Family Description:
Members of this family of green algae also have coenocytic bodies, and, depending on the species, these can have either erect, crowded branches or be encrusting. The numerous, discoidal chloroplasts are tiny and lack pyrenoids. There is no asexual reproduction (that is, no zoospores are formed); meiosis is gametic. Gametes are formed by meiosis in gametangia on the diploid thallus; they are biflagellate and of two sizes.

Species description:
The holdfast of the of Sea Staghorn is a small cushion supporting erect branches to 30 cm (12 in) tall. The cylindrical branches, which are 3-8 mm (0.1 to 0.3 in) wide, are repeatedly branched dichotomously (that is, at each fork, they divide into 2 equal branches). The entire thallus is rather spongy in texture, very dark, almost blackish-green, in color, and usually has a rather unpleasant, pungent odor.

This is our best example of a siphonous alga; that is, the entire organism is formed of interwoven and confluent filaments that are in fact a single multinucleate cell. Like Rockweed (Fucus gardneri), Codium is characterized by gametic meiosis, hence there is no haploid phase in the life cycle. In the closely related Codium fragile subsp. tomentosoides in the Gulf of Maine, both male and female gametes are produced in the same sex organ. Female gametes are twice the diameter of male gametes and outnumber them 6:1. In our subspecies (Codium fragile subsp. fragile), female and male gametangia occur on separate plants.

Codium fragile subsp. tomentosoides was carelessly introduced to the east coast of the United States about 1957 and has spread rapidly since. It is a costly pest to shellfish farmers because it settles and grows on mussels, scallops and oysters, increasing their drag in the currents. During storms, the burdened shellfish are torn from their substratum and perish.

Sea Staghorn is typically a species of the outer coast . It is eaten by the Branched Placida (Placida dendritica), a tiny and specialized ascoglossan sea slug, although it is low in caloric value (2.62 Calories per gram of dry weight). Unlike some other ascoglossans that feed on Sea Staghorn, the Branched Placida digests the chloroplasts of the alga. Recently, it has been shown that Branched Placida on Sea Staghorn are smaller than those feeding on the related Bryopsis corticulans, and that the individuals from Sea Staghorn had more numerous, shorter cerata on wider bodies than did ones from Bryopsis. When the Branched Placida was experimentally transplanted, its outward appearance changed in response to the new diet.

In Oregon, workers showed that the growth of Sea Staghorn is limited by short supplies of nitrogen during at least part of the year. Due to its growth form, Codium is unable to take up transient high concentrations of ammonium and respond by rapid growth. It can grow fast at low light levels, however, and is a superior competitor in the low intertidal and subtidal areas.

In New York, Codium fragile subsp. tomentosoides has been shown to harbor cyanobacteria ("blue-green algae") among its filaments. When conditions are favorable for the growth of the cyanobacteria, a small amount of the nitrogen in the Codium comes from the process of nitrogen fixation occuring inside the cyanobacteria. Elsewhere in the northeastern United States, a species of the nitrogen-fixing bacterium Azotobacter also occurs with Codium fragile and fixes nitrogen, but in this case the bacterium occurs on the outer surface of the alga, not internally among the filaments.

Species of filamentous red algae in the genus Ceramium grow on the branches of Sea Staghorn.

Source:  North Pacific Seaweeds