Phylogeography of Geothelphusa freshwater crabs: unexpected dual dispersal routes via land and sea

AimDispersal is an important factor that determines the potential for colonization to pioneer sites. Most decapods employ seaward migration for reproduction with a planktonic larval phase. However, true freshwater crabs spend their entire life cycle in freshwater. Therefore, it is expected that genetic regionality can be easily detected. In this study, we focused on the genetic structures of true freshwater crabs, Geothelphusa crabs. Herein, we reveal the evolutionary history and dispersal patterns of freshwater crustaceans, for which there is limited knowledge to date. LocationJapanese Islands TaxonGeothelphusa dehaani (Decapoda, Potamidae) MethodsWe collected and genetically analyzed 283 specimens at 138 localities from freshwater habitats across the Japanese Islands. Phylogenetic analyses were conducted on 1,796 bp of the combined dataset (mtDNA COI, 16S, and nDNA ITS1, histone H3) and 569 bp of the mtDNA COI dataset. The demographic history of G. dehaani was simulated using Approximate Bayesian Computation analysis. A salt tolerance experiment was conducted to confirm the survival rate in seawater. ResultsThe resulting of phylogenetic relationships detected 10 clades that were highly likely to be monophyletic. These 10 clades genetically exhibited an explicit pattern of geographical differentiation. Also, we confirmed the salt tolerance ability of these Japanese freshwater crabs. Main conclusionsThe highlights of this study were the discovery of several cryptic species/lineages or undescribed species, and the completely different heterogeneous dual dispersal pathways detected within a single species; i.e., both land and ocean routes. As a result of phylogenetic analysis, it was concluded that Japanese crabs are basically genetically divided by straits. However, strong evidence for dispersion via ocean currents was also detected (i.e., a "sweepstake"), and it was also determined that G. dehaani could survive in seawater. This is the first observation of such a unique mode of expansion of a species distribution area.