E-Flora BC: Electronic Atlas of the Flora of British Columbia

Family Description:
Most species are flattened and one to six cells thick. Many have veins or a midrib. Tetrasporangia are tetrahedrally divided and usually occur in sori. Cystocarps are embedded in the plant. Carpospores escape through an ostiole.

Species description:
This bright, rose red to purplish red seaweed has flattened and erect, fan-shaped blades that are thin and somewhat delicate. The blade has a distinct midrib at the base, but this becomes progressively more difficult to see in upper parts, where it is replaced by a fine network of veins. At the base, Hidden Rib is anchored by a holdfast that can be either discoidal or somewhat branched. Each blade is rather deeply cleft into elongated lobes that become gradually narrower near their tips. The most distinctive feature of this species, however, is the dense fringe of tiny, semi-circular proliferations that develop along the sides of these lobes (this fringe does not extend out onto the tips of the lobes and is not present in all specimens).

Male and female gametophytes are separate individuals in Hidden Rib. Spermatia and tetrasporangia are produced in patches along the lobe margins or in the fringe of proliferations.

Hidden Rib grows on rocks in areas exposed to moderate surf.