It is also the only group within the diverse infraorder Brachyura (which contains 7000 valid species and subspecies in 93 families) that is endemic to this extreme environment. This is remarkable, as only one other brachyuran species from a very distant family is endemic to hydrothermal vents, but from shallow waters (i.e., Xenograpsus testudinatus in Xenograpsidae); and just a handful of opportunistic brachyuran species have been observed at deep-sea hydrothermal vents. Understanding the evolution of this important deep-sea hydrothermal vent taxon requires knowledge on the phylogenetic relationships among its members. As presently diagnosed, the family Bythograeidae consists of six genera and fourteen described species. The family is most diverse at the eastern Pacific vent systems (the East Pacific Rise, Galapagos Rift, and the Pacific-Antarctic Ridge), where it is represented by eight species belonging to three endemic genera: Cyanagraea praedator; Allograea tomentosa; Bythograea thermydron; Bythograea microps; Bythograea laubieri; Bythograea vrijenhoeki; Bythograea intermedia; and Bythograea galapagensis (although the last two are very likely a synonymy). The Mid-Atlantic Ridge is inhabited by Segonzacia mesatlantica. The Western Pacific back-arc basins are inhabited by Austinograea williamsi, Austinograea alayseae, Gandalfus puia, and Gandalfus yunohana. In addition, an undescribed Austinograea species (A. affinity williamsi) is suspected in the Western Pacific Lau back-arc basin. Finally, Austinograea rodriguezensis inhabits the Central Indian Ridge. The northeastern Pacific ridges (Explorer, Juan de Fuca, and Gorda) are the only major spreading centers that lack bythograeids. Some bythograeid species occur in large numbers at specific vent sites (e.g., B. thermydron and A. williamsi), whereas others appear to be rare (e.g., A. tomentosa and B. galapagensis).