Reduced genetic diversity and alteration of gene flow in a fiddler crab due to mangrove degradation

Abstract

The fiddler crab Austruca occidentalis is a dominant species in mangrove forests along the East African coast. It enhances soil aeration and, through its engineering activities, makes otherwise-inaccessible food available for other marine organisms. Despite its importance, the habitat of A. occidentalis is threatened by human activities. Clearing the mangroves for salt farming and selective logging of mangroves trees continue to jeopardise mangrove ecosystems in the Western Indian Ocean. This study aims to use partial mitochondrial COI gene sequences and nuclear microsatellites to determine whether salt farming activities in mangroves have a negative impact on the genetic diversity and gene flow of A. occidentalis collected along the Tanzania coast. The level of genetic diversity for both mitochondrial DNA and nuclear microsatellites are relatively lower in samples from salt ponds compared to natural mangrove sites. Analysis of molecular variance (AMOVA) among all populations showed low but significant differentiation (COI: Fst = 0.022, P < 0.05; microsatellites: Fst = 0.022, P < 0.001). A hierarchical AMOVA indicates lower but significant genetic differentiation among populations from salt ponds and natural mangroves sites (COI: Fct = 0.033, P < 0.05; microsatellites: Fct = 0.018, P = < 0.01). These results indicate that salt farming has a significant negative impact on the genetic diversity of A. occidentalis. Since higher genetic diversity contributes to a stable population, restoring the cleared habitats might be the most effective measures for the conservation of genetic diversity and hence adaptive potential to environmental change in this species.

Genome-wide SNP analyses reveal population structure of Portunus pelagicus along Vietnam coastline

“…Historic migration results strongly supported the Panmixia model based on the highest Bezier approximation score (ln = -115475.9) in which migration was maintained among all sites with random mating between crab individuals ( Table 5 ). The analyses disclosed that there was no mating restriction between crab individuals in the history supposed to be over 1000s of years [32,83]. The populations were able to share genetic material either through larval dispersal due to currents or via migration of adult crabs.…”

“…This heterozygosity deficiency is also recorded in P . pelagicus populations in Malaysia [50], and in other marine and freshwater organisms due to widespread habitat loss, degradation and fragmentation [32,96,97]. Significant relatedness and sib-ships have been observed in marine populations due to biophysical larval behavior [98,99], self-recruitment [31,100], and overexploitation/restocking [101].…”

“…The management for this species should be careful to ensure that overfishing and habitat degradation do not further affect the vitality of existing populations. Immediate actions such as a seasonal ban on catching crabs in the autumn and late summer to increase successful spawning [32], establishment of marine reserves to reduce genetic losses [101], and coastal pollution control to increase numbers of breeding individuals and larval dispersal [28,33]. Moreover, gear regulation, habitat monitoring and restoration might be one of the most effective ways to manage healthy populations.…”

“…Historic gene flow between populations was estimated using the Bayesian inference implemented in MIGRATE-n v3.6.11 [82]. MIGRATE-n’s implementation of coalescent theory measures migration 4 × Ne generations in the past [32,83]. Sample sizes were reduced for each population to obtain 200 loci genotyped in 100% of individuals used for the analysis.…”

“…Recently, genetic studies have increasingly been applied to improve understanding of stock size, gene flow, distribution and migration patterns of subpopulations in mixed fisheries [24–27]. Population information including connectivity across species distribution range, exchange rate and source-sink dynamics are important for understanding potential impacts of bio-physical factors [28–31], human-induced fragmentation [32] and pollution [33,34], or overexploitation [35–37]. Among the wide-range of molecular approaches, restriction site-associated DNA sequencing (RADseq) is well known for its ability to identify and score thousands of single nucleotide polymorphisms (SNPs), which are randomly distributed across the target genomes using next generation sequencing [38–40].…”