Family Description:
These are all very finely branched red seaweeds. Plants are often uniseriate (monosiphonous) filaments, which are corticated by small cells or rhizoids in some species. These filaments are the main axis of the plant. Branching is opposite, alternate, or verticillate (with branches arranged in whorls around the axis). Many branches have determinate growth, and are usually called branchlets, but a few continue to grow to produce new axes like the one bearing them (these continue to be called branches). Tetrasporangia are cruciately or tetrahedrally divided. In some cases, bisporangia (sporangia containing two spores) or polysporangia (sporangia containing many spores) are borne in place of tetrasporangia. Cystocarps are naked or at most surrounded by involucral filaments. Most plants in this family are quite small and require a microscope or very good hand lens to see these diagnostic features.

Species description:
This diminutive red seaweed, at 2.5 cm (1 in) tall, is scarcely larger than Dwarf Skein. Its really distinctive feature is that it is alternately branched througout its two or three orders of branching. Also, all branches are uncorticated (lack a "bark").

The lower cells of the main axis are only slightly longer than wide. The blunt-tipped branchlets are up to twenty cells long and carry the rather crowded ultimate branchlets alternately along two sides.

The sex organs also occur along both sides of the branchlets. The spermatangia are somewhat more elongated than in other closely related species, while the sporangia contain many spores (that is, they are polysporangia). Alternate Skein grows epiphytically on other species of algae.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Callithamnion vancouverianum