Blades are undivided; they have a central midrib or thickening, and are borne on a simple or dichotomously branched stipe. Sori occur on sporophylls, which develop in opposite rows along the stipe near the intercalary meristem at the junction of stipe and blade. As currently constituted, the family contains only three genera: Alaria, Pterygophora, and Lessoniopsis. The last two genera are monotypic; that is, they consist of only a single species each.
This is one of our most abundant algae, both on the outer coast and in more protected waters with significant tidal currents. It almost invariably grows on rocks. As in other kelps, the sporophyte is the conspicuous generation.
The holdfast is well developed (although somewhat smaller than that of other kelps) and well branched, with many rootlike haptera providing a sturdy attachment to rocks. The holdfast gives rise to a single cylindrical stipe, about 5 mm (0.2 in) in diameter and to 7 cm (almost 3 in) long. From this stipe arise first a single, terminal vegetative blade and, soon, up to 40 lateral sporophylls. The vegetative blade is rather lanceolate and can reach 3 meters (about 10 feet) long and 20 cm (almost 8 in) wide. Running up the center of the vegetative blade is a single thick, solid midrib about 12 mm (about 0.5 in) wide; the rest of the blade is thin and hangs limply when not supported in the water. The 20 to 40 specialized sporophylls start forming in spring; they arise laterally in two rows along the upper part of the stipe below the vegetative blade. Initially, they are only slightly thicker than the vegetative blade, but become noticeably thicker at maturity. They measure up to 25 cm (almost 10 in) long at maturity and are elliptical in shape with broadly rounded tips. In late spring and summer the vegetative blade and sporophylls erode, leaving the holdfast and stipe, which can overwinter but rarely resprout the following spring. Ribbon Kelp, therefore, is considered to be an annual.
Individuals from southern British Columbia/northern Washington can survive for a week in water at 18°C (64°F) but die at higher temperatures.
Ribbon Kelp is an excellent edible for human use. The midrib can be chopped into bite-sized pieces and added to salads or oriental dishes, to spaghetti sauce, or even deep fried to make a substitute for potato chips. This kelp is a good source of vitamins B6 and K, iodine and potassium, and is over 6% protein when harvested in good condition. Lou Barr of Auke Bay, Alaska, has seen Sitka Black-tailed Deer eat Ribbon Kelp when they are forced down into intertidal areas to feed during times of extremely heavy snowfall. How well they digest it is unknown. It has moderate caloric value (3.31 Calories per gram of dry weight).
Interestingly, the sporophylls contain various natural chemicals that discourage herbivores although the vegetative blade does not. Herbivores therefore tend to eat only the vegetative blade, leaving the reproductive potential of this alga intact.At Yakutat Bay, Alaska, (and perhaps elsewhere in our area), low intertidal Ribbon Kelps can contain the bodies of an associated brown alga, Streblonema pacificum. The S. pacificum filaments penetrate into the sporophylls of Ribbon Kelp and form circular dark brown patches.
Source: North Pacific Seaweeds