Phylogenomic analyses of brachyuran crabs support early divergence of primary freshwater crabs.

Recent fossil calibrated molecular phylogenies have revealed that the Brachyura underwent rapid radiation during the Cretaceous and Early Tertiary, resulting in many early diverging lineages separated by short internodes that remain difficult to resolve. Here we present the first phylogenomic analyses of Brachyura using transcriptome data from 30 brachyuran species and 22 families. Analyses were carried out on a dataset containing 372 putative homologous loci (246,590 bps) and included data from 21 newly generated transcriptomes. With minor exceptions, all phylogenetic analyses recovered a congruent, highly resolved and well supported brachyuran phylogeny. Consistent with previous work, this phylogeny suggests that primary freshwater crabs diverged early in brachyuran evolution, falling sister to Thoracotremata, thus supporting recent proposal for establishment of subsection Potamoida for primary freshwater crabs. The interfamilial relationships among heterotremes were well resolved in our analyses but those within Thoracotremata remained problematic. Phylogenomic analyses clearly provide a powerful means for resolving brachyuran relationships, but future studies would benefit greatly from increased taxon sampling of transcriptome data.