Hawai'i Coral Reef Network

Seaweeds of Hawaii
by William H. Magruder and Jeffrey W. Hunt

This is a MOP - Maui CC project by Kim Roseman and Carl Stepath in 1999. This website is by permission of William Magruder for educational purposes and no reproductions are permitted. This permission is fully revocable at any time.

(Green Seaweeds)

Boodlea composita

B. composita forms thick, rounded cushions I to 4 cm high and 2 to 8 cm broad that are made of a network of fine branching filaments. It has a delicate crispy texture, and the color is grass green, but may become yellowish green when exposed to bright sunlight. This seaweed is found in tide pools, on reef flats, and in lower intertidal habitats along rocky coastlines.

Bornetella sphaerica

B. sphaerica is easily recognized by its pea-like shape and hexagonal surface pattern. Usually dark or grass green in color, it can become yellowish when exposed to bright sunlight. This seaweed reaches a diameter of I cm, and grows on rocky substrates in a variety of subtidal habitats, where it is often found under larger seaweeds.

Bryopsis sp.

The delicate soft featherlike appearance and dark green color of this seaweed are easily recognized. Bryopsis is found on shallow reef flats, in tide pools, and in lower intertidal habitats of coastlines with low wave action. It grows to heights of 2 to 12 cm.


Caulerpa racemosa

C. racemosa is found in tide pools and on reef flats, where it often forms spreading light green to slightly bluish mats up to 4 cm thick. It has cylindrical prostrate branches from which rhizoids grow and anchor it to the bottom, and upright branches that are covered with small spheres creating a grapeclusterlike appearance.


Caulerpa serrulata

C. serrulata has prostrate branches with anchoring rhizoids and upright branches with a serrated zig-zag appearance. It grows from 2 to 8 cm high, is gray-green in color, and is found in sandy areas on reef flats and in tide pools.

Caulerpa sertularioides

C sertularioides has upright branches that are featherlike in appearance and prostrate branches with rhizoids that anchor it in the sandy areas of reef flats and tide pools. It is light green in color and from 3 to 10 cm high. This seaweed closely resembles C taxifoliM but can be distinguished by the shape of its upright branches, which are round in all parts.

Caulerpa taxifolia

C taxifolia has featherlike upright branches and prostrate branches with bunches of anchoring rhizoids. It is usually green in color, is 3 to 15 cm high. and grows in the sandy areas of tide pools and reef flats. This seaweed closely resembles C sertularioides except that the upright branches are somewhat flattened with angular edges, giving them a square or rectangular appearance.

Chaetomorpha antennina

C antennina forms stiff tufts of unbranched filaments 2 to 15 cm high composed of large cells. Its overall color is grass green, but close examination will reveal alternating green and white bands. The tips become pale green or yellowish when bleached by bright sunlight. This seaweed grows intertidally on rocky coastlines exposed to large breaking waves.

Chlorodesmis hildenbrandtii

C. hildenbrandtii usually forms soft, thick mats from I to 5 cm thick that are dark green to yellowish-green in color. It has soft fine branches less than I mm in diameter that often trap large amounts of sand. This seaweed grows in tide pools and on reef flats. C hildenbrandtii is one of the few green seaweeds that produces a noticeable odor.

Cladophora fascicularis

C fascicularis is a large grass green seaweed that can be from 5 to 50 cm long. It has a bushy appearance produced by many long filaments that have clusters of short branches that all branch in the same direction. This seaweed is found near the zero tide level on protected rocky coastlines and in tide pools.

Cladophora patula

C. patula is a coarse, stiffly branched seaweed that is grass green in color and from l to 5 cm high. It has large easily visible cells from 1 to 12 mm long and grows on rocky coastlines in shaded areas, on reef flats, and in between the branches of coral heads at deeper depths.

Cladophoropsis luxurians

C. Iuxurians forms spreading mat-like turfs from I to 3 cm thick. The turf is produced by many branches that intertwine, attaching themselves to the bottom wherever they make contact. This seaweed is grass green in color and is commonly found growing on reef flats.

Codium arabicum

C. arabicum forms very dark green spongy masses from 0.5 to 3 cm thick and up to 15 cm wide. It is characterized by rolling bumps or convolutions on its surface. This seaweed is commonly found in lower intertidal habitats and on reef flats.

Codium edule (wawoe'iole)

C. edule forms spongy, mat-like masses made of many intertwined dark green cylindrical branches that are from 3 to 8 mm in diameter. The branches attach to whatever they contact, and it is not uncommon to pick up a specimen and have the bottom covered with pieces of shells, sand, or small rocks. This seaweed is commonly found on reef flats, in lower intertidal habitats, and in tide pools.

