Lomentaria clavellosa is a red (dark-brown to purplish red) alga, which is usually 100-200 mm tall (occasionallyl 400 mm tall). The growth form varies with wave exposure. Plants from an exposed site can grow in a symmetrical pyramid-shape- from a disk-like holdfast, with multiple symmetrical branching and re-branching. Plants from a sheltered site had dense but irregular branching, but still tapered near the tips. The tips are club-shaped. This seaweed grows a an epiphytes on other algae in sheltered low-intertidal and shallow subtidal areas. This alga is native from Norway and Iceland to the Mediterranean and Black Seas. We consider it cryptogenic in Madeira, the Azores, and Morocco. On the East Coast of North America, it was first collected in 1963 in Boston Harbor, and is now established rom Casco Bay, Maine, to Long Island Sound. This seaweed has also been introduced to Brazil and Argentina. Likely vectors include ballast water and hull fouling..