.5–1 cm wide, blunt at the apex; tapering toward apex; leaves on axes and branches about 4 cm long and 0.3 cm wide, with undulate margin and some spines. Midribs percurrent, occurring on the surface of leaf. Cryptostomata arranged in two rows. Vesicles ovate, 0.2–0.3 cm wide, with petioles ½ shorter than vesicle length. Plant dioecious. Receptacles arise in clusters between leaf axils or vesicles. Male receptacles cylindrical, 2–3 times furcate. About 2 cm long and 0.12 cm wide. Female receptacles triquetrous at the base to compressed, simple to one furcate, about 1 cm long and 0.15 cm wide. Maturation in spring to early summer in the southwestern Vietnam. Habitat:— Sargassum cinereum grows on rocks in the lower intertidal to subtidal zone at two locations on the Phu Quoc Island. Specimens examined:— Duong Dong, Phu Quoc, lower intertidal, 16 Feb. 2019, V. T . Nguyen (NVT2497, NVT2498, NVT2499); An Thoi, Phu Quoc, subtidal, 17 Feb. 2019, V. T . Nguyen (NVT2504, NVT2505, NVT2506, NVT2511). Taxonomic remarks:— Sargassum cinereum is distinguished by ashy-grey(cinerascent) color, dioecious reproduction, leaves having midrib that vanishes from the middle part. The type locality of this species is Hong Kong (Silva et al. 1996). This species was first described in Phu Quoc by Pham (1985). Although it was reported in northern and central Vietnam (Nguyen et al. 1997, Nguyen 2007), its occurrence was doubtful because later sampling trips didn’t collect the species and voucher specimens were absent in any institute and personal herbaria (Nguyen 2014). Specimens from Duong Dong and An Thoi are similar in most of characteristics, but the subtidal specimens from An Thoi is larger than lower intertidal specimens from Duong Dong. Specimens from the Phu Quoc Island are very similar in the shape of leaves with cuneate bases and dentate margins and shapes of receptacles to specimens from Malaysia and Thailand (Ajisaka et al. 1999, Noiraksar & Ajisaka 2008). Two sequences of internal transcribed spacer 2 of nuclear gene from Thailand specimens are released on GenBank, but yet to wait for publication. This study is the first report S. cinereum in Vietnam., 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 page 283, DOI: 10.11646/phytotaxa.600.5.3, http://zenodo.org/record/8129374, {"references":["Agardh, J. G. (1848) Species genera et ordines algarum. Seu descriptiones succinctae specierum, generum et ordinum, quibus algarum regnum cnstituitur. Volumen Primum. Algas fucoideas complectense. C. W. K. Gleerup, Lund, 363 pp.","Silva, P. C., Basson, P. W. & Moe, R. L. (1996) Catalogue of the benthic marine algae of the Indian Ocean. University of California Publications in Botany 79: 1 - 1259.","Pham, H. H. (1985) The plants of Phu Quoc Islands. Ho Chi Minh Publishing House, Hochiminh, 183 pp.","Nguyen, H. D. (1997) Sargassaceae in Vietnam resources and utility. Agriculture Publishing House, Hanoi, 199 pp.","Nguyen, H. D. (2007) Flora of Vietnam. Science and Technology Publishing House, Hanoi, 117 pp.","Nguyen, V. T. (2014) Seaweed diversity in Vietnam, with an emphasis of the brown algal genus Sargassum. Ph. D. thesis. Ghent University, Ghent, 191 pp.","Ajisaka, T., Phang, S. M. & Yoshida, T. (1999) Preliminary report of Sargassum species collected from Malaysian coasts. In: I. A. Abbott (Ed.) Taxonomy of economic seaweeds with reference to some Pacific species. Vol. 7. California Sea Grant College Program, La Jolla, CA. pp. 23 - 41.","Noiraksar, T. & Ajisaka, T. (2008) Taxonomy and distribution of Sargassum (Phaeophyceae) in the Gulf of Thailand. Journal of Applied Phycology 20: 963 - 977. https: // doi. org / 10.1007 / s 10811 - 008 - 9327 - 3"]}

Sargassum confusum C. Agardh (1824: 301) (Fig. 3A–F) Thallus 60–80 cm high, attached by a lobed holdfast, up to 2 cm in diameter. Main axes somewhat compressed, up to 2 cm long, producing 3–5 primary branches. Primary branches cylindrical to slightly compressed, about 0.2 cm, 0.4 cm wide; lateral branches bearing spines, about 15 cm long, alternately branched at an interval of 3–5 cm. Basal leaves up to 7 cm long and 0.5–0.8 cm wide, hard, linear, with entire or sparsely serrated margin. Midrips narrow at the base of leaves and diluted in the middle part of leaves. Cryptostomata sparse. Vesicles numerous, small, spherical or somewhat obovate. Plant dioecious. Male receptacles cylindrical, slightly curved, sometimes with small protuberances, simple to once