The Division Rhodophyta, also known as red algae, contains the most species of limu. They can range in form from filamentous tufts to calcified crusts. In the marine environment red algae never dominate the landscape or form dense beds in the way some brown algae do. It is difficult to identify many red algae species strictly from outward appearance, this is why classification of this group of limu is done from a cellular level (the cells of all species take on a specific characteristic which acts as their identifier.)
Gracilaria spp
The species within the genus Gracilaria can take on many different forms. Many have terete branches while others have compressed or flattened branches. There are nine species that can be seen in Hawai’i with only three being common.
Gracilaria salicornia
Gracilaria salicornia (also referred to as gorilla ogo) is considered one of the most successful invasive algae on reef flats. It is much hardier than most native species and can its mat form allows for a fast growth rate. Thalli consist of sold cylindrical to compressed branches that are irregularly arranged. Its color ranges from yellow in the sunny areas to a greenish brown in the shade. It is found in tidepools and on reef flats and up to 4 meters of depth. This species was first found in 1971 in Hilo Bay, Hawai’i and was introduced to Waikiki later in the 1970’s.
Gracilaria doyti
Pants are erect and stand about 2-6 cm tall. The form dense tufts from fused holdfasts that attach the plant to the substrate. Branching is irregular and thick near the base and becomes more dichotomously divided upward where the braches form long segments. This plant can be found in small, shallow depressions on intertidal limestone or lava benches where the most wave force is encountered.
Acanthophora spp
There are only two species of Acanthophora that can be seen in Hawaiian waters. Each has a very different morphology form each other, making them look like they are not remotely related at all.
Acanthophora spicifera
Acanthophora spicifera (also known as pickleweed) is another successful invasive algae, introduced from Guam that out competes many other algae species like Laurencia and Hypnea. Plants can stand up to 40 centimeters tall with cylindrical branches. The main branches have many short branches that give the plant a spiny appearance. The color can be highly variable from shades of red or purple to a orange or brown color. Color can be reflective of the type of environment, for example in shallow areas with low water motion plants are a yellow or brown color that reflects the sandy or silty bottom. This plant is abundant on calm reef flats, tidepools, and in the intertidal.
Asparagopsis taxiformis
This is the only species of the genus Asparagopsis that can be found in Hawaiian waters. Plants have creeping stolons that have fluffy pink or grayish uprights. It is one of many red algae that has a life history where the alternate phases are completely different and until the connection between the two became known, the tetrasporophyte phase was known as a completely different species (Falkenbergia). It is found in the intertidal and shallow subtidal and is attached to rocks. The Falkenbergia phase is often an epiphyte on other algae. This limu is also probably the most highly valued algae used as food by Hawaiians, known as limu kohu or lumu lipehe.
The middle right photo shows how this algae can grow very thick on rocks, turning them red (photo credited to Maxine Pauley).
Coelothrix irregularis
This is the only species from the genus Coelothrix that can be found in Hawai’i. Plants are wiry, stiff, narrow branches that are horizontal and erect and are attached to the substratum at numerous points. This species can be found growing in spiraling clumps on rock or dead coral and is often iridescent. It is generally over looked because it tends to grow intermixed with other turf algae species
Galaxaura spp
There are three species Galaxaura in Hawai’i, all of which are commonly found on reefs. It is characterized by a calcified, dichotomously branched body with cylindrical branches. This genus is unusual because both the gametophyte and sporophyte phase of the plant take on a different appearance. In the gametophyte phase, the surface of the branches appear smooth and orange while in the sporophyte phase the branches are covered in a dense layer of hairs that are often arranged in whorls and the plant appears red.
Galaxaura rugosa
This is a common species found in the shallow subtidal and in tidepools. Both the sporophyte and gametophyte stages phases can be seen year round in similar habitats. Size can range up to 15 centimeters.
Top left and right photos credited to Heather Hillard (these plants appear to be in the gametophyte stage).
