Plant lax, from 3 - 30 cm long (most less than 15 cm) in
tangled, bushy clumps. Axes extend through entire length of plant, but main
branches absent; dichotomous branching throughout. Axils rounded, with sides
symmetrically developed, with side branches at times growing almost
horizontally for 1 - 5 mm before bending up or curling downward. This
curling tendency produces the characteristic tangled branches. Branches
coarser below (1.5 mm diam.) more slender above (0.25 mm), tapering abruptly
to acute tips. Spine-like branchlets 1 mm long, few to many. Holdfasts
small, inconspicuous, or lacking. Commonly yellowish, but deep red
Medulla appear parenchymatous around central axial cell;
cortical filaments with few divisions on radii, outer layer pigmented.
Tetrasporangia zonately divided, in raised nemathecia, usually on ultimate
branches; spermatangia borne in chains in slightly swollen nemathecia at
base of branchlets. Cystocarps conspicuous, rounded, without discharge pore.
Hypnea cervicornis is found in intertidal tidepools and
on shallow reef flats, where it varies in color from bright yellow in areas
with bright sunlight to dark red in shaded areas. This alga is either
attached to rock or coral rubble or ephipytic on other algae, such as common
species of Sargassum.
Hawai‘i: Northwest Hawaiian
Islands, O‘ahu, Maui, and Hawai‘i Island.
Worldwide: Eastern Atlantic,
Caribbean, Indian and Pacific Oceans.
Before the introduction of
Acanthophora spicifera and
Hypnea musciformis, H. cervicornis was one of the most common
intertidal reef algae on Hawaiian reefs, and had a close association with
Laurencia species. The native species were both quite common on
reef flats with low to moderate wave action. A. spicifera entered the
same niche and competed successfully with Laurencia spp. and
enhanced the productivity of H. cervicornis. H. musciformis’
successfully invaded into the same niche, and replaced H. cervicornis
as the species most often found attached or entangled on the upper branches
of A. spicifera. H. cervicornis appears to be not as
widespread as it once was, but is still epiphytic on many reef algae. This
alga resembles its primary competitor, H. musciformis, except for the
lack of the flattened broad hooks associated with the invader.
H. cervicornis is considered an economically important
alga for its production of carrageenan, an important thickener in food and