Codium reediae ('a 'ala 'ula)

This dark green seaweed has somewhat flattened branches from I to 2 cm wide that are mostly branched in the same plane. It is upright, being attached to the bottom in only one place. C. reediae grows in fairly calm water habitats, such as deep tide pools, and on deep reef flats, where it reaches 10 cm

Dictyosphaeria cavernosa

Because of its large round cells, D. cavernosa is frequently called the "green bubble alga." When it is small it is a hollow sphere, but as it grows it ruptures, becoming convoluted and cup shaped. This seaweed is grass green in color, and often forms extensive mats from 1 to 10 cm thick over large areas on shallow reef flats. It can also be found in tide pools and at deeper depths.

Dictyosphaeria verslaysii

D. versluysii also has bubblelike cells, but unlike D. cavernosa it is completely solid in the middle and always remains rounded. Grass green or sometimes slightly bluish in color, from 1 to 2 cm high and I to 5 cm wide, this seaweed is commonly found on reef flats and in tide pools.

t. enteromarpha s p. ('ele'ele)

Enteromorpha characteristically consists of a flattened hollow tube that may be branched or unbranched, and is grass green in color. It can be from I to 10 mm wide and from 3 to 20 cm long. This seaweed can tolerate a

wide range of salinities, often being found in brackish estuaries and in sandy areas with fresh-water seepage. There are several species of this genus in Hawaii, but they are difficult to tell apart without a microscope.

Halimeda discoidea

H. discoidea has flat ribless segments I to 3 cm wide, and reaches lengths of 15 cm. The outer segments are dark green, but the inner ones are calcified and appear whitish. Due to the nature of its holdfast, this is one of the few seaweeds that is able to anchor itself in deep sand as well as to hard bottoms. It accomplishes this through a sediment-filled bulbous mass of rhizoids that is embedded in the sand. H. discoidea is commonly found in sandy areas on reef flats and in deeper water. It is estimated that a significant amount of the sand in some of Hawaii's beaches is the result of the weathering of dead Halimeda segments.

Halimeda opuntia

This calcareous green to whitish seaweed is found in the quiet water of inner reef flats, or more commonly, between the branches of corals in deeper water. H. opuntia attaches to the bottom in several places, often has a rib in the middle of its flat 0.5 to 2 cm wide segments, and can reach a length of 30 cm, although it is usually smaller.

Microdictyon japonicum

M. japonicum is a single flattened blade consisting of a meshwork of fine connecting branches. It is grass-green in color, from 1 to 6 cm wide and 1 to 4 cm high. This seaweed can be found in the low intertidal area of rocky coastlines, on reef flats, and in between the branches of corals at deeper depths.

Microdictyon setchellianum

Like M. setchellianum, this grass green seaweed is also a flattened blade made of a meshwork of branches, but the branches are usually coarser, and several blades are usually arranged in a rosette. Commonly found on reef flats, M. setchellianum grows to 8 cm wide and 5 cm high.

Neomeris annulata

This is surely one of the most distinctive seaweeds in Hawaii. N. annulata has a fingerlike form, grows to 4 cm high, and is found in tide pools, on reef flats, and in subtidal habitats, where it often grows in groups in depressions in the rock. The tips of this seaweed are green in color with a fuzzy appearance that is produced by many fine hairs. The lower part is calcified and white.

Siphonocladus tropicus

S. tropicus has many branches that radiate outward from a central stalk, with each branch covered by many small spine-like projections. Reaching a height of 7 cm, it is usually green but often collects fine sediment and may appear whitish. Although not abundant, this seaweed is quickly noticed due to its unusual form. It commonly grows on reef flats.

Ulva fasciata (palahalaha)

Ulva fasciata (palahalaha)

U fasciata, one of the commonest seaweeds in Hawaii, is often called "sea lettuce" in other parts of the world. It is a flat, often twisted blade from 1 to 10 cm wide and 5 to 100 cm long that is grass-green in color. This seaweed is commonly found on intertidal rocks, in tide pools, and on reef flats, and is often abundant in areas that are high in nutrients.

Ulva reticulata

U. reticulata is also a grass green flat blade, which when very small closely resembles U. fasciata. As it gets larger, however, cells in the center of the blade produce reproductive spores or gametes and fall out, creating distinctive holes or reticulations. It also separates from its point of attachment and then grows tangled on other algae, reaching lengths up to 3 m, although it is usually much smaller. This seaweed is commonly found on reef flats, where at times it almost completely covers the other seaweeds.

Valonia aegagropila

V. aegagropila is composed of many elongated grass green tubes that usually have only their ends visible. This seaweed forms hemispherical cushions from ~ to 8 cm high and 5 to 15 cm wide, and is found on reef flats or in tide pools, where in some areas it almost completely covers large areas of the bottom.

Valonia ventricosa

This distinctive egg-shaped seaweed is a single liquid-filled sack that can be as large as 5 cm across. Dark to silvery green in color, V. ventricosa is found in turFs in lower intertidal habitats along rocky coastlines, in tide pools, on reef flats, and in between the branches of corals at deeper depths.

Return to cover page


Last update: 1/25/2005