Five-ribbed Kelp (Costaria costata), photographed along the Mendocino coast on 20 August 2012.
The blades can be up 1.5-2 meters (5-6.5 ft) long and 35 cm (1 ft) wide. Note the strong longitudinal ribs running parallel to the edges of the blade. Three of the ribs project on one side of the blade, and two of the ribs project on the opposite side.
The stipe is somewhat compressed with prominent grooves (see below).
The holdfast is made up of many narrow, branched haptera (root-like structures). This holdfast was attached to coralline algae.
The areas in between the ribs are strongly puckered (see next photo). In algal terminology, this is called bullate = a local outward bulging, blistering, or puckering of the surface (definition from Marine Algae of California by Abbott and Hollenberg).
An interesting puzzle: Although Costaria costata can be found from Alaska to southern California, and I've seen it both north and south of Bodega Head, I have yet to find it on Bodega Head itself. Have you? If so, I'd love to hear about it!