University of Hawai`i

Botany Department

          Marine Algae of Hawai`i


Web Publications




Native Algae


Invasive Marine Species


Keys, Methods, and Glossary



Graduate Students


UH Botany Department


UH Classes



ReefWatcher's Field Guide to
Alien and Native Hawaiian Marine Algae

ReefWatchers is a volunteer organization affiliated with University of Hawaii Sea Grant College Program, School of Ocean Engineering, Science and Technology.


The Hawaiian Islands are the most distant from any other land mass on earth. Because of this geography, their flora and fauna have evolved in isolation and are unique: about 25% of Hawaii's reef fish, coral, and algae species occur nowhere else. Their isolated evolution, combined with human alteration of near-shore habitats, has made Hawaii's marine habitats, especially coral reefs, particularly vulnerable to alien invasions.

Four species of alien algae have become invasive in Hawaii: Acanthophora spicifera, Hypnea musciformis, Kappaphycus spp., and Gracilaria salicornia. Acanthophora spicifera arrived as an accidental "visitor" on the bottom of a shipping barge in 1950; the latter three species were intentionally introduced on Oahu and Molokai in the 1970's for experimental aquaculture for the agar industry. These "weedy" species have now spread from their initial sites of introduction and are competing with native marine flora and fauna.

Most alien algae populations are currently confined to discreet areas and may still be able to be controlled by removal and/or enhancement of native grazer populations. Other areas, particularly coral reefs, remain at risk from the spread of aliens from these sites.

This field guide is designed to help ReefWatchers protect Hawaii's reefs. It includes information about invasive alien algae, native seaweeds (so important in the reef food web, as well as for cultural usage), and native algae that may become invasive in altered habitats (e.g. from nutrient pollution or overfishing).

Algae are major contributors to our coral reefs. Just as plants are on land, algae are the primary producers in the ocean. Algae exist in many forms: as the larger, more-recognizable macroalgae in this guide, as well as lesser known encrusting coralline, turf, and filamentous forms. Even the corals are photosynthetic factories, infused with zooxanthellae, a microscopic alga.


How to Use the Field Guide  

Organization: The algae are listed in alphabetical order.  Under the name of the alien algae (red) is the native most commonly confused with that particular alien species.    Other native species included are commonly found in the same habitats on the Big Island.


Colored Names: The color of the algal names specifies whether the algae in alien, native or of unknown origin (see legend below).


What's in a name: Most of our Hawaiian algae do not have common names, except for the edibles that have Hawaiian names. The latin name is comprised of two parts: the genus name groups related algae while the species name is unique to that particular plant.    Example: (genus) Halimeda opuntia (species)


Group Classifications: The alien and native macroalgae included in this guide are from 3 major groups of marine algae. The group classifications are based upon photosynthetic pigments and give the algae their characteristic colors: the Rhodophyta (red algae), Chlorophyta (green algae), and Phaeophyta (brown algae).


Features: Key features of each alga are listed, providing a quick reference for identification in the field. Unique or defining characteristics, color and habitat are noted.

Additional references: For more information refer to:


"Marine Red Algae of the Hawaiian Islands" by Dr. I.A. Abbott, Bishop Museum Press, Honolulu, Hawaii, 1999.

Guide to Invasive Marine Algae of Hawai‘i:

Alien and Invasive Algae website:



 Color Legend:       Alien        Native      Unknown origin

Acanthophora spicifera  (Rhodophyta)
Native Look-alike:  Laurencia spp.

Spinelike branches

Red, brown to yellow in bright sunlight

Intertidal, shallow reef flats

Most successful alien

Description: Solid, cylindrical branches covered with many distinctive small spinelike branches, branched sparingly to repeatedly. Grows to 20 cm high. Color varies from red, brown to dark green but turns yellow in bright sunlight.

Habitat: Acanthophora spicifera is abundant on calm, shallow reef flats, tidepools, and on rocky intertidal benches. This alga attaches to hard substrates such as rocks, basalt ledges, or dead coral heads. It may also be found free-floating, due to its brittle, easily-broken nature.