Laurencia spp
There are ten species of Laurencia found in Hawai’i and they are widely distributed and commonly found in the intertidal to subtidal on reefs. Plants are errect or decumbent, depending on species, with terete or compressed axes.
Laurencia mcdermidiae
Frond of this plant are 5-6 cm tall. The plant itself is a bright green color except for at the bases of the frond which are typically reddish. It typically grows on rock in the low intertidal, generally on exposed areas. This plant is endemic to Hawaii.
Liagora spp
Liagora is a common genus on Hawaiian reefs in the deeper subtidal with ten species representing it in Hawai’i. It is recognized by its dichotomously divided and calcified body. All species are very similar in appearance which makes it difficult to positively identify which species you are seeing without using a microscope. All species are common in the shallow subtidal and the intertidal zone.
Hypnea spp
Species in the genus Hypnea can be found throughout the world but there are only seven that can be found in Hawaiian waters. They can generally be characterized by being cartilaginous and having short lateral branches.
Hypnea musciformis
This species is locally common on reefs, identified by clumps of loosely intertwined, cylindrical branches that have a characteristic hook shape. This hooked tip allows for the plant to entwine itself around other algae and serve as the main anchor for the plant (the holdfast is either inconspicuous or it does not have one at all). These characteristics make it a common epiphyte on other reef algae such as Sargassum or Acanthophora. It is usually a red color but can be a yellow or brown color depending on light and nutrient conditions. It is commonly found in the calm intertidal and shallow subtidal reef flats as well as tide pools and rocky beaches. This species was purposely introduced to Kane’ohe Bay in 1974 from Florida as a experiment in mariculture. Since then it has spread quickly throughout all the Hawaiian islands. Its rapid growth rate and the ability act as an epiphyte on other algae have helped this species become on the most prevalent invasive seaweeds in the Hawaiian Islands.
(Photos credited to Heather Hillard)
Dasya spp
There are six species of Dasya found in Hawai’i. Theses plants are characterized by having branches covered by numerous pigmented filaments that give it a shaggy appearance. Because of this, species within the genus Dasya are considered one beautiful species of algae in Hawai’i.
Dasya iridescens
This species of Dasya is common throughout the main Hawaiian Islands and up to Midway Atoll of the Northwestern Hawaiian Islands. It can be found on reef flats and the low intertidal. As well as the characteristic “hairy” appearance, this species also displays a blue iridescence which makes it very conspicuous on the reef.
(Photos credited to Heather Hillard)
Amansia spp
The genus Amansia is represented by three species in the Hawaiian Islands. Plants have distinct rosettes of flattened branches with short serrations along stem-like branches. There is only one species that is common in Hawai’i, Amansia glomerata.
Amansia glomerata
This species grows in the low intertidal and shallow subtidal, mostly on hard surfaces but it can also act as an epiphyte on larger algae like Halimeda. Because this is a perennial plant, it acts as a favorite substratum for other smaller epiphytic algae. At depths of about 4 meters, crustose coralline algae and other small filamentous red algae can be found growing on Amansia glomerata.
Martensia spp
There are only two species that represent this genus in Hawaii. When seen underwater they can impart a bluish iridescence which makes it one of the more beautiful algae to observe while on the reef. The plant body is thin and membranous but has meshlike margins.
Martensia fragilis
This species can be found in tidepools and subtidally down to depths of 50 meters. It generally grows on rock and can occasionally be epiphytic on other algae. This species is generally uniform in color, as compared to the other species of Martensia, Martensia flabelliformis, which bears a more mottled appearance.
(Photos credited to Heather Hillard)
Porteria hornemannii
There is one species of Portieria found on Hawaiian reef. P. hornemannii has a distinctive flattened body with an alternate branching pattern. The most telltale feature that allows for easy recognition of this species is the curling pattern on the tips. This species can also have a pungent odor which helps with identification. This species can be found throughout the low intertidal and subtidal down to depths of 33 meters It prefers to grow on limestone or basalt rock that is exposed to surf.
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