Sargassum agardhianum is a native species of Sargassum which is irregularly dispersed in dense clumps from Point Dume (LA county), CA to Punta Eugenia, Baja California in the low intertidal zone to upper subtidal in dense clumps (MAC). Very common on Catalina island, rarer on mainland.
Thalli 5-40 cm tall, attached by an irregular, lumpy holdfast, often forming dense strands. Small blades of main axes found in clumps; blades have midribs and serrated margins. Single pneumatocysts (often absent) are small with a terminal spine. Can be confused with non-native, Sargassum muticum, but S. agardhianum is smaller, has denser blades, and has pneumatocysts with a spine.