A new family Leptograpsodidae n. fam. is erected to include an unusual genus of grapsoid crabs, Leptograpsodes Montgomery, 1931, represented by a burrowing species from South Australia L. octodentatus (H. Milne Edwards, 1837), with setal pouches on the coxae of pereiopods 2 to 4 that are evidence of a high degree of terrestrial adaptation. With nearly 600 extant species, the superfamily Grapsoidea MacLeay, 1838 encompasses a diverse range of morphologies and ecologies. Genetic data question its monophyly, whereas the inter- and intrarelationships of included taxa have not been recently examined through a morphological review using a reliable complex of characters (primarily, proepistome, thoracic sternum, male pleon, male genital region, male gonopore, and penis). The reevalution of the morphological characters of L. octodentatus in order to reappraise its proper taxonomical status has afforded us the opportunity to review most of grapsoids and, in particular, the Gecarcinidae H. Milne Edwards, 1837 (see Appendix), Grapsidae MacLeay, 1838, Varunidae H. Milne Edwards, 1853 (especially Cyclograpsus H. Milne Edwards, 1837), and Xenograpsidae N. K. Ng, Davie, Schubart & Ng, 2007. The discovery that Leptograpsodes shares some characters with Gecarcinidae, especially a stridulatory apparatus similar to that of Discoplax A. Milne-Edwards, 1867, a feature not previously recorded in either genus, has revealed the need to re-examine the taxonomy of Gecarcinidae. This resulted in an unexpected outcome. Discoplax must be restricted to its three troglobitic species: D. longipes A. Milne-Edwards, 1867 (type species), D. gracilipes Ng & Guinot, 2001, and D. michalis Ng & Shih, 2015. Cardisoma Latreille, 1828 sensu stricto only accommodates C. guanhumi Latreille, in Latreille, Le Peletier, Serville & Guérin, 1828 (type species), C. armatum Herklots, 1861, C. carnifex (Herbst, 1796), and C. crassum Smith, 1870. A new genus, Tuerkayana n. gen. is hereby established to include two species previously assigned to Cardisoma: T. rotundum (Quoy & Gaimard, 1824) n. comb. (type species), T. hirtipes (Dana, 1851) n. comb., plus two others previously regarded as Discoplax: T. celeste (Ng & Davie, 2012) n. comb. and T. magnum (Ng & Shih, 2014) n. comb. Compared to Varunidae, Leptograpsodes bears a superficial resemblance to Cyclograpsus H. Milne Edwards, 1837, a paraphyletic genus that must be restricted to its type species C. punctatus H. Milne Edwards, 1837 and only its closest congeners. The validity of the subfamilial taxon Heliceinae Sakai, Türkay & Yang, 2006 is recognised. The putative sister-group relationship of Leptograpsodes and Xenograpsus supported by molecular analyses of Schubart (2011) and Ip et al. (2015) is not confirmed by morphology, even if the two families share some traits. The synapomorphies of Leptograpsodes justify its separation from all grapsoid lineages as currently conceived. The use of previously overlooked traits, notably related to the thoracic sternum, proves to represent an optimal support for the brachyuran systematics and phylogeny, and presently to redefine the grapsoid taxonomical categories. The complete fusion of thoracic sternites 3 and 4 without any visible suture, even laterally, is proposed as a synapomorphy of Grapsoidea; this character state could be extented to other Thoracotremata in the future. Species of three genera Leptograpsodes, Discoplax and Epigrapsus are reported to exhibit stridulating structures (suborbital pars stridens and plectrum on inner margin of P1 merus). Some nomenclature issues, notably the authorship of the family-group name Gecarcinidae that is here credited to H. Milne Edwards, 1837, are addressed in the Appendix.
Varunidae, Xenograpsidae, Heliceinae, Cyclograpsinae, Cyclograpsus, Discoplax, Cardisoma, thoracic sternum, new combinations, new genus, new family.