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:
A very dark, chocolate brown alga, Sea Cabbage grows on mid to low intertidal rocks in outer coastal waters where there is considerable exposure to surf, although it becomes dominant mostly in areas of moderate wave exposure. In protected areas on Amchitka Island in the Aleutians, it grows best in the mid intertidal zone and is excluded from lower areas on the shore by the growth of other kelps. In the most exposed locations, it is usually outcompeted by species of Laminaria. The sporophyte is perennial and probably lives two to three years.

A richly branched holdfast is usually attached directly to the base of the blade with no stipe separating the two as in other kelps. The blade is thick, but rather fragile, and becomes torn with age; it can reach a length of 150 cm (5 feet) and a width of 80 cm (32 in) under favorable conditions, but is usually much smaller. Individuals growing at exposed sites tend to have narrower, flatter blades than those growing at more protected sites.

In some parts of its range, Sea Cabbage forms an extensive canopy beneath which other species of algae (such as corallines and other red algae) can thrive and grow, protected from desiccation. In Washington State, it forms extensive beds in the presence of grazers such as sea urchins, because they do not eat it. Where grazers are absent, however, other kelps outcompete and replace Sea Cabbage. Because it is a perennial, established beds can persist for years until a disturbance opens up space for its competitors. In more exposed areas, most individuals can be removed by winter storms during most years.

The Black Katy chiton (Katharina tunicata) feeds heavily on Sea Cabbage in British Columbia and Washington State, prefering it strongly to other brown algae. There, this chiton chews on the holdfasts of young adult kelps, weakening them so that they are more easily lost during storms. Sea Cabbage has rather low caloric value (2.85 Calories per gram of dry weight).

At Botanical Beach on the west coast of Vancouver Island, British Columbia, Sea Cabbage grows maximally from March to August and becomes fertile in late fall and winter.

Laboratory experiments have shown that individuals from southern British Columbia/northern Washington can withstand water temperatures only up to 15°C (59°F), making Sea Cabbage among the most thermally sensitive of any local algae.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Hedophyllum sessile