Sargassum confusum C. Agardh. A. Habit 1824

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/812937