Comparing the effect of larval dispersal strategies on morphological versus genetic differentiation in two neotropical fiddler crabs

The dispersal mode of estuarine invertebrates has a strong effect on genetic variability and phenotypic expression, potentially maintaining gene flow among widely separated populations or limiting connectivity among close ones. Understanding the dispersal strategies effects can support adequate strategies for environment management and understand the evolutionary history of the studied species. A comparative study on the effect of larval dispersal strategies (export vs retention) along the overall geographic distribution ranges of adult populations of two co-distributed fiddler crab species was performed. Geometric morphometric techniques were used to evaluate morphological differentiation in the shape of the carapace and major cheliped propodus in 10 populations of Minuca mordax and five populations of Leptuca leptodactyla. Potential genetic differentiation was assessed using DNA sequence data from up to 170 fiddler crab specimens, corresponding to the mitochondrial gene cytochrome c oxidase subunit 1 (COX1). Patterns of genetic and morphological characteristics diverged among populations of both species. In terms of morphology, populations of both species showed intraspecific variation in the shape of the carapace and, to a lesser extent, in the major cheliped propodus. This observed intraspecific variation followed a clear geographic pattern of morphometric differentiation, in which the geographic origin is associated with a phenotypic response to the environmental diversity along the extended Brazilian coast. In contrast, there are shared haplotypes in populations separated by more than 4000 km, but with significant Φ_ST values among most populations of L. leptodactyla and M. mordax, suggesting a moderate to high level of gene flow. Therefore, it can be concluded that the split of the Central South Equatorial Current has not acted as an efficient long-term barrier to the dispersal of larvae in both species. In M. mordax, a slight limitation of genetic exchange can be observed between the southernmost populations and the remaining ones. This first evidence of genetic structuring within this widely distributed species is consistent with its documented strategy to retain larvae in the estuarine environments. However, occasional transport of larvae to more open coastal waters seems to maintain stepwise gene flow between neighboring populations. Overall, our results still provide evidence of more pronounced genetic homogeneity in a species with a larval export strategy (L. leptodactyla) compared to a species with a larval retention strategy (M. mordax). This suggests that larval dispersal strategies, coupled with the microhabitat in which the respective species occur, have a noticeable effect on the degree of gene flow. Most likely, these biological aspects are further influenced by the estuarine and coastline geomorphology.