NAGA, WorldFish Center Quarterly Vol. 26 No. 4 Oct-Dec 2003 4 articles Genetic diversity in wild stocks of the giant freshwater prawn (Macrobrachium rosenbergii ): Implications for aquaculture and conservation P.B. Mather and M. de Bruyn Abstract The giant freshwater prawn ( Macrobrachium rosenbergii ) is cultured widely around the world but little is known about the levels and patterns of genetic diversity in either wild or cultured stocks. Studies have suggested that genetic diversity may be relatively low in some cultured stocks due to the history of how they were founded and subsequent exposure to repeated population bottlenecks in hatcheries. In contrast, wild stocks have an extensive distribution that extends from southern Asia across Southeast (SE) Asia to the Pacifi c region. Therefore, wild stocks could be an important resource for genetic improvement of culture stocks in the future. Understanding the extent and patterns of genetic diversity in wild giant freshwater prawn stocks will assist decisions about the direction future breeding programs may take. Wild stock genetic diversity was examined using a 472 base-pair segment of the 16S rRNA gene in 18 wild populations collected from across the natural range of the species. Two major clades ("eastern" and "western") were identifi ed either side of Huxley’s line, with a minimum divergence of 6.2 per cent, which implies separation since the Miocene period (5-10 MYA). While divergence estimates within major clades was small (maximum 0.9 per cent), evidence was also found for population structuring at a lower spatial scale. This will be examined more intensively with a faster evolving mtDNA gene in the future. Introduction Freshwater aquaculture has expanded rapidly in the Asia-Pacific region over the last 30 years, both in terms of the size of the industr y and the diversity of species that are cultured. W hile marine penaeid prawns remain the major crustacean group used in culture in the region, freshwater prawns of the genus M acrobrachium have seen a dramatic increase in production. The giant freshwater prawn ( M . rosenbergii) is the largest species in the genus (Fig. 1) and is by far the most important culture species. In Asia, har vesting of wild stocks also forms a significant industr y and production from the wild increased from 5 246 t in 1984 to approximately 130 000 t in 1998, with a value exceeding US $800 million (FAO 2000). Countries producing the largest volume of M . rosenbergii (1998 figures) include China (48 per cent), Bangladesh (37 per cent) and Thailand (6 per cent). W hile Asia accounts for more than 98 per cent of global production, culture of this species has expanded rapidly both within Asia and more widely in regions far removed from the species’ natural distribution (e.g. Brazil) (FAO 2000) and it is now cultured in at least 43 countries across five continents. The natural distribution of M . rosenbergii extends from Pakistan in the west to southern Vietnam in the east, across SE Asia, and south to northern Australia, Papua N ew Guinea, and some Pacific and Indian O cean Islands. M . rosenbergii lar vae require brackish water for sur vival and early development (N ew and Singholka 1985). Fully mature females migrate from freshwater to estuarine areas to spawn, where free-swimming larvae hatch from eggs attached to the female's abdomen. The larvae metamorphose into post-larvae after three to six weeks and then migrate upstream towards freshwater. W hile there has been little effort to determine if M. rosenbergii is capable of marine dispersal, lar vae have been raised to post-lar vae stage in 100 per cent artificial seawater, suggesting that the species may be capable of at least limited marine dispersal (Smith et al. 1976; Sandifer and Smith 1979). M . rosenbergii has been cultured in SE Asia using modern aquaculture techniques since the early 1960s. Early work by Fujimura (1966) at the Anuenue Fisheries Research Center in Hawaii resulted in the development of the successful mass- rearing techniques employed today for commercial-scale hatcher y production of post-lar vae. Brood stock from Malaysia were introduced to the Center in 1965, but consisted of only 12 individuals (Hedgecock et al. 1979). Later, brood stock from H awaii (mostly) and SE Asia were introduced into many regions where M. rosenbergii was not indigenous, including Fig. 1. M acrobrachium rosenbergii, the giant freshwater prawn