Ecology: Acanthophora spicifera is the most widespread and successful alien alga in Hawai`i. A. spicifera is often found in communities with common native and alien intertidal species, such as Hypnea and Laurencia spp. The brittle nature of the branches often results in fragmentation, which contributes to frequent, large free-floating populations and its widespread distribution. A. spicifera was first found in Pearl Harbor and Waikiki in the early 1950's, most likely entering Oahu on a barge from Guam. The alga's adaptability has enabled it to spread throughout the state: it is found in brackish water ponds, salty tidepools, on basalt ledges and in sandy bottomed habitats attached to coral rubble. It is now found on all of the main Hawaiian Islands and is a common component of the intertidal environment throughout the state.

Competition between A. spicifera and native algae and invertebrates is likely, but impacts on community structure and diversity have not yet been extensively quantified.







Ahnfeltiopsis coccina  limu aki`aki  (Rhodophyta)

Thick erect, cylindrical blades

Thick clumps attached to basalt

Bright yellow in sun to dark brown in shade

High wave exposure

Description: Plants thick, erect, 2-60 cm tall. Branches cylindrical, to 3 mm in diameter, forked branching. Forms thick clumps. Color variable; yellow in sun to dark reddish brown in shade.

Habitat: Forms bands at high, intertidal zone in locations with heavy wave exposure. Attaches to to basalt rock; can form thick mats or may only be found in cracks and crevices. Abundant where found, common on Hawai`i, Maui, Kaui, but rare on Oahu.

Edible seaweed. 




(Melan) Amansia glomerata*   (Rhodophyta)

Semi-transparent blades in rosettes

Blades with midribs, serrated edges

Tough, stem-like stalk

Dark maroon red

High wave exposure

Description: Plants have semi-transparent, strap-shaped blades arranged in rosettes at top of tough, stem-like stalks, giving plant top-heavy look. Stalk bare below rosettes, may branch several times. Blades less that 1 cm long, with midribs, edges serrated to smooth, and tips rounded. Plant averages 6 cm tall. Dark maroon red.

Habitat: Melanamansia glomerata grows on rock and coral rubble, and is often found in low light habitats such as crevices or among coral fingers. M. glomerata is often covered with epiphytic crustose coralline seaweeds, giving it a pink appearance.


*Name under revision; currently Melanamansia




Avrainvillea amadelpha  (Chlorophyta)

Wedge-shaped, loosely woven spongy blades

Densly clustered blades

Green to gray-green

Calm, sandy bottoms

Description: Plant consists of wedge shaped blades that are thin, diaphanous, 1-4 cm wide, and 1-3 cm tall. Each blade is attached by stalk to a compact basal holdfast, forming dense clusters. Blades are asymmetrical, surfaces felt-like, edges smooth to lacerated. Green to green-gray. Clumps are often covered with silty sand, appearing muddy brown.

Habitat: Avarainvillea amadelpha is abundant in habitats of shallow, sandy bottoms with low water motion, 1-10 meters deep. Forms dense clumps often covered with silt and sand. In larger, more mature communities, other macroalgae will be found attached to blades.

Ecology: In Hawai`i, A. amadelpha can be found in abundance on the shallow reef flats on Oahu's south shore where is has disturbed and replaced native seagrass beds, and specimens have been collected from deeper water up to 90 m depth. At this time it is not known if this alga has been introduced or is a native. It is possible that A. amadelpha is a natural component of the deep-water community in Hawai'i and is now emerging in shallow water.





Caulerpa racemosa   (Chlorophyta)

"Grape-like"- spherical branchlets

Thick clusters

Light green

Tidepools and reef flats

Description: Upright branches 1-15 cm high, covered in small, bead-like branchlets 2-4 cm diameter. Light to bright green in color. Attaches to substrate by creeping runners that are often branched. Looks like small bunches of grapes; forms mats up to 4 cm thick.

Habitat: C. racemosa forms intertwined mats in tidepools and on reef flats. Horizontal runners tightly anchor mats to rocks and sand and in calm to moderately heavy surf areas.





Cladophora sericea   (Chlorophyta)

Very fine, delicate

Wispy, feather-like

Bright green

Low intertidal to subtidal

Forms large blooms in west Maui

Description: Very fine, filamentous, has a very soft appearance and is heavily branched. Light to bright green. Has main axis that may branch, with lateral branches arranged opposite of one another. Variable; feather-like, or may form highly branched filaments that tangle together forming large clumps of bright green wisps.

