Family Description:
In this family, the thallus has a single axis from which arise pinnate or irregularly arranged branches that are not compact or spongy. The sporophytic phase, which is filamentous, forms multiflagellate zoospores, usually in globular to ovoid zoosporangia separated from the rest of the multinucleate thallus by a wall. The zoospores have a characteristic crown of flagella. They germinate into the alternate, gametophytic phase, which can be spherical or filamentous. This phase produces biflagellate gametes of two sizes on separate "female" and "male" thalli.

Species description:
The sporophytic phase of Green Sea Felt forms filamentous tufts up to just 1.5 cm (0.6 in) high that grow on exposed shores on bare rocks, on encrusting coralline algae, or on the Giant Pacific Chiton (Cryptochiton stelleri). Within the tufts, basal, horizontal branches support numerous erect branches 50 to 65 µm in diameter; the erect branches are not organized along a central axis, and branch frequently. Chloroplasts within the branches lack pyrenoids. Stalked zoosporangia form laterally on the upright branches and are ovoid and up to 0.2 mm in length. Each zoosporangium will ultimately form 20 to 30 zoospores each with a crown of flagella; the flagella are located at the front end of each zoospore, and their beating pulls this asexual cell through the water in search of a place to settle and commence growth to form a gametophyte.

The gametophytic phase is spherical and attaches to encrusting coralline algae via a short stalk. It is usually up to 10 mm (0.4 in) high. Seaweed biologists originally thought that this phase of Green Sea Felt was an entirely different kind of alga, and named it Halicystis ovalis, but we now know that this is indeed the gametophyte of Green Sea Felt. Within this spherical structure, there are many peripherally located chloroplasts and nuclei. Areas where female gametes are forming are recognizable as a dark green band. Male areas are yellowish tan and occur on separate plants. The region destined to become the gametangium is separated from the remaining protoplasm by a cell membrane. The contents of this region divide up to form a mass of either female (larger, darker) or male (smaller, paler) gametes, each with two flagella. The gametes are released explosively from this gametangium.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Derbesia pacifica
Halicystis ovalis
Vaucheria marina