E-Flora BC: Electronic Atlas of the Flora of British Columbia

Family Description:
Members of this family have bodies that are either tubular and one cell thick or bladelike and two cell layers thick. Most cells contain a single nucleus and a single chloroplast. The chloroplast is flattened to cuplike in shape, and has one to several pyrenoids.
Typically, the alternation of generations shown by members of this family is isomorphic, with sporophytic generations similar to gametophytic generations. When asexual spores are formed, they are motile and bear four flagella. Gametes are biflagellate.

Species description:
This species has a small, cushionlike holdfast that attaches to stones in protected bays; it is abundant in Southeast Alaska. It is bright green in early spring (April) but darkens with age, particularly when removed from the water and dried. Initially, the blade forms a hollow sac but it soon splits open to form a large sheet up to 25 cm (10 in) high and 18 cm (about 7.5 in) broad and sometimes larger. Although only one cell thick (like Monostroma), it is nearly opaque. No newspaper reading through this species!

Dark Sea Lettuce has very high caloric value at 4.56 Calories per gram dry weight. It is eaten by sea urchins.