Phascolosoma esculenta is a newly developed mariculture species in south-eastern China. However, its wild populations are diminishing because of marine environmental pollution, beach reclamation, and overexploitation. To investigate the genetic diversity and genetic structure of the wild populations of P. esculenta, the entire sequence (1136 bp) of its mitochondrial cytochrome b (Cyt b) gene were sequenced and analyzed in 80 organisms sampled from four areas. A total of 59 variable sites and 49 haplotypes (61.25% of the total samples) were detected from the sequence, indicating a high level of polymorphism. High haplotype diversity (0.976±0.007) and low nucleotide diversity (0.00439±0.00027) were detected. Topologies of neighbor-joining trees were shallow and the clustering of haplotypes was not correlated with their geographical distributions. Both AMOVA and pairwise Fst analyses showed that the genetic differentiation between populations was relatively weak, which might be influenced by its life history, paleoclimate, and recent human activity. The neutrality test and mismatch-distribution analysis revealed that the expansion of P. esculenta populations occurred approximately 0.046 million years ago.
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