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Genetic diversity and geographic differentiation of the giant tiger shrimp, Penaeus monodon, in Thai waters (Satun, Trang, Phangnga, and Ranong in the Andaman Sea and Chumphon and Trat in the Gulf of Thailand) were examined by COI polymorphism (N = 128). We observed 28 COI mitotypes across all investigated individuals. The sequence divergence between pairs of mitotypes was 0.00-20.76%. A neighbor-joining tree clearly indicated lineage separation of Thai P. monodon and large nucleotide divergence between interlineage mitotypes but limited divergence between intralineage mitotypes. High genetic diversity was found (mean sequence divergence = 6.604%, haplotype diversity = 0.716-0.927, pi = 2.936-8.532%). F-statistics (F(ST)) and an analysis of molecular variance (AMOVA) indicated that the gene pool of Thai P. monodon was not homogeneous but genetically differentiated intraspecifically (P < 0.05). Six samples of P. monodon could be allocated into three different genetic populations: Trat (A), Chumphon (B), and the Andaman samples Satun, Trang, Phangnga, and Ranong (C). Contradictory results regarding patterns of geographic differentiation previously reported by various molecular approaches were clarified by this study.

ABSTRACT. Genetic diversity and population differentiation of the blue swimming crab, Portunus pelagicus, in Thailand were analyzed by RAPD analysis. One hundred and twelve RAPD fragments were generated from 109 individuals of P. pelagicus using OPA02, OPA14, OPB10, UBC122, and UBC158 primers. The percentage of polymorphic bands in each geographic sample and that of each primer across overall samples were 72.7-85.0 and 92.0-100%, respectively. Large numbers of polymorphic bands found in the RAPD analysis suggested high genetic diversity of Thai P. pelagicus. The mean genetic distance between samples across all primers was 0.0929-0.2471. Significant geographic heterogeneity was observed across samples overall and between all pairs of geographic samples (P < 0.01 for θ and P < 0.0001 for the exact test), indicating strong genetic differentiation of P. pelagicus in Thai waters, despite its high potential of dispersal. Limited gene flow levels (0.44-1.19 individuals per generation) of Thai P. pelagicus were also observed. A fine scale level of differentiation suggested that P. pelagicus from each geographic sample in Thai waters should be regarded as a separate genetic population and treated as a different exploited stock.

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