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


Sandfish (Holothuroids) or sea cucumbers in general play important ecological roles in the ecosystem. One role is remineralising large quantities of organic nutrients through their feeding and burying activities, which increases the benthic productivity of coral reefs. Further, the role of sea cucumbers in the dissolution of CaCO3 sediment provides an important source of alkalinity and may play a role in buffering ocean acidification at least at local scales on coral reefs.

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 × 10−4−17 × 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 (FST = 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.


By Valeli J. Bugota and Cyrus Rumisha