Sargassum microcystum J. Agardh (1848: 323)

Tu, Nguyen Van, 2023, Morpho-anatomical study of Sargassum (Fucales, Phaeophyceae) diversity reveals noteworthy collections and range extensions from southwestern Vietnam, Phytotaxa 600 (5), pp. 281-290 : 286

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).

Figures

FIGURE 5. Sargassum microcystum J.Agardh.A. Habit of a voucher specimen (NVT2468).B. Holdfast.C.Leaves.D.Vesicles.E. Receptacles. F. Section of female receptacle.