Abstract
Fiddler crabs (Brachyura, Ocypodidae), like many other marine organisms, disperse via planktonic larvae. A lengthy pelagic larval duration is generally assumed to result in genetic connectivity even among distant populations. However, major river outflows, such as of the Amazon or Orinoco, or strong currents may act as phylogeographic barriers to ongoing gene flow. For example, the Mona Passage, located between Puerto Rico and Hispaniola, has been postulated to impair larval exchange of several species. In this study, Cox1 mtDNA data was used to analyze population genetic structure of two fiddler crab species from the western Atlantic, comparing the continental coastline and Caribbean islands. The results indicate genetic homogeneity in Minuca rapax among Atlantic (continental) populations (Suriname, Brazil), whereas Caribbean populations show significantly restricted gene flow among the constituent islands and towards continental populations. Our data support the hypothesis of the Mona Passage hindering larval exchange. Contrastingly, Caribbean Leptuca leptodactyla populations appear to be devoid of detectable variation, while Atlantic-continental (i.e. Brazilian) populations show much higher haplotype and nucleotide diversities and display slight genetic differentiation among populations within the Atlantic region, though not statistically significant. Both species show a pronounced divergence between regions, supporting the presence of a phylogeographic barrier.
Highlights
Ongoing gene flow among widespread populations is essential for genetic homogeneity within a species, while its disruption leads to genetic differentiation and heterogeneity
As expected from previous studies on the genetic structure of multiple fiddler crab species in this area [36, 39, 46], M. rapax exhibits a clear lack of genetic structuring along the Atlantic mainland coast
While we found strong evidence in favor of the Mona Passage impeding gene flow between western and eastern M. rapax populations, it remains untested whether the passage has a similar effect on L. leptodactyla due to a lack of appropriate samples available to our research at the time this study took place
Summary
Ongoing gene flow among widespread populations is essential for genetic homogeneity within a species, while its disruption leads to genetic differentiation and heterogeneity. Being rather limited in spatial expansion as adults, a dispersive planktonic larval stage is part of the reproductive strategy of many coastal marine organisms [1,2,3]. The longer the timespan that larvae spend in the plankton, the greater the distances they can be transported by water currents, and the larger the geographic range within which gene flow counteracts genetic structuring. Clinal Genetic Diversity in Western Atlantic Fiddler Crabs
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