A comprehensive molecular phylogeny of the brachyuran crab superfamily Xanthoidea provides novel insights into its systematics and evolutionary history.

Maximum likelihood and Bayesian phylogenies for the brachyuran crab superfamily Xanthoidea were estimated based on three mitochondrial and four nuclear genes to infer phylogenetic relationships and inform taxonomy. Habitat data was then used in conjunction with several diversification rates analyses (BAMM, BiSSE, HiSSE, and FiSSE) to test evolutionary hypotheses regarding the diversification of xanthoid crabs. The phylogenies presented are the most comprehensive to date in terms of global diversity as they include all four constituent families (Xanthidae, Panopeidae, Pseudorhombilidae, and Linnaeoxanthidae) spanning all oceans in which xanthoid crabs occur. Six Xanthoidea families are recognised. Panopeidae and Xanthidae sensu stricto are the two largest family-level clades, which are reciprocally monophyletic. Pseudorhombilidae is nested within and is here treated as a subfamily of Panopeidae. Former subfamilies or tribes of Xanthidae sensu lato are basally positioned clades in Xanthoidea and are here assigned family-level ranks: Garthiellidae, Linnaeoxanthidae, Antrocarcinidae, and Nanocassiopidae. The genera Linnaeoxantho and Melybia were recovered in separate clades with Linnaeoxantho being sister to the family Antrocarcinidae, while Melybia was recovered within the family Panopeidae. The existing subfamily classification of Xanthidae and Panopeidae is drastically restructured with 20 xanthid and four panopeid subfamilies provisionally recognised. Diversification-time analyses inferred the origin of Xanthoidea and Garthiellidae in the Eocene, while the other families originated during the Oligocene. The majority of genus- and species-level diversification took place during the Miocene. Ancestral state reconstruction based on depth of occurrence (shallow vs. deep water) shows some ambiguity for the most recent common ancestor of Xanthoidea and Nanocassiopidae. The most recent common ancestors of Antrocarcinidae and Panopeidae were likely deep-water species, while those of Garthiellidae and Xanthidae were probably shallow-water species. Several shifts in net diversification rates were detected but they were not associated with depth-related habitat transitions.

Evolutionary history of true crabs (Crustacea: Decapoda: Brachyura) and the origin of freshwater crabs.

Crabs of the infra-order Brachyura are one of the most diverse groups of crustaceans with approximately 7,000 described species in 98 families, occurring in marine, freshwater, and terrestrial habitats. The relationships among the brachyuran families are poorly understood due to the high morphological complexity of the group. Here, we reconstruct the most comprehensive phylogeny of Brachyura to date using sequence data of six nuclear protein-coding genes and two mitochondrial rRNA genes from more than 140 species belonging to 58 families. The gene tree confirms that the "Podotremata," are paraphyletic. Within the monophyletic Eubrachyura, the reciprocal monophyly of the two subsections, Heterotremata and Thoracotremata, is supported. Monophyly of many superfamilies, however, is not recovered, indicating the prevalence of morphological convergence and the need for further taxonomic studies. Freshwater crabs were derived early in the evolution of Eubrachyura and are shown to have at least two independent origins. Bayesian relaxed molecular methods estimate that freshwater crabs separated from their closest marine sister taxa ~135 Ma, that is, after the break up of Pangaea (∼200 Ma) and that a Gondwanan origin of these freshwater representatives is untenable. Most extant families and superfamilies arose during the late Cretaceous and early Tertiary.

Phylogenetics of the brachyuran crabs (Crustacea: Decapoda): the status of Podotremata based on small subunit nuclear ribosomal RNA.

The true crabs, the Brachyura, are generally divided into two major groups: Eubrachyura or 'advanced' crabs, and Podotremata or 'primitive' crabs. The status of Podotremata is one of the most controversial issues in brachyuran systematics. The podotreme crabs, best recognised by the possession of gonopores on the coxae of the pereopods, have variously been regarded as mono-, para- or polyphyletic, or even as non-brachyuran. For the first time, the phylogenetic positions of the podotreme crabs were studied by cladistic analysis of small subunit nuclear ribosomal RNA sequences. Eight of 10 podotreme families were represented along with representatives of 17 eubrachyuran families. Under both maximum parsimony and Bayesian Inference, Podotremata was found to be significantly paraphyletic, comprising three major clades: Dromiacea, Raninoida, and Cyclodorippoida. The most 'basal' is Dromiacea, followed by Raninoida and Cylodorippoida. Notably, Cyclodorippoida was identified as the sister group of the Eubrachyura. Previous hypotheses that the dromiid crab, Hypoconcha, is an anomuran were unsupported, though Dromiidae as presently composed could be paraphyletic. Topologies constrained for podotreme monophyly were found to be significantly worse (P < 0.04) than unconstrained topologies under Templeton and S-H tests. The clear pattern of podotreme paraphyly and robustness of topologies recovered indicates that Podotremata as a formal concept is untenable. Relationships among the eubrachyurans were generally equivocal, though results indicate the majoids or dorippoids were the least derived of the Eubrachyura. A new high level classification of the Brachyura is proposed.