Family Description:
Members of this family have a single, terminal blade that is divided in some species. The stipe is rarely branched. Blades are smooth or undulate, entire or with holes, of uniform thickness or with a central, thickened region (midrib). Sori develop directly on the blades.

Species description:
Unlike most other laminarians, Three-ribbed Kelp fastens to rocks with an unbranched, discoidal holdfast. It occurs more commonly on cobbles than on either bedrock or boulders, often with several individuals growing together. From the holdfast, a stipe of up to 25 cm (about 10 in) leads to a single undivided and linear blade up to 4 m (13 feet) long and 18 cm (7 in) wide. The entire plant is light yellowish-brown (or sometimes reddish-brown) in color, much lighter than most other kelps. Three shallow folds or longitudinal wrinkles extend down the center of the blade, hence the common name of this species, and no other local alga has this distinctive feature. These folds become more pronounced as the plant ages and can be nearly 2.5 cm (1 in) deep in mature individuals.

This kelp has an unusual sweet smell rather like cut cucumber. If your sense of smell is good, you can detect this odor while still some distance away from the alga, but if you are in doubt then tear the blade in half and sniff the broken central folds where the aroma should be unmistakable.

We have seen this kelp in both inshore and outer coastal waters. This is an annual kelp with a fairly restricted seasonal distribution; it reaches its peak of development in May in the Juneau, Alaska, area and by late June appears tattered and falling apart. At more favorable sites, mature individuals can become quite thick and leathery with deep folds. Laboratory experiments have shown that individuals from southern British Columbia/northern Washington State have an upper temperature tolerance between 15° and 18°C (59° to 64°F), making this species among the least thermally tolerant of any local algae.

Source:  North Pacific Seaweeds