Habitat: Cladophora sericea occurs in small amounts with other algae or in abundant loose strands attached to coral, basalt substrate, and other hard surfaces on reef flats low intertidal to 8 m deep.

Ecology: Cladophora sericea is native to Hawai`i and is found on most reefs. This green alga is usually a small part of the biomass of the diverse, highly competitive intertidal community.

C. sericea has demonstrated weedy characteristics in Hawai`i. Approximately 10 years ago the alga became exceedingly abundant on leeward reefs in Maui and large blooms now occur regularly. During the blooms, large masses of the alga drift in the water column, snagging on coral and rock outcroppings, smothering organisms beneath. Rotting algae on the beaches and extensive amounts of algae drifting in the nearshore environment prevent people from enjoying ocean-related activities.





Codium edule   limu wawae'iole   (Chlorophyta)

Soft, spongy to touch

Forked branches

Dark green

Lies prostrate, frequently attached to substrate and rubble

Description: Fleshy, felt-like, dark green plants, 1-2 cm diameter. Soft, spongy to touch. Branches repeatedly forked. Branches lie prostrate and attach to anything on the substrate, forming mats incorporating bits of coral rubble, rocks and shells.

Habitat: Codium edule is common throughout the islands and is found intertidally to subtidally, 2-4 m deep, but most commonly subtidally.

C. edule is a popular edible seaweed sold in markets in Hawai`i. Its Hawaiian name means rat's foot, after the appearance of the thin, cylindrical branches.




Dictyosphaeria spp. (Clorophyta)



Convoluted mass

Large, "bubble" cells

Description: Dictyosphaeria cavernosa is referred to as the "Green Bubble Alga" due to its large round cells. It forms hollow spheres when small; when larger spheres burst becoming convoluted and cup-shaped, forming large mats. D. versluysii also has bubble-like cells but is completely solid and remains rounded, 1-5 cm wide. Grass green to bluish in color.

Habitat: Dictyosphaeria spp. is found attached to rocks or coral rubble on shallow, calm reef flats and in tidepools. Older D. cavernosa plants can form large convoluted mats, 1-10 cm thick, that may cover large subtidal areas to 59 meters deep. In Kane`ohe Bay it overgrows and kills finger coral. D. versluysii does not show weedy tendencies; it forms small clusters of solid "bubbles" scattered among turfs on hard substrates.


Dictyosphaeria cavernosa
Feature:  Hollow cup-shaped "bubbles"



Dictyosphaeria versluysii

Solid, rounded masses

Gracilaria salicornia  (Rhodophyta)
Native Look-alike:  Codium edule



Brittle, easily fragments

Short, pointed side branches

Yellow to dark red

Calm, intertidal and shallow reef flats

Large floating blooms in west Maui

Description: Plants brittle, cylindrical, 2-5 cm in diameter. Branching irregular, often forked at tips and constricted at base of forks. Tips bluntly rounded. Varies in color from a bright yellow at the tips to orange or green and then even dark brown at the base.

Habitat: Gracilaria salicornia is found in tidepools, on reef flats, and intertidal to subtidal 4 meters deep, attached to limestone and basalt substrates. This alga grows in three-dimensional mats tightly adherent to hard substrata up to 40 cm in thickness. In calm environments it can also grow in an upright and more openly branching form.

Ecology: G. salicornia's three dimensional growth form allows it to grow over the top of other benthic organisms (native algae, corals and other invertebrates), thus becoming particularly disruptive and ecologically dominant in some habitats.

G. salicornia has been present for several decades inside and outside the break wall and in Kapoho Bay in the Hilo area on the Big Island of Hawai`i; the origin of these populations is unknown. G. salicornia was intentionally transported from Hawai`i to Kane`ohe Bay and Waikiki in O`ahu in the 1970's and later to near Pukoo, Moloka`i, where it was planted in open reef cultures for experimental aquaculture and research. Evidence suggests that G. salicornia has significantly altered benthic community structure and species diversity where it has spread throughout much of Waikiki, and reports suggest that it is now common on much of Molokai's south shore from Kamalo to Kaunakakai.






Galaxaura marginata  (Rhodophyta)

Flattened, calcified blades with raised margin

Chalky pink to red

Intertidal to subtidally

Description: Plants to 12 cm tall, distinctly flattened, pink to wine red, more or less dichotomous (forked) branching. Blades strap shaped, flat and smooth, 1-2.5 cm wide with margins of blades slightly raised. Chalky appearance due to calcification. Often forms large clumps attached by single holdfast.

