Genetic diversity and connectivity in the East African giant mud crab Scylla serrata: Implications for fisheries management

Abstract

The giant mud crab Scylla serrata provides an important source of income and food to coastal communities in East Africa. However, increasing demand and exploitation due to the growing coastal population, export trade, and tourism industry are threatening the sustainability of the wild stock of this species. Because effective management requires a clear understanding of the connectivity among populations, this study was conducted to assess the genetic diversity and connectivity in the East African mangrove crab S. serrata. A section of 535 base pairs of the cytochrome oxidase subunit I (COI) gene and eight microsatellite loci were analysed from 230 tissue samples of giant mud crabs collected from Kenya, Tanzania, Mozambique, Madagascar, and South Africa. Microsatellite genetic diversity (He) ranged between 0.56 and 0.6. The COI sequences showed 57 different haplotypes associated with low nucleotide diversity (current nucleotide diversity = 0.29%). In addition, the current nucleotide diversity was lower than the historical nucleotide diversity, indicating overexploitation or historical bottlenecks in the recent history of the studied population. Considering that the coastal population is growing rapidly, East African countries should promote sustainable fishing practices and sustainable use of mangrove resources to protect mud crabs and other marine fauna from the increasing pressure of exploitation. While microsatellite loci did not show significant genetic differentiation (p > 0.05), COI sequences revealed significant genetic divergence between sites on the East coast of Madagascar (ECM) and sites on the West coast of Madagascar, mainland East Africa, as well as the Seychelles. Since East African countries agreed to achieve the Convention on Biological Diversity (CBD) target to protect over 10% of their marine areas by 2020, the observed pattern of connectivity and the measured genetic diversity can serve to provide useful information for designing networks of marine protected areas.

Fig 1

A. Map of the East African coast showing sample sites. SEC = South Equatorial Current, EACC = East African Coast Current, MC = Mozambique Current, NEMC = Northeast Madagascar Current, SEMC = Southeast Madagascar Current, AC = Agulhas Current. Main ocean currents were drawn according to [31]. B. Bar charts showing the likelihood of individual genotypes of belonging to different groups inferred by STRUCTURE analysis. C. Haplotype network of partial cytochrome oxidase subunit I sequences. Each circle represents a haplotype. Size of each circle is proportional to the number of individuals carrying each haplotype. The central haplotype represents 109 sequences. Hatch marks = mutations. EA = sites on mainland East Africa, West coast of Madagascar, and Seychelles. ECM = East Coast of Madagascar.

Fig 2. Pairwise mismatch distribution showing a unimodal distribution of the cytochrome oxidase subunit I haplotypes in the East African giant mud crab Scylla serrata.