Phylum: Ochrophyta   Family: Desmarestiaceae

Desmarestia aculeata  (Linnaeus) J.V. Lamouroux, 1813

Desmarest's Prickly Weed; Landlady's Wig

The thallus is round, over a metre in length. The branching is irregular and flattened. Found growing on rocks in the lower intertidal zone

References: 
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland