Desmarestia aculeata (Linnaeus) J.V.Lamouroux
Thallus cylindrical, bushy, dark olive brown, to 1 m long. Frond rigid, much branched, branching irregularly alternate. All axes and branches flattened, tough and opaque. In spring and summer branchlets are soft to touch, yellow brown in colour, clothed in yellow green tufts of deciduous hairs (above); at other times the most obvious feature is the alternately arranged sharply pointed spines. A short basal stalk arises from a discoid holdfast. Many discoid chloroplasts per cell. Unilocular sporangia developed from and immersed in the cortical cells; plurilocular sporangia unknown on macrothallus.
Habitat: Infralittoral; epilithic.