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A specimen of Hypnea spp. collected from the Indian River Lagoon.  Photo by K. Hill, Smithsonian Marine Station;  courtesy of D. and M. Littler, NMNH. 

 
Close-up of Hypnea spp. collected from the Indian River Lagoon.  Photo by K. Hill, Smithsonian Marine Station;  courtesy of D. and M. Littler, NMNH. 

Species Name: 
Hypnea cervicornis J. Agardh

Common Name:
Hooked Red Weed (Note, this is a collective term not specific to this species.

Species Description:
Hypnea cervicornis is a Rhodophycean algae with a wiry, entangled form. It is yellow in color under conditions of strong sunlight, but becomes a darker brown-red color when shaded. Its branches are short and pointed, with branchlets that appear tendril-like. H. cervicornis lives attached to rocks or corals via a holdfast, or as an epiphyte on other plants. It grows 3-30 cm tall to depths of 10 m (Littler and Littler 1989).

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Worldwide in tropical and subtropical zones.

IRL Distribution:
In the Indian River Lagoon, H. cervicornis is common in Thalassia testudinum beds.

III. LIFE HISTORY AND POPULATION BIOLOGY

Abundance:
Abundant

Reproduction:
The pattern of spore germination in the Rhodophyta has been used as a diagnostic trait to taxonomically group species. Hypnea, and other species in the Order Gigartinales, have carpospores (sexual stage) and tetraspores (asexual stage) which settle and attach to substrata. Spores of H. cervicornis have no resting stage, as in some other algal groups. The attached spores undergo repeated segmentation to produce a morula-like stage from which the holdfast and shoot emerge (Mshigeni 1976).

Release of sexual carpospores, but not asexual tetraspores, was observed to co-occur with release of a gelatinous secretion which enveloped the spores. A gelatinous mass of aggregated spores is likely to sink considerably faster than individual spores, thus potentially making carpospores better adapted for attachment to substrata (Coon et al. 1972, Mshignei 1976). The predominance in Hawaii of Hypnea tetrasporophytes (which develop from carpospores) over gametophytes (which develop from tetraspores) appears to lend credence to this observation.

IV.  PHYSICAL TOLERANCES

Temperature:
Survives well in the tropics and subtropics in the shallow intertidal zone, and withstands dessication when exposed to air during low tides.

V.  COMMUNITY ECOLOGY

Trophic Mode:
Autotrophic

Habitat:
H. cervicornis is common in the intertidal zone, and was found to occur higher above other Hypnea species, (H. chlordacea, H. nidifica) thus, it is more often exposed to air and direct sunlight at low tide than other species (Mshigeni 1977). Mshigeni (1976) noted that in Hawaii, Hypnea species show some differentiation in the holdfast (attachment disc) based on habitat type. H. cervicornis has a somewhat smaller holdfast than some other species in the genus. Mshigeni (1976) associated this trait with H. cervicornis' typical habitat: calm, sandy substrata. Other species, with larger holdfasts, tended to be found in higher energy zones.

Associated Species:
 In the IRL, is often associated with Thalassia testudinum.

VI. SPECIAL STATUS

Special Status:
None.

Economic Importance:
Hypnea cervicornis is considered an economically important seaweed because it produces carrageenan, a binding and smoothing agent used in many commercial products such as toothpaste, ice cream, pet foods, etc. In addition, H. cervicornis is important to the medical and pharmaceutical industries, because agar, an important derivative of carrageenan, is a principal component of bacterial culture media.

Report by:  K. Hill, Smithsonian Marine Station
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Page last updated:  Sept. 18,  2001