KEY TO THE CONFIRMED SPECIES OF SARGASSUM C. AGARDH IN NORTHWESTERN LUZON, PHILIPPINES

1. Thallus typically with stoloniferous holdfasts; branches terete, usually with numerous branched protuberances; vesicles small, numerous, spherical to ovoid ................................................... Sargassum polycystum C.Agardh

— Thallus without stoloniferous holdfasts; branches smooth, terete, slightly compressed, to compressed; vesicles large, globose to obovoid, ribbed, sometimes mucronated ........................................................................... 2

2. Branches smooth and compressed; blades linear-lanceolate to linear oblanceolate, distinctly serrate-dentate; vesicles large, ovoid to obovoid, to 7 mm long ............................... Sargassum aquifolium (Turner) C.Agardh

— Branches smooth, terete to slightly compressed; blades highly variable in shape, margins serrate to biserrate, ‘duplicated’ especially towards the apices, duplication sometimes appearing turbinarioid; vesicles globose, to 5.4 mm in diameter ......................................................................... Sargassum ilicifolium (Turner) C.Agardh