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 minute, dark red seaweed lives on other algae, so you will find it if you search such places carefully. In order to see its diagnostic features, however, you will most surely need to use a microscope.
Several main axes are up to 2 cm (0.8 in) tall, and are anchored to the host seaweed via a prostrate branch at the base. The lower cells of each axis are two to three times as tall as wide. Each cell of the main axis bears two opposite branchlets near the upper end of the cell. Lateral branches replace branchlets where they occur (and, a lateral branch does not have a branchlet opposite it on the same axial cell). The basal cell of each branchlet is distinctly smaller than the other cells, and this makes a good diagnostic feature for identifying this species easily. The ultimate branchlets occur along only the upper sides of the supporting branchlets and are bent towards the branchlet tips.

Tetrasporangia are supported by mostly one-celled stalks (that is, they are not sessile) and occur on the upper side near the base of each branchlet.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Antithamnion pygmaeum