Salinity tolerance explains the contrasting phylogeographic patterns of two swimming crabs species along the tropical western Atlantic

Patterns and processes of species diversification in the oceans are still not fully understood. Traditionally, studies have been using the pelagic larval duration (PLD) to explain the genetic structure and phylogeographic history of marine taxa. However, this trait has given inconsistent results, especially when there is a physiological barrier. Phylogeographic studies comparing species that have similar PLD but differ in other important traits can indicate which ones drive intraspecific evolution. To test our hypothesis, we selected two species with similar distribution and PLD and different salinity tolerance to explore the role of Amazon-Orinoco plume (the biggest freshwater discharge into the ocean worldwide) in the diversification of western Atlantic species. We amplified mtDNA markers (COI and 16S rRNA) ofCallinectes ornatus(less tolerant to low salinity) andC. danae(tolerant to low salinity) from both sides of the Amazon-Orinoco plume (four biogeographical provinces). Then, we performed genetic structure, historical demography, divergence time, and biogeographic modelling analyses. Our results show contrasting phylogeographic and demographic patterns that can be explained by salinity tolerance. The Amazon-Orinoco plume represents a barrier forC. ornatus, which has two evolutionary units (ESUs). The plume is not a barrier forC. danae, which has no genetic structure. Furthermore,C. ornatusis formed by an ancestral Caribbean group that dispersed to the southwestern Atlantic after the establishment of the Amazon-Orinoco plume.Callinectes danaehas undergone demographic changes during the Last Glacial Maximum, probably due to the loss of estuarine habitats due to sea level fall, whileC. ornatuswas not affected because it is absent in this type of environment. Therefore, we show that ecological traits of marine taxa, like salinity tolerance, are more reliable predictors of genetic variation than the usually used larval dispersal potential.