Infraorder BRACHYURA Latreille, 1802

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Infraorder BRACHYURA Latreille, 1802

Compiler and date details

May 2012 - Peter Davie, Queensland Museum, Brisbane

Introduction

Brachyurans are known as the 'true-crabs', and, with more than 1070 species in Australian waters, total almost twice as many species as the next most diverse decapod group, the Caridea. A few families include species of commercial importance, most significant of which are the swimming crabs (Portunidae), with among them the Mud Crab (Scylla species) and the Blue-swimmer or Sand Crab (Portunus armatus). Other families that include species of lesser fisheries importance are the Menippidae (the Giant Crab, Pseudocarcinus gigas, the Hypothalassiidae (Deepwater Spiny Crab, Hypothalassia species); and the Geryonidae (Chaceon species).

As some indication of how much we still have to discover about crabs in general, and Australian crabs in particular, in the Zoological Catalogue of 2002 there were the 42 families of crabs recognised from in Australia. This number has now grown to 67 as the relationships between groups are better understood, and many subfamilies have been given full family status. In 2002 there were also only 969 species in 369 genera, and in the intervening 10 years this has grown by over 10 per cent to 1069 species in 419 genera.

The family structure used here follows the Systema Brachyurorum of Ng et al. (2008), and Ahyong et al. (2011). Explanations for the latest changes will be developed in the revised edition of the Systema Brachyurorum likely to be published in late 2012.

Despite the high profile of crabs in the public consciousness, phylogenetic relationships among families are only now coming under intense scrutiny. More critical analyses have only really been undertaken over about the last 35 years, and only sporadically. Until the mid-seventies, most authors followed a classificatory system dating back to that of Borradaile (1903), refined by Balss (1957). Guinot (1977, 1978) was the first to change radically our perceptions of phylogeny with her re-evaluation of family relationships based on a suite of new characters. The keystone to her system is the changing disposition of the genital openings which move from the plesiomorphic coxal position (Podotremata), into a hybrid coxo-sternal position (Heterotremata), and finally onto the sternum proper (Thoracotremata). Thus, she rejected the older classification of Oxystomata, Oxyrhyncha, Cyclometopa, and Catametopa, based on more plastic carapace morphologies. While Guinot's classification was not fully embraced immediately by all workers, it gradually received growing support and acceptance. In particular, the position of her "podotreme" groups as the most primitive crabs is well supported (as opposed to the Eubrachyura proposed by de Saint Laurent (1980a, 1980b) to include both the Heterotremata and Thoracotremata). However, the most recent work now indicates that the Podotremata is probably itself paraphyletic, and some workers are suggesting the use of three separate Sections: the Dromiacea, the Raninoida, and the Cyclodorippoida, as all being primitive sister groups to the Eubrachyura, instead of the collective Podotremata.

Diagnosis

Carapace progressively shortened and widened; fused to epistome. Eyes stalked, compound; sometimes reduced. Antennules with peduncle 3-segmented; flagella often reduced or vestigial. Antennal peduncles usually with one or two segments; usually without exopod; flagella short, sometimes absent. Mandibles with or without palp; molar and incisor processes usually not distinct. Maxillulae usually with endopodal palp. Maxillae biramous; usually with bilobed endites. Maxillipeds with flagella often reduced or absent; third maxilliped often with ischium and merus flattened, operculate. Pereiopods: first chelate; fifth (or fourth and fifth) sometimes subchelate; fifth occasionally paddle-shaped; fifth rarely markedly reduced or absent. Sexes separate; gonopores on coxae or sternite of third pereiopods in female, on fifth in male. Abdomen with male first and second pairs of pleopods modified as gonopods, third to fifth absent; in females second to fifth usually developed, rarely first pair also developed; uropods in both sexes usually absent, sometimes present in Dromioidea; telson usually reduced. Segmentation: head with 5 + 3 thoracic (maxillipeds); thorax with 5, abdomen with 6 or fewer (some frequently coalesced), excluding telson.