furcate, about 0.8 cm long and 0.08–0.1 cm wide. Female receptacles triquetrous, simple to once furcate, about 0.3 cm long and 0.1 cm wide. Maturation likely in late spring to early summer in the southwestern Vietnam. Habitat:— Sargassum confusum grows on rocks in the lower intertidal to subtidal zone. Specimens examined:— Duong Dong, Phu Quoc, lower intertidal to subtidal, 01 Mar. 2018, V.T . Nguyen, S.M. Boo (NVT2039, NVT2040, NVT2070, NVT2071, NVT2072), Duong Dong, Phu Quoc, lower intertidal, 05 Mar. 2020, V.T . Nguyen (NVT2500, NVT2532). Taxonomic remarks:— Sargassum confusum is distinguished by having its main branches usually armed with spines, coriaceous leaves in texture, small vesicles of less than 5 mm in diameter. The type locality of this species is Japan (Yoshida 1983). Because S. confusum is similar to S. gracilimum in its dark colour when dried, more rounded vesicles, and cylindrical and longer receptacles, identification of S. confusum needs careful examination of specimens. Yoshida et al. (2002) argued against the occurrence of S. confusum in Vietnam because of the lack of voucher specimens. Cho et al. (2012) analyzed ITS2, cox 3, psa A, and rbc L from S. confusum from Korea and revealed its close relationship with S. fulvellum (Turner) C.Agardh (Agardh 1820: 34), S. miyabei Yendo (Yendo 1907: 112), and S. muticum (Yendo) Fensholt (Fensholt 1955: 306) that are considered natives to northeastern Pacific (Yoshida 1983, Cho et al. 2012). This is the first report on the occurrence of S. confusum in Vietnam., 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 page 284, DOI: 10.11646/phytotaxa.600.5.3, http://zenodo.org/record/8129374, {"references":["Agardh, C. A. (1824) Systema algarum. Literis Berlingianis, Lundae, 312 pp.","Yoshida, T. (1983) Japanese species of Sargassum subgenus Bactrophycus (Phaeophyta, Fucales). Journal of the Faculty of Science, Hokkaido University 13: 99 - 246","Yoshida, T., Dai, N. H., Ajisaka, T. & Noro, T. (2002) Verification of Sargassum species identified with subgenera Bactrophycus, Phyllotrichia, and Schizophycus in Vietnam. In: I. A. Abbott & K. McDermid (Eds.) Taxonomy of economic seaweeds with reference to some Pacific species. Vol. 8. California Sea Grant College Program, La Jolla, CA. pp. 95 - 102.","Cho, S. M., Lee, S. M., Ko, Y. D., Mattio, L. & Boo, S. M. (2012) Molecular systematic reassessment of Sargassum (Fucales, Phaeophyceae) in Korea using four gene regions. Botanica Marina 55: 473 - 484. https: // doi. org / 10.1515 / bot- 2012 - 0109","Agardh, C. A. (1820) Species Algarum, Vol. 1, Part 1, Fucoideae, 168 pp.","Yendo, K. (1907) The Fucaceae of Japan. Journal of College of Science, Imperial University of Tokyo 21: 1 - 174.","Fensholt, D. E. (1955) An emendation of the genus Cystophyllum (Fucales). American Journal of Botany 42: 305 - 322."]}

Tu, Nguyen Van (2023): Morpho-anatomical study of Sargassum (Fucales, Phaeophyceae) diversity reveals noteworthy collections and range extensions from southwestern Vietnam. Phytotaxa 600 (5): 281-290, DOI: 10.11646/phytotaxa.600.5.3, URL: http://dx.doi.org/10.11646/phytotaxa.600.5.3

Sargassum microcystum J. Agardh (1848: 323) (Fig. 5A–F) Thallus 30–80 cm high or more, attached by a discoidal to conical shaped holdfast, warty, up to 1 cm in diameter. Main axes cylindrical, up to 0.4 cm long, producing 3–5 primary branches. Primary branches cylindrical or somewhat compressed at lower part, smooth, 0.15–0.2 cm in diameter. Lateral branches coarse. Leaves thick, crisp, lanceolate, with short petioles, obtuse to round at the apex, with irregularly denticulate margin. Midrib inconspicuous, vanishing to the apex, 2–4 cm long and up to 0.4–1 cm wide. Cryptostomata conspicuous, scattered over leaf surface. Vesicles numerous, 0.15–0.2 cm long, spherical to obovate, bearing a fine spine or dentate wing. Plant dioecious. Receptacles in racemose branchlets. Male receptacles cylindrical, about 0.5 cm long and 0.1 mm wide. Females receptacles triquetrous, about 0.35 cm long and 0.15 cm wide. Maturation in spring in the southwestern Vietnam. Habitat:— Sargassum microcystum grows on rock and dead coral in the lower intertidal to subtidal zone. Specimens examined:— Ganh Dau, Phu Quoc, subtidal, 15 Feb. 2019, V. T . Nguyen (NVT2468, NVT2469, NVT2471, NVT2492, NVT2493), Duong Dong, lower intertidal, 16 Feb. 2019, V. T . Nguyen (NVT2517). Taxonomic remarks:— Sargassum microcystum is distinguished from