Gracilaria edulis (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

1, habit; 2, upper portion of branchlets; 3, rhizome-like creeping part, showing holdfasts and the bases of erect fronds; 4, cross-section of a thallus; 5, longitudinal section of a cystocarp; 6, longitudinal section of the pericarp of a cystocarp; 7, cross-section of a tetrasporophyte with tetrasporangia; 8, cross-section of a male gametophyte of which the adjacent spermatangial conceptacles coalesc to form Polycavernosa-type spermatangial conceptacles.

Gracilaria edulis (S.G. Gmelin) P.C. Silva

Protologue: Univ. Calif. Publ. Bot. 25: 293 (1952).

Family: Gracilariaceae

Chromosome number: 2n= probably 48, a number found for several Gracilaria spp. and in agreement with the assessment of nuclear genome size for Philippine material of the present species

Synonyms

• Fucus edulis S.G. Gmelin (1768),
• Polycavernosa fastigiata C.F. Chang & B.M. Xia (1989),
• Hydropuntia fastigiata (C.F. Chang & B.M. Xia) M.J. Wynne (1989).

Vernacular names

• Indonesia: sayur karang, janggut monyet, bulung embulung (Java, Bali)
• Malaysia: agar-agar karang (Johore), sare (collectively with other Gracilaria spp.) (Malay, Kelantan)
• Philippines: gargararao (general name for all Gracilaria and all Gracilariopsis in Ilocos Province)
• Thailand: sarai woon.

Origin and geographic distribution

G. edulis occurs in the Indian Ocean (East Africa, Mauritius, Laccadive Islands, India, Sri Lanka, Nicobar Islands, Andaman Islands) and in the Pacific Ocean (China, Japan, Micronesia, north-eastern Australia). In South-East Asia it is found in Burma (Myanmar), Thailand, Vietnam, Malaysia, Singapore, Indonesia, the Philippines and Papua New Guinea.

Uses

G. edulis is collected by coastal communities for food and used either as a salad after blanching with hot water or as a source of home-prepared agar. It is also used for animal feed, as fertilizer and in waste water purification plants. In traditional medicine it is used especially for poulticing knee joints and to treat unhealthy sores.

Production and international trade

A pilot seaweed farm for growing G. edulis in Maung Shwe Lay Gyaing in Burma (Myanmar) has ceased production by 1998 due to lack of local demand for an agar industry. The alga is now only grown in that country in small bays and inlets and in polycultures with the shrimp Panaeus monodon in shallow ponds.

Properties

G. edulis is an agarophyte producing an algal galactan which can be extracted in hot water and which gels at room temperature. The agar yield and quality vary depending on habitat, season, and reproductive phase. The agar gel strength ranges from 120-1100 g/cm². For the Philippines a gel strength of 890 g/cm² has been recorded. The melting and gelling temperatures range from 69.0-93.1 °C and 35.0-54.0 °C respectively. Alkali (3-10% NaOH) pretreatment doubles or triples the agar yield and also increases gel strength, although assessments on cultured plants do not report such an increase. Differences in NaOH concentration of between 3 and 10% for Indonesian G. edulis agar do not result in considerable increase of gel viscosity, but mean gelling and melting temperatures of 1.5% agar change between 45-54 °C and 69-82(-85) °C respectively. For this material gel strength of 1.5% agar augments between 200-500(-900) g/cm² after pre-treatment with 3-10% NaOH, while hardness for the same material rises between 40-100(-150) g/mm² under these circumstances. Flexibility stays at low values: below 2 g/100 mm². The latter are the lowest values found for any commercial Gracilarioid agar. A sulphate value of 0.08% was found for G. edulis agar from Thailand that had not been pretreated with alkali. Constituents found in G. edulis material from India are as follows in g/100 g of dry seaweed: water 14.9, protein 7.6, fat 0.4, ash 43.4 (maximum in May), and in mg: calcium 830, phosphorus 53, iron 290, iodine 50. The energy value per 100 g dried seaweed is 1200 kJ. Per 100 g agar powder of G. edulis (from Sri Lanka) the following compounds are present (in g): water 18.5, ash 5.05, sulphate 2.27, sodium 2.7, potassium 1.5, calcium 0.92. Extracts of G. edulis (as G. lichenoides) show some antibiotic activity against pathogenic bacteria as well as against the protozoan Trichomonas foetus.

Description

• Thalli up to 27 cm tall, brownish-red, each arising from a discoid holdfast; branching dense and fastigiate, divaricate, dichotomous to trichotomous, up to 7 orders and with long branch intervals; branches 1-1.5 mm in diameter, cartilaginous, flexuous, with or without a constriction at their bases or with only a slight constriction, cylindrical, ending in pointed apices.
• Thalli in transverse section consisting of roundish thin-walled medullary cells, 100-300 μm in diameter, and 1-2 rows of small cortical cells, 5 μm in diameter, with an abrupt transition from medulla to cortex.
• Tetrasporangia ovoid to oblong, cruciate, 8 μm × 16 μm, scattered over surface of thalli.
• Spermatangia in deep, pot-like conceptacles with multiple cavities, which are arranged in groups of up to 10.
• Cystocarps globose, up to 2 mm, with rostrate tips and constricted at the bases; pericarp thick, consisting of 9-14 rows, cells of the outer rows oval, inner cells horizontally compressed, basal absorbing filaments robust with many branches.

