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.
Highlights
The giant mud crab (Scylla serrata) is widely distributed in the Indo-Pacific and it is the only Scylla species found at African shores [1]
The extension of the South Equatorial Current (SEC) northwest of Madagascar reaches the African coast around 11 ̊S, where it splits into the northward East African Coast Current (EACC) and the southward Mozambique Current (MC) [32]
Significant departure from the Hardy-Weinberg equilibrium (HWE) was shown by the loci SCY38 and SCY12 at sites 6 and 10, respectively
Summary
The giant mud crab (Scylla serrata) is widely distributed in the Indo-Pacific and it is the only Scylla species found at African shores [1]. The crabs provide an important source of income and food to coastal communities in East Africa [2]. Adult and juvenile mud crabs inhabit muddy estuaries and mangrove ecosystems where they can be found buried in mud or taking shelter in burrows at low tide [3,4]. The planktonic larvae undergo a series of up to five moults for a period of two to three weeks [6]. During this period, the larvae are susceptible of being transported by currents and tides to coastal areas where they settle in sheltered areas among mangroves and seagrass. Knowledge of the patterns of connectivity between sites is crucial for the identification of genetically meaningful management units
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