Habitat: Galaxaura marginata is found in shaded areas of tidepools, and low intertidal to subtidal, where it grows among coral fingers and over rubble. G. marginata is more common subtidally on open coasts.




Halimeda opuntia  (Chlorophyta)

Thick kidney-shaped blades


Green to chalky white

Large spreading plants subtidally

Description: Blades are kidney-shaped calcified thick segments, 0.5-2 cm wide, with midrib down middle of segment. Plants are large and spreading, to 30 cm, attached to substrate frequently with rhizoids (small filamentous root-like structures). Green to chalky white.

Habitat: Halimeda opuntia is found attached to hard substrates subtidally between rocks or under coral overhangs. H. opuntia is the most widespread species of Halimeda and is the major producer of carbonate sands in many tropical reef areas.





Hypnea musciformis (Rhodophyta)
Native Look-alike:  Pterocladiella spp.



Tips flattened, with "hooks"

Short, pointed side branches

Yellow to dark red

Calm, intertidal and shallow reef flats

Large floating blooms in west Maui

Description: Rounded main branches, 1-3 mm diameter, with short, pointed side branches. Tips of branches are flattened and have characteristic "hooks" or tendril-like projections. Length from 3-30 cm. Bright yellow in sun to dark red in shaded areas.

Habitat: Hypnea musciformis is common on calm intertidal and shallow subtidal reef flats, tidepools and on rocky intertidal benches. Most often found low intertidal to shallow subtidal reef flats, attached to sandy flat rocks. In bloom stage, may be found free-floating.

Ecology: H. musciformis was introduced to Kane`ohe Bay in 1974 for kappa-carageenan mariculture. The alga is often found as an epiphyte on reef algae such as Sargassum spp. and Acanthophora spicifera. H. musciformis forms large, odiferous unialgal mats, and during the winter can represent 2/3 of the biomass of drift algae on windward and leeward beaches on Maui. In peak blooms, thousands of pounds wash up on Maui beaches. This species has been collected from all the main Hawaiian Islands except Hawai`i and Kaho`olawe. Samples have also been collected from lobster traps in deep water off Maro Reef and Necker Island in the Northwestern Hawaiian Islands.

The success of this alga in Hawai`i is likely due to a rapid growth rate, ability to epiphytize other algae and easy fragmentation. Dispersal may well have been enhanced via interisland travel on fouled boat hulls.

Soon after its introduction, it was identified as a food source for the green sea turtle, Chelonia mydas. H. musciformis can make up a signficant part the their diet, sometimes representing 99-100% of the seaweed mass found in their stomachs.






Laurencia spp.   limu mane`one`o (Rhodophyta)



Pits in end of branches

Pink-purple to red or green

Intertidal to subtidal

Description: There over 16 species of Laurencia in Hawai`i. Plants are usually erect and fleshy with variable branching patterns, each order of branching shorter than the preceding. Most Laurencia species have cylindrical branches, but a few intertidal species are compressed. Branch tips club-shaped with pits in the tips. Plants usually range in size from a few cm to 20 cm, and are pink-purple to red but can have yellow and even green portions.

Habitat: Laurencia spp. are found in clumps or as components of turfs attached to eroded coral or basalt rocks intertidally to subtidally, and are often associated with Acanthophora spicifera.

Some Laurencia spp. are used as condiments by Hawaiians.



Liagora spp.  (Rhodophyta)

Calcified, often soft and slippery

Forked branching

Tan to pink to brown

Intertidal to subtidal

Description: Liagora spp. can often be recognized by their cylindrical, forked branches and white, calcified appearance. Otherwise, they are highly variable. The amount of calcification can vary from chalky to slippery, and structure from limp to stiff. Can be up to 45 cm tall, and color often tan to pink to brown. Plants moderately to highly branched, attached to substrate by single holdfast.

Habitat: Commonly found in tidepools, low intertidal habitats and on reef flats. Attaches to rock and eroded coral. Some species may be annuals, appearing seasonally.