Growth and development

Average daily growth rates of G. edulis in experimental rope cultivation in the Philippines reached 9.0%, which is much higher than rates recorded for other areas.

Other botanical information

Material often identified as G. coronopifolia J. Agardh from Vietnam turns out to be G. edulis.

Ecology

G. edulis is commonly found in association with G. changii (B.M. Xia & I.A. Abbott) I.A. Abbott, C.F. Zhang & B.M. Xia. It grows abundantly on rocks, coral, mangrove roots, fish cages and on intertidal mud flats. Plants growing on fish cages have thicker primary branches and form much-branched fastigiate tufts. Entangled or loose clumps are formed on muddy substrate. In violent seas the plants readily break into pieces, which may form the starting points of new plants when they come into contact with a suitable substrate.

Propagation and planting

G. edulis can be planted using similar techniques as those used for G. changii, i.e. vegetatively from cuttings or from tetraspores or carpospores. Thalli can be attached to monofilaments or rafts and cultured in ponds or the open sea. In rope culture, compact planting (5 g of plant material every 10 cm) results in higher yields than a wide planting pattern (10 g of plant material every 20 cm).

Phycoculture

The most productive methods of phycoculture of G. edulis are on shallow reef-flat areas or in shallow tanks. Tests on Guam recorded rates of 5.4-7.6% and 4.5-5.1% growth/day respectively. In laboratory experiments growth was stimulated by adding phosphate and nitrate, but the agar yield diminished and agar quality was only slightly improved (less sulphate). In the Philippines co-farming Eucheuma denticulatum (Burm.f.) Collins & Herv. with G. edulis shows potential. The depth of water above the cultivated plants has considerable effect on the yield: maximum yield was obtained at a depth of 40-50 cm, and significantly less at depths of 0-25 cm or more than 1 m. Raft cultivation in India showed highest growth rates for plants tied to shallow ropes. Regrowth of thalli from the remaining frond bases after the first harvest is often not as good as the initial growth.

Diseases and pests

Epiphytic algae like Chaetomorpha, Cladophora and Hypnea spp. may be found on the thalli of G. edulis. Especially in tank culture these epiphytes can almost totally cover the G. edulis plants. The epiphytes are removed by introducing grazers such as the topshell Trochus niloticus or the opisthobranch Stylocheilus longicauda or they are cleaned manually at weekly intervals. On coral reefs and in some cultures G. edulis is grazed by herbivorous fish. An adelphoparasite of yet unknown identity has been recorded growing on and in G. edulis from Manado (Sulawesi, Indonesia).

Harvesting

In India it is recommended that G. edulis be harvested by cutting the thalli just above the base. The first harvest is done three months after planting, and further harvests are made quarterly.

Yield

In north-eastern Australia the agar yield of G. edulis, when extracted without alkali treatment, ranges from 12.5-26.0% (dry weight). This is lower than that of most commercial Gracilarioid agarophytes, but the agar yield of alkali-treated material is within the same range as in other Gracilaria spp.

Handling after harvest

Harvested plants of G. edulis should be cleaned and dried, packed in bags and stored in a dry place.

Prospects

Although G. edulis is collected for agar extraction, the potential for commercial exploitation is limited by the low agar yield, low gel strength and low gelling temperature.

Literature

• Gerung, G.S., Terada, R., Yamamoto, H. & Ohno, M., 1999. An adelphoparasite growing on Gracilaria edulis (Gracilariophyceae, Rhodophyta) from Manado, Indonesia. In Abbott, I.A. (Editor): Taxonomy of economic seaweeds 7. pp. 131-136.
• Nelson, S.G., Tsutsui, R.N. & Best, B.R., 1980. A preliminary evaluation of the mariculture potential of Gracilaria (Rhodophyta) in Micronesia: growth and ammonium uptake. In: Abbott, I.A., Fostert, M.S. & Eklund, L.F. (Editors): Pacific seaweed aquaculture. California Sea Grant College Program, La Jolla, California, United States. pp. 72-79.
• Raju, P.V. & Thomas, P.C., 1971. Experimental field cultivation of Gracilaria edulis (Gmel.) Silva. Botanica Marina 14: 71-75.
• Rebello, J., Ohno, M., Ukeda, H. & Sawamura, M., 1997. Agar quality of commercial agarophytes from different geographical origins: 1. Physical and rheological properties. Journal of Applied Phycology 8: 517-521.
• Thomas, P.C. & Subbaramaiah, K., 1990. Studies on the shedding of tetraspores in Gracilaria edulis (Gmel.) Silva from Mandapam Region. Phycos 29: 141-147.

Sources of illustration

Original drawings of the authors Phang, S.-M. & Lewmanomont, K. (habit, cross-section of thallus); Xia, B. & Zhang, J., 1999. Flora algarum marinarum sinicarum, vol. 2, Rhodophyta, 5. Academiae Sinicae Edita, Beijing, China. Fig. 27, p. 46 (all other drawings). Redrawn and adapted by P. Verheij-Hayes.

Authors

• S.-M. Phang, K. Lewmanomont & P. Gronier