Low genetic diversity and lack of genetic structure among populations of the sandfish Holothuria (Metriatyla) scabra on the Tanzanian coast

The tropical sandfish Holothuria (Metriatyla) scabra Jaeger, 1833 has been severely depleted in the Western Indian Ocean because it is easy to catch and in high demand. In response, Mainland Tanzania closed the fishery; however, the fishery remains open in the semi-autonomous islands of Zanzibar. Yet, it is unknown whether the aforementioned contrasting management measures are consistent with the genetic stock structure of the fishery. Therefore, this study analysed partial sequences (706 base pairs) of the cytochrome oxidase subunit I gene (COI) from four sandfish populations from Tanzania to assess the extent of genetic diversity and population structure. The populations showed low haplotype (0.17-0.44) and nucleotide diversities (0.025-0.084%), as well as small mutation-scaled effective size (9 x 10(-4)-17 x 10(-4)) compared to other marine macroinvertebrates from Tanzania, suggesting that the fishery has not recovered despite the ban that was imposed 16 years ago. Furthermore, Analysis of Molecular Variance (AMOVA) revealed a low and non-significant genetic differentiation index (F-ST = 0.0047, p > 0.05), indicating a lack of population structure. The lack of population structure was further supported by phylogenetic analysis, which grouped together all COI haplotypes of sandfishes from both Zanzibar and Mainland Tanzania. This suggests that populations in Mainland Tanzania and Zanzibar are genetically homogeneous and that Zanzibari fishers may be targeting the same stock that is protected in Mainland Tanzania. Therefore, the fishery should be managed as a single unit and management measures between Mainland Tanzania and Zanzibar should be harmonized.