The following two taxa apparently lie within the circumscription of Sargassum polycystum but have not yet been transferred or reduced to synonymy:
Sargassum pergracile Greville, 1849 [1848–1849]: 108, 109, pl. IV: lower right figs. 5, 6, nom. inval. (type locality: India).—Greville, 1850: 94–95, pl. IX: lower right figs. 5, 6.
Sargassum brevifolium Greville var.? pergracile Grunow, 1915 [1915–1916]: 442–443 (`pergracilis').—K. Srinivasan, 1965: 243.— K. Srinivasan, 1967: 137, fig. 24.— V. Krishnamurthy & H. Joshi, 1970: 14.
INDIAN OCEAN DISTRIBUTION: India, Indonesia (Sunda Strait), Singapore, Sri Lanka.
Note: Sargassum pergracile is an invalid provisional name (see Art. 34.1(b)). Greville wrote, "Should it [Sargassum brevifolium var. b] prove distinct, it may bear the name of Sargassum pergracile''. Sargassum brevifolium var. pergracile, although based on S. pergracile Greville, is valid because Grunow accepted it.