Martensia fragilis (Rhodophyta)

Delicate, flat blade

Alternating blade and mesh portions

Iridescent pink or blue

Low intertidal to subtidal

Description: Plant has very delicate flat blades, iridescent pink, purple or blue colors. Blades may be whole or branched in a semi-forked pattern. Blades have solid blade and guaze-like mesh portions that are very delicate and may contain holes from being torn. 1-8 cm tall.

Habitat: Martensia fragilis form small to large clumps attached to coral rubble or other algae. It is common in intertidal pools and low intertidal to shallow subtidal crevices.


Padina spp.    (Phaeophyta)

Flat calcified blade with rings

Chalky white to light yellow-brown

Bright yellow in sun to dark brown in shade

Intertidal and subtidal

Description: Padina spp. have flat calcified brown blades that may be rolled into a circle or flat and broad. Attaches to substrate with a small holdfast. Degree of calcification can vary between species but is often visible in horizontal circular rings, upper surface is usually more calcified than the lower. Size varies from a few cm to greater than 20-30 cm tall. Chalky white to light yellow-brown.

Habitat: This genus is common in Hawai`i and can be found in both intertidal and subtidal zones. P. japonica (right) is common in tide pools and on basalt benches. Other species can found in silty disturbed areas.



Pterocladiella capillacea  (Rhodophyta)

Strap-shaped blades

Opposite branching, pinched at base

Dark red to green

Often single species mats

Description: Plants firm, wiry, to 7 cm high. Flattened branches taper near tips and branchlets are constricted at base. Branching patterns often regular and opposite but may be highly variable in response to environmental factors. Dark, reddish brown to light green.

Habitat: Pterocladiella capillacea and other Pterocladiella spp. are very common on nearshore intertidal reef flats, tidepools and subtidally, attached to basalt and other hard substrates in areas of high wave exposure. May form extensive unialgal masses seasonally, with peak growth in Hawai`i in December.


Sargassum echinocarpum  (Phaeophyta)

Thick, spiny blades

Gas bladders

Golden to dark brown

Intertidal, wave exposure

Description: Blades very thick and stiff, margins with spiny projections. Plants 5-70 cm tall with blades 0.3-2 cm wide. Golden to dark brown holly leaf-like blades with mid rib and flattened stems. Gas bladders (small round hollow beads) attached to stalks.

Habitat: Sargassum echinocarpum is commonly found on wave-swept rocky intertidal benches, in tidepools, and on reef flats. Often associated with Turbinaria ornata and turfs in intertidal bench communities.




Turbinaria ornata  (Phaeophyta)

Stiff, erect turban-like blades with spines

Light yellow brown

Very common

High intertidal to subtidal

Description: Stiff erect seaweed with distinctive angular turban-like blades and a central stalk with a row of stiff spines around blade margin. Light yellow brown color and grows 2-20 cm tall.

Habitat: Very common. Found in tidepools and on reef flats and crests on rocky intertidal coastlines, mid-intertidal to 30 m deep. Plant often isolated or in small groups, but occasionally forms large, low mats in high intertidal. Shows seasonal tendencies in upper intertidal. 



Ulva fasciata  limu palahalaha  (Chlorophyta)


Flat, thin, sheet-like

Thick clumps attached to basalt

Bright green

Intertidal and reef flats, areas of high wave exposure

Description: Large, flat twisted blade, 1-10 cm wide and 5-100 cm long. Blades broadened at base, upper portions may be divided deeply into many ribbon-like segments; margins smooth, often wavy. Plants thin, sheet-like, up to 1 meter long, consisting of wide blades attached to substrate with a small holdfast. Grass green in color.

Habitat: Ulva fasciata, also known as sea lettuce, is very common. Found on intertidal rocks, in tide pools, and on reef flats. Abundant in high nutrient water, near stream mouths and run-off pipes. 




This booklet is the product of the collaborative efforts of Dr. Cindy Hunter, Dr. Isabella Abbott, Dr. Karla McDermid, and Jen Smith, all dedicated to the understanding and preservation of Hawai`i's reef ecosystems. The main author, photographer and designer is Linda Preskitt, with contributions of text from Dr. Cindy Hunter, Jen Smith and Sara Peck.

This booklet and workshop materials have been funded by the Hawaii Community Foundation and University of Hawaii Sea Grant College Program, School of Ocean Engineering, Science and Technology. 


Sponsored by:



Webmaster L. Preskitt