University of Hawai`i

Botany Department

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Gracilaria salicornia

(C. Agardh) Dawson 1954

Invasive Alien



Class Order Family
Rhodophyta Rhodophyceae Gracilariales Gracilariaceae

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Gracilaria salicornia is one of the most successful invasive algae on reef flats. It appears competitively linked with the native G. coronipfolia and G. parvispora, but its mat form allows for a more robust growth rate, and it is hardier than the natives.







Distribution of Gracilaria salicornia in Hawaiian Islands





Gracilaria salicornia on reef flat at Waikiki, O‘ahu.




Thalli consist of solid, brittle, cylindrical to compressed branches, 2 - 5 mm in diameter. Axes 3 - 18 cm long and 1.5 mm broad, with branches usually irregularly arranged. Both axes and branches are regularly or irregularly constricted or continuous, with both conditions occurring on the same plant or neighboring plants. Plants often prostrate and overlapping, with lateral branches running along substrate, spreading in mats to 30 cm or broader, with rocks and pebbles between branches, or erect with an inconspicuous discoid holdfast and occasional secondary attachments.


Gracilaria spp. are extremely variable in Hawaiian waters. Although normally cylindrical, the branches are frequently found flattened, and sometimes plants are compressed throughout.


Structural Features

Cortex 1-2 layered, cells 4-6 by 10-12 mm, basal hair cells common; medullary cells relatively small (to other Gracilaria sp.) Tertrasporangia scattered over surface, 16-20 by 40-45 mm. Spermatangia in pits. Cystocarps globose, constricted at base, 1.4 - 1.8 mm diameter, with few to many tubular nutritive cells; pericarp cells in relatively straight anticlinal rows of oval to rounded cells.



Gracilaria salicornia is found in tidepools and on reef flats, intertidal to subtidal 4 meters deep, attached to limestone and basalt substrates. Intertidal plants are often without constrictions, subtidal plants with constrictions.



Hawai‘i:  O‘ahu, Hawai‘i Island.

Mechanism of Introduction:  First found in 1971 in Hilo Bay, Hawai‘i. Introduced to Kane‘ohe Bay and Waikiki in the 1970’s.

Worldwide:  Wide spread throughout the warm Indian and Pacific Oceans.



Gracilaria salicornia is very successful in calm, protected waters. It has spread over 5 kilometers from its point of introduction on O‘ahu since it was introduced in 1978. This alien algae is usually sterile in Hawaiian waters, but has been found to propagate sexually as well as asexually. Its widespread dispersal is accomplished primarily through fragmentation. Molecular fingerprinting shows a high degree of genetic similarity within a community, supporting the idea that dispersal occurs by cloning through the fragmentation process. This species is thought to compete with the native reef algae, such as G. coronopifolia, for substrate on the reef flat. Compared to other Gracilaria species, G. salicornia appears more flexible to light adjustments and seems to have a higher growth rate. High abundance appears to be associated with moderate water motion. This species successfully competes with other macroalgae by forming large, intricate mats that cover the substrate and inhibit settlement of other algae.


When other more desirable cultured Gracilaria species or the wild G. coronopifolia are not available for consumption, G. salicornia is used as a substitute. Its "crunchiness" is gaining favor, and this species has been sold under the name "robusta" in O‘ahu.



Abbott, I.A., 1999. Marine Red Algae of the Hawaiian Islands. Bishop Museum Press, Honolulu, Hawai‘i.

Doty, M.S. 1986. Experiments with Gracilaria in Hawai‘i, 1983-1985. Hawai‘i Botanical Science Paper, no. 46, University of Hawai‘i, Honolulu, Hawai‘i.

Larned, S.T., 1998. Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae. Marine Biology, 132: 409-421.

Magruder, W.H., and J.W. Hunt, 1979. Seaweeds of Hawai‘i. Oriental Publ. Co., Honolulu, Hawai‘i.

Nishimura, N.J. 2001. Assessment of Genetic Variability in the Invasive Red Alga, Gracilaria salicornia (C. Agardh) Dawson. Using Multi-locus DNA Fingerprinting. Masters Thesis, University of Hawai‘i at Manoa, Honolulu, Hawai‘i.

Rodgers, S.K, and E.F. Cox, 1999. Rate of spread of introduced rhodophytes Kappaphycus alvarezii, Kappaphycus striatum, and Gracilaria salicornia and their current distributions in Kane‘ohe Bay, O’ahu, Hawai’i. Pacific Science 53: 232-241.

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