other species by having numerous small vesicles without rhizoids and secondary axes that are smooth and somewhat compressed on the lower parts. The syntype localities are “marfi Indico”, Moluccas in Indonesia, Manila in the Philippines, and Singapore (Silva et al. 1996: 685). Yendo (1907) described leaves of S. microcystum as being narrow; however, specimens from Phu Quoc have typical lanceolate leaves, and basal leaves are ovate and longer than upper ones, as described by Pham (1967). Cryptostomata are conspicuous, a feature not described by Pham (1967)., 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 page 286, DOI: 10.11646/phytotaxa.600.5.3, http://zenodo.org/record/8129374, {"references":["Agardh, J. G. (1848) Species genera et ordines algarum. Seu descriptiones succinctae specierum, generum et ordinum, quibus algarum regnum cnstituitur. Volumen Primum. Algas fucoideas complectense. C. W. K. Gleerup, Lund, 363 pp.","Silva, P. C., Basson, P. W. & Moe, R. L. (1996) Catalogue of the benthic marine algae of the Indian Ocean. University of California Publications in Botany 79: 1 - 1259.","Yendo, K. (1907) The Fucaceae of Japan. Journal of College of Science, Imperial University of Tokyo 21: 1 - 174.","Pham, H. H. (1967) Contribution a lʹetude des algues littorales du Vietnam: le genre Sargassum. Annales de la Faculte des Sciences de Saigon 1967: 259 - 332."]}

Sargassum feldmannii Pham-Hoàng (1967: 297) (Fig. 4A–F) Thallus 30–40 cm high, attached by a discoidal holdfast that is up to 1 cm in diameter. Main axes cylindrical, short, up to 0.3 cm wide, producing 2–3 primary branches. Primary branches compressed, smooth, 0.3–0.4 cm in diameter; lateral branchs short, raising at an interval of 1–3 cm. Leaves thick, elliptic to linear, 2–3 cm long and up to 1 cm wide, obtuse or rounded at the apex, sometimes double serrated or forming a cup–shape, with short cylindrical petioles, bearing some spines at top. Midribs vanishing near the apex. Cryptostomata numerous, irregularly scattered over leaf surfaces. Vesicles on leaves, spherical to elliptic, normally arranged at the central or distal part of the vesicle–leaves, with sharply dentate margins, 0.6–0.8 cm wide. Plant monoecious. Receptacles forming dense cymes, branched, cylindrical and warty, up to 0.7 cm long. Maturation likely in late spring in the southwestern Vietnam. Habitat:— Sargassum feldmannii grows on rocks and dead corals in the subtidal zone in the Phu Quoc and Tho Chu islands. Specimens examined:— Ganh Dau, Phu Quoc, subtidal, 15 Feb. 2019, V. T . Nguyen (NVT2461, NVT2462), Bai Dong, Tho Chu, subtidal, 21–23 Mar. 2019, V. T . Nguyen (NVT2563, NVT2564, NVT2634, NVT2667, NVT2669, NVT2670). Taxonomic remarks:— Sargassum feldmannii is distinguished from other species by having compressed branches, vesicles with terminal leaves that are thick, inwardly curved branches, and serrated margins of leaves. The type locality of this species is Nha Trang, Vietnam (Pham 1967). It differs from S. aquifolium (Turner) C.Agardh (Agardh 1820: 12) by its large leaf size, and the shape of its vesicles (Nguyen 2014). Sargassum feldmannii has short lateral branches, usually curved, thick leaves to stem compared to S. carpophyllum J.Agardh (Agardh 1848: 304). Based on morpho-anatomical examination of specimens from Phu Quoc Island, S. feldmannii is distinct from S. crassifolium J.Agardh (Agardh 1848: 326), a species currently treated as a synonym of S. aquifolium (type locality, Sunda Strait, Indonesia, Silva et al. 1996)., 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 page 285, DOI: 10.11646/phytotaxa.600.5.3, http://zenodo.org/record/8129374, {"references":["Pham, H. H. (1967) Contribution a lʹetude des algues littorales du Vietnam: le genre Sargassum. Annales de la Faculte des Sciences de Saigon 1967: 259 - 332.","Agardh, C. A. (1820) Species Algarum, Vol. 1, Part 1, Fucoideae, 168 pp.","Nguyen, V. T. (2014) Seaweed diversity in Vietnam, with an emphasis of the brown algal genus Sargassum. Ph. D. thesis. Ghent University, Ghent, 191 pp.","Agardh, J. G. (1848) Species genera et ordines algarum. Seu descriptiones succinctae specierum, generum et ordinum, quibus algarum regnum cnstituitur. Volumen Primum. Algas fucoideas complectense. C. W. K. Gleerup, Lund, 363 pp.","Silva, P. C., Basson, P. W. & Moe, R. L. (1996) Catalogue of the benthic marine algae of the Indian Ocean. University of California Publications in Botany 79: 1 - 1259."]}