Family Description:
Members of this family are simple, bifurcate, dichotomously branched, or proliferous blades or sacs. The medulla is composed of large, globose cells (no filaments). Reproduction is as described for the order.

Species description:
One of our most easily recognized species, Sea Sac is an annual that grows in clumps on rocks or on other algae such as Graceful Coral Seaweed (Corallina vancouveriensis) and Black Pine (Neorhodomela larix). It can form rather isolated patches (in more protected waters) or extensive mats, usually intermixed with other species such as Sea Cauliflower, Leathesia difformis (in exposed, outer coastal areas). Each individual is an almost perfect ellipsoid up to about 15 cm (6 in) high and about 2 cm (0.8 in) wide. Young Sea Sacs are thin-walled and bright purplish red, but as they increase in size they become progressively more bleached to pale yellow, especially at the tips. The outer surfaces of the sacs are smooth and shiny and dotted with 5 to 15 tiny pores mainly on the upper third of each sac, but you will need a strong field lens to see them. These pores admit sea water to the sacs, which are filled to capacity except often for a bubble of oxygen at the tip, formed by rapid photosynthesis. The bubble of gas within the sac helps hold it erect when immersed, and the internal water helps keep the sac cool (because water requires relatively a lot of heating to raise its temperature) and moist during low tide (because water can seep out of the pores when the sacs are emersed, although air cannot then enter). Three hours of emersion is enough to kill sacs experimentally emptied of their water, but filled sacs survive. When reimmersed, the sacs reinflate to their ellipsoidal form due to the elasticity of their walls, with sea water re-entering the pores. Basally, each sac tapers to a short, weak stipe that is anchored to the rock with a small, discoidal holdfast. The stipe can be weak and still not break because the sacs have such a superb hydrodynamic shape, and drag on the sacs by water currents is very low. These individuals are either male gametophytes or tetrasporophytes since females are microscopic. Sea Sacs appear in early spring and then degenerate after spores are released in the fall.

Studies performed in Washington State have shown that Sea Sac spores have a strong preference for settling on rough surfaces (up to 98% of spores settled on the fine tips of projecting substrata rather than in the valleys between tips).
On the west coast of Vancouver Island, British Columbia, workers found that photosynthesis in sacs exposed by low tide was only half that of sacs immersed in sea water. The uptake of carbon dioxide from water held within the sac was almost 70% as much as that taken up from the air and about 30% of that taken up from sea water outside the sacs, so the researchers concluded that water held inside the sacs is a significant source of carbon dioxide for the alga.

Individuals collected from southern British Columbia/northern Washington State withstood a week of immersion in water at a temperature of 18°C (64°F) but died when placed in warmer water.
Sea Sacs are an often ignored edible. They can be eaten raw or added to soups. In nature, they are eaten by limpets, which benefit from their high caloric value (4.12 Calories per gram of dry weight).

Gammarid amphipods sometimes chew a hole near the base of a Sea Sac, then enter the watery interior and use the sac as a protective residence.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Halosaccion americanum
Ulva glandiformis