The red alga Gracilaria contributes approximately 66% of the total agar production, according to the most recent estimates. This contribution is likely to increase as culti vation expands and technologies are developed to increase the gel strength of Gracilaria. Although more than 150 species of Gracilaria have been reported from different parts of the world, the taxonomy of the genus is still in flux. Gracilaria is widely distributed all over the world, but most of the species are reported to be from subtropical and tropical waters. Major Gracilaria producing countries are by far, according to 2004 FAO data, Chile and China, followed by Taiwan, South Africa, Namibia, the Philippines, and Vietnam. Morphologically, the thallus of Gracilaria is cylindrical, compressed, or bladelike and irregularly branched, giving a bushy appearance. Gracilaria has a typical Polysiphonia type or triphasic life history. The male and female gametophytes in the early stages appear identical without the aid of a magnifying lens. Subsequently, the latter can be easily identified by the presence of cystocarps, which appear as distinct hemispherical lumps all over the thalli. The cystocarp releases a large number of carpospores (2n) that give rise to the tetrasporophyte plants (2n). Each diploid tetrasporophytic plant is morphologically similar to the haploid gametophytic plants (i.e., they are iso morphic). The tetrasporophyte phase produces haploid tetraspores by meiosis within cortical sporangia. The tetrasporangia ultimately give rise to tetraspores that germinate into male and female plants, thus completing the triphasic life cycle.
Gracilaria is cultivated commercially through a num ber of methodologies.
Site selection for Gracilaria seedling cultivation is critical. Sites should be located near seawater sources for open water cultivation. For pond cultivation, sites should be located near both seawater and freshwater sources to insure salinity control. Sites should also be protected from strong winds. Gracilaria tolerate a wide range of salinities (10-24 psu), but it is important to check other ecological conditions such as temperature, light, and pH (>7.5-8.0). Healthy branches of Gracilaria from natural stock must be selected for successful farm ing. Thalli of Gracilaria are usually vegetatively propagated for successful large scale production; how ever, in some instances spores (either carpospores or tetraspores) may also be used to seed substrates for some farms.
Gracilaria cultivation is mainly practiced in three dif ferent ways: open water cultivation, pond culture, and tank culture. Open water cultivation is practiced in estuaries, bays, and upwelling areas. Gracilaria has been cultivated in ponds on a large scale only in China and Taiwan. Ponds are generally located in areas not exposed to strong wind, situated near the sources of both freshwater and seawater. Several species of economically important marine organisms (e.g., shrimp, crabs, fish, and prawns) are co cultured in the same pond at the same time - a type of polyculture integrated multitrophic system. The use of tanks may provide the greatest productivity per unit area and is more efficient than any other type of farming. In this type of system, several steps can be precisely controlled and managed to reduce the labor input, although this type of system has high operational (especially energy) costs. Tank systems may hold promise for the processing of nutrient enriched waters from fed aquaculture systems (i.e., from finfish or shrimp aquaculture within an integrated multitrophic aquaculture system). Figure 4 illus trates the most common cultivation techniques used. Figures 4a-4d show the bottom stocking method with rocky substrata, insertion of Gracilaria in soft sediment, and bottom stoking with Gracilaria attached to a plastic tube, usually filled with sand. Figures 4e-4g show the method of Gracilaria cultivation attached to ropes in long line systems or in raft systems. The FAO has published several technical papers on Gracilaria cultivation.
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