Sargassum cinereum J. Agardh (1848: 305)

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 : 283

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.

Figures

FIGURE 2. A. Sargassum cinereum J.Agardh.A. Habit of a voucher specimen (NVT2497). B. Holdfast. C. Leaves. D. Vesicle. E. Receptacle. F. Section of male receptacle.