Euryplacidae Stimpson, 1871

Ng, Peter K. L. & Castro, Peter, 2007, On a new genus and species of euryplacid crab (Crustacea: Decapoda: Brachyura: Goneplacoidea) from the Philippines, Zootaxa 1549, pp. 43-53 : 43-44

Family Euryplacidae Stimpson, 1871

Remarks. The taxonomy of the Goneplacidae sensu lato has always been contentious, with many workers using it as a “dumping ground” for problematic taxa. Over the years, some workers have started to reappraise the status of the various subfamilies traditionally placed in the family. The reappraisal of the Goneplacidae sensu lato started with the pioneering work of Guinot (1969a – c, 1971, 1979) when she introduced suites of characters that demonstrated the family grouping was heterogeneous. In recent years, the structure of the Goneplacidae has been discussed and re-examined by a number of authors ( Karasawa & Kato 2003a, b; Števčić 2005; Karasawa & Schweitzer 2006; Castro in press, Ng & Manuel-Santos in press) and some problems have been solved. Castro (in press) and Ng & Manuel-Santos (in press) reviewed in depth the taxonomic changes that have taken place and the various taxa that should now be regarded as part of the Goneplacoidea. This recognition of Goneplacoidea for taxa allied to the Goneplacidae sensu stricto has been necessary (see Števčić 2005; Karasawa & Schweitzer 2006), although its composition differs markedly from that proposed by Števčić (2005). Readers should consult Castro (in press) and Ng & Manuel-Santos (in press) for a full discussion of the taxonomic complexities of the superfamily.

With regard to the Euryplacidae Stimpson, 1871 , which had been regarded as a subfamily of the Goneplacidae sensu lato until recently, we regard the following characters as diagnostic for euryplacids: the male genital openings are coxo-sternal; the abdomen is distinctly T-shaped with somites 4–6 and the telson distinctly transversely narrowed, the press-button of the male abdominal locking mechanism is on the anterior edge of thoracic sternite 5 (near suture 4/5), the adult female abdomen is relatively narrow and does not completely cover the surface of the thoracic sternum, the G1 is very slender, elongated and simple, with the distal part tapering to a narrow apex and the distal surface lined with prominent spinules (dorsoventrally flattened, distal part truncated, with spinules only along margins in Sotoplax ; dorsoventrally flattened, with distal part only slightly expanded, without spinules in Chasmophora ); the G2 is short, always less than a third the length of the G1; and the vulva of the female is relatively small and lacks a vulvar cover (see also Castro (in press) for a summary of the differences between euryplacids and other taxa) and the thoracic sternite 8 is not visible (from ventral view) when the abdomen is closed.

The carapace characters enumerated by Števčić (2005) and Kato & Schweitzer (2006), although broadly useful, are not reliable as indicators of brachyuran phylogeny because there is too much convergence. Although carapace characters are useful in paleontological studies, sometimes being the only available characters, too much emphasis on them can lead to erroneous conclusions. Many previous cases show that the use of general carapace features can be misleading (see Guinot 1969a – c, 1971, 1978, 1979, 1989; Ng 1987; Ng & Clark 2000a, b; Castro et al. 2004; Ng 2003). To this effect, even though the carapace of Xenocrate new genus, is similar to that of some species of Carcinoplax H. Milne Edwards, 1852 , family Goneplacidae MacLeay, 1838 sensu Karasawa & Kato, 2003 (see also Castro in press), it otherwise possesses all the diagnostic features of the Euryplacidae as presently defined: male abdomen that is proportionately more slender (especially somites 4–6) and T-shaped; female abdomen that is relatively narrow, leaving a good part of the thoracic sternum exposed; relatively more slender G1 with a pointed apex and the distal part armed with many prominent spinules; and G2 that is much shorter than the G1 (less than a third the length of the G1) Xenocrate is undoubtedly an euryplacid. In addition, a ridged carapace, the slightly bilobed front, the presence of a large tooth on the inner margin of each suborbital margin, and the absence of a vulvar cover in Xenocrate are features that are diagnostic of euryplacids rather than of goneplacids.

Twelve genera are currently recognised as members of the Euryplacidae : Chasmophora Rathbun, 1914 , Eucrate De Haan, 1835 , Eucratodes A. Milne-Edwards, 1880 , Euryplax Stimpson, 1859 , Frevillea A. Milne- Edwards, 1880, Heteroplax Stimpson, 1858 , Machaerus Leach, 1818 , Nancyplax Lemaitre, García-Gómez , von Sternberg and Campos, 2001, Psopheticoides Sakai, 1969 , Sotoplax Guinot, 1984 , Trapezioplax Guinot, 1969 , and Trizocarcinus Rathbun, 1914 (see Guinot 1969a – c, 1971, 1978; Lemaitre et al. 2001; Castro in press). Surprisingly, only three euryplacid genera, Eucrate, Heteropla x and Psopheticoides , occur in the Indo- West Pacific, with the others being wholly Atlantic or eastern Pacific in distribution. Goneplax maldivensis Rathbun, 1902 , from the Indian Ocean is also clearly a euryplacid (see Guinot 1969b: 518; Castro in press). Števčić (2005) referred Rathbun’s species to a new genus, Otmaroplax, but his name is a nomen nudum. Karasawa & Schweitzer (2006) also list eight exclusively fossil genera in the family. A revision of the family Euryplacidae by the authors is now in progress and will help clarify the taxonomy of the group. Certain genera, like Chasmophora , are unusual and their taxonomic position will need to be re-appraised.