Family Description:
Most members of this family have an alternation of heteromorphic generations in which the gametophyte is the upright phase we recognize, and the tetrasporophyte is a crust. The cruciately divided tetrasporangia occur in chains, often in nemathecia (bands of sori). Gametophytes are cylindrical or flattened, and branched or bladelike. Most gametophytes have a medulla made up of large, colorless cells. Mastocarpus is the exception; it is sometimes placed in its own family, the Petrocelidaceae.
At one time, the genus Gigartina contained both species with isomorphic life phases and species with heteromorphic life phases like Turkish Washcloth. Later, botanists decided that the heteromorphic species were different enough to warrant their own genus, so they took the heteromorphic species out of Gigartina and resurrected the genus Mastocarpus for them. Turkish Washcloth is thus now a species of Mastocarpus, and all but a handful of the other species have been placed in the genus Chondracanthus.

Species description:
The gametophytes of Loose Ahnfelt's Seaweed are so openly branched that they remind us of shrubs that a gardener has neglected to prune properly. The rather stiff branches are cylindrical, about 1 mm in diameter, and arise from a branch that creeps horizontally along the rocks.
This species varies from dark, purplish red or brownish red to almost black, although younger parts are somewhat lighter in color. It is erect and can grow to 25 cm (10 in) tall.

The tetrasporophyte is crustose and tetrasporangia are intercalary. Special pores develop on the surface of the crust so that the spores can be released from the submerged reproductive structures.

Loose Ahnfelt's Seaweed grows on rocks, often where it is partly immersed in sand.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Ahnfeltia concinna
Ahnfeltia gigartinoides