Gracilaria eucheumatoides (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

1, habit; 2, cross-section of a thallus; 3, longitudinal section of a thallus; 4, longitudinal section of a cystocarp; 5, longitudinal section of the pericarp of a cystocarp; 6, cross-section of a tetrasporophyte with tetrasporangia.

Gracilaria eucheumatoides Harv.

Protologue: Proc. Amer. Acad. Arts Sci. 4: 331 (1860). ("eucheumioides")

Family: Gracilariaceae

Chromosome number: 2n= probably 48, although assessment of nuclear genome sizes of G. eucheumatoides from the Philippines suggests a size which comes close to that of representatives of the genus Gracilariopsis E.Y. Dawson, which probably has 2n= 64.

Vernacular names

• Indonesia: duyung (Bangka, Lingga)
• Philippines: canot-canot (Ilokano), caocaoyan, cawat-cawat.

Origin and geographic distribution

G. eucheumatoides is found along tropical and subtropical coasts in the Indian and Pacific Oceans. In South-East Asia it has been recorded from Burma (Myanmar), the eastern coast of Thailand, Vietnam, Malaysia, Indonesia, the Philippines and Papua New Guinea.

Uses

G. eucheumatoides is used as a source of agar and as food (salad, or stew mixed with vegetables or as dessert gel). In Indonesia it is also used as a medicine to treat stomach ailments, goitre and urinary diseases. Other uses include animal feed, fish bait and insect repellent.

Production and international trade

No information on the production of G. eucheumatoides is available. It does not lend itself to farming due to its slow growth and very strict ecological requirements. All production is from wild populations.

Properties

G. eucheumatoides contains a good quality sugar-reactive agar, a hydrocolloid used in the food and in pharmaceutical and biotechnological industries. The general quality of agar from G. eucheumatoides is less than that of G. manilaensis H. Yamam. & Trono or Gracilariopsis heteroclada C.F. Zhang & B.M. Xia, but this quality can be improved to meet gel standards of the food industry by adopting an optimized alkali pretreatment in the agar extraction. Extraction methods involving alkali pretreatment (10% NaOH) at high temperatures (90 °C) for 2 hours resulted in a good agar yield with a maximum gel strength of 432 ± 43 g/cm² for a 1.5% agar solution. Higher extraction temperatures and longer extraction times may result in even higher gel strengths, but a substantially lower yield. The alkali-treated Gracilaria agars contain higher 3,6-anhydrogalactose contents and lower sulphate levels. Strongest gels were obtained in July, but there is no distinct seasonal pattern for gel strength in this alga, while high agar yields are generally correlated with lower gel strength. Viscosity of the G. eucheumatoides agar (measured at 75 °C) increases progressively from September (lowest value: 10 cP) to May (highest value: 320 cP), and thus is higher during the rainy season (May-October), while gelling and melting temperatures of the gel are generally higher during the dry season (December-March). The seasonal variations in contents of sulphate and 3,6-anhydrogalactose are generally small. The alga contains in g per 100 g dry matter: water 12.9, protein 7.9, fat 0.05, starch/sugars 58.4 and fibre 3.0.

Description

• Thalli greenish to purplish, cartilaginous in texture, thick and fleshy, forming loose or thick prostrate clumps, 3.4-8 cm long, attached to substrate by means of many haptera; branching irregular, dichotomous to alternate; branches flattened, irregular in width, up to 1 cm broad and with coarse, sharp teeth or short marginal spines.
• Medullary cells 130-132 μm in diameter, cell walls 3-6 μm thick, cell transition to cortex cells (20-30 μm to 7-13.5 μm × 4.5-9.0 μm) gradual.
• Gland cells elongate, 15-20 μm × 35-50 μm.
• Tetrasporangia cruciate, spherical, elongate in transverse section, 24-40 μm in diameter.
• Cystocarps globose, rarely constricted at its base, with pericarp 215-365 μm thick, differentiated into two layers.

Growth and development

G. eucheumatoides is known to be very slow growing.

Ecology

G. eucheumatoides occurs on rocks in the intertidal and subtidal zones in open sea or coral areas. It does not grow on soft substrates or in unattached populations.

Propagation and planting

G. eucheumatoides is not grown in phycoculture.

Phycoculture

There is no specific cultivation method for G. eucheumatoides. However, of the three methods used for other Gracilaria spp. the bottom-stocking method may be appropriate. This method involves the transfer of vegetative thalli, which are naturally attached to small stones and shells, to areas where an increased density is required. 10 mm mesh nylon, laced over rocks to keep thalli in place, can be used.

Harvesting

Selection of the best harvesting time for G. eucheumatoides depends on the intentions of the collector. To obtain a maximum agar yield harvesting is done during other months of the year than for obtaining optimum gel strength. If the intention is to obtain both a maximum agar yield and an optimal gel strength, a July harvest is recommended in the northern Philippines. This suggestion, however, is based on experiments executed in a single locality only, while experiments in other localities nearby may result in different recommendations.

Yield

The agar yield of G. eucheumatoides from samples from natural stock in the northern Philippines ranged from 20-29% (dry weight). It was lowest in March and highest in May.

Handling after harvest

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

Prospects

G. eucheumatoides is a good source of sugar-reactive agar.

Literature

• Abbott, I.A., 1992. New records and a reassessment of Gracilaria (Rhodophyta) from the Philippines. In: Abbott, I.A. (Editor): Taxonomy of economic seaweeds 4. pp. 111-118.
• Critchley, A.T., 1993. Gracilaria, Gracilariales, Rhodophyta: an economically important agarophyte. In: Ohno, M. & Critchley, A.T. (Editors): Seaweed cultivation and marine ranching. Japan International Cooperation Agency, Yokosuka, Japan. pp. 89-112.
• Istini, S., Zatnika, A. & Sujatmiko, W., 1998. The seaweed resources of Indonesia. In: Critchley, A.T. & Ohno, M. (Editors): Seaweed resources of the world. Japan International Cooperation Agency, Yokosuka, Japan. pp. 92-98.
• Villanueva, R.D., Montaño, N.E., Romero, J.B., Aliganga, A.K.A. & Enriquez, E.P., 1999. Seasonal variations in yield, gelling properties, and chemical composition of agars from Gracilaria eucheumoides and Gelidiella acerosa (Rhodophyta) from the Philippines. Botanica Marina 42: 175-182.
• Villanueva, R.D., Pagba, C.V. & Montaño, N.E., 1997. Optimized agar extraction from Gracilaria eucheumoides Harvey. Botanica Marina 40: 369-372.

Sources of illustration

Trono, G.C. & Ganzon-Fortes, E.T., 1980. An illustrated seaweed flora of Calatagan, Batangas, Philippines. University of the Philippines Marine Science Center & Filipinas Foundation, Manila, The Philippines. Fig. on p. 91 (habit); Chang, C.F. & Xia, B.M., 1976. Studies on Chinese species of Gracilaria. Studia Marina Sinica 12: Figs. 32 & 33, p. 134 (all other drawings). Redrawn and adapted by P. Verheij-Hayes.

Authors

• G.C. Trono Jr