XANTHIDAE MacLeay, 1838

Mendoza, Jose Christopher E., Clark, Paul F. & Ng, Peter K. L., 2012, The identity of Pilumnoplax acanthomerus Rathbun, 1911 (Crustacea: Decapoda: Brachyura: Xanthidae), with new records from the central and western Pacific *, Zootaxa 3367 (1), pp. 211-221 : 213-215

XANTHIDAE MacLeay, 1838

Linnaeoxanthinae Štev č i ć, 2005

Linnaeoxanthinae Števčić, 2005: 45.

Melybiidae Števčić , in Martin & Davis, 2001: 112 (nomen nudum).

Melybiidae Števčić, 2005: 88 .

Diagnosis. Carapace subhexagonal, wider than long; dorsal surface slightly convex, regions poorly indicated. Eyes relatively large. Supraorbital margin with distinct notch. Basal antennal article subrectangular, flagellum long. Posterior margin of epistome with median part straight. Male thoracic sternum wide; median line present only at level of sternites 7, 8. Chelipeds long, unequal; merus, carpus spinose; chelae flattened. Merus, carpus and propodus of ambulatory legs spinose on anterior margin; merus of P2, P3 with subdistal spine on posterior margin. G1 relatively stout; G2 about one-third length of G1, terminal segment short. Male gonopore on coxa of P5.

Remarks. See genus.

Linnaeoxantho Štev č i ć, 2005

Pilumnoplax . — Rathbun 1911: 237 (partim).

Neopilumnoplax Serène, 1968: 90 (partim) (nomen nudum) (not Neopilumnoplax Serène , in Guinot, 1969: 689). Linnaeoxantho Števčić, 2005: 45 .

Linnaeoxanthus [sic]. — Ng et al. 2008: 203. — De Grave et al. 2009: 43.

Type species. Pilumnoplax acanthomerus Rathbun, 1911 , by monotypy; gender feminine.

Diagnosis. Carapace subhexagonal, wider than long; dorsal surface regularly finely granulate, slightly convex, regions poorly indicated. Front wide, bilobate, ventrally deflexed, with distinct row of granules posterior to frontal margin. Eyes relatively large, corneas well developed. Supraorbital margin with distinct notch. Basal antennal article subrectangular, flagellum long. Anterolateral margin much shorter than posterolateral margin; with 4 acute projecting teeth, including exorbital angle, fourth tooth small, spiniform. Buccal cavity widened anteriorly, not completely covered by third maxillipeds. Posterior margin of epistome with median part straight. Male thoracic sternum wide; median line present only at level of sternites 7, 8. Chelipeds long, unequal; merus, carpus spinose; chelae flattened; palm longer than fingers, fingers stout, grooved, with small tufts of setae; fixed finger with large tooth near tip, dactylus keeled on superior margin. Ambulatory legs moderately long, merus, carpus and propodus spinose on anterior margin; merus of P2, P3 with subdistal spine on posterior margin. G1 relatively stout, distal half somewhat flattened, spinose, gently curved outward. G2 about one third length of G1, terminal segment short. Male gonopore on coxa of P5, just anterior to coxo-sternal condyle.

Remarks. Rathbun (1911) originally assigned her species to Pilumnoplax (junior synonym of Eucrate ), and by Serène (1968), with doubt, to Neopilumnoplax ( Table 1). This species has been attributed to a number of families including Goneplacidae MacLeay, 1838 , by Serène (1968), Euryplacidae , by Manning & Holthuis (1981) and Mathildellidae , by Karasawa & Kato (2003). But according to Guinot (1969), Ng & Manuel-Santos (2007) and Castro & Ng (2010), it does not belong to any of these families. Serène & Lohavanijaya (1973) also briefly mentioned that, due to its short G2, the species does not belong to Goneplacidae-Carcinoplacidae. Goneplacids, euryplacids and mathildellids all have freely articulating abdominal somites 3–5 ( Castro & Ng 2010: fig. 1; Ng & Manuel-Santos 2007: fig. 10D–F) (vs. fused, Fig. 4C ). Furthermore, euryplacids have a G1 tapering almost to a point ( Castro & Ng 2010: figs. 11E, 14) (vs. relatively stout and blunt, Fig. 4F–H ), and a long penis protected by a penial groove or tube ( Castro & Ng 2010: figs. 16E, 17G) (vs. short, unprotected penis, Fig. 4B ). Mathildellids also have a slender, tapering G1 ( Ng & Manuel-Santos 2007: 45) (vs. stout, blunt G1, Fig. 4F–H ), and a G2 as long as the G1 ( Ng & Manuel-Santos 2007: 45) (vs. G2 about half as long as G1, Fig. 4I ).

Despite the superficial carapace resemblance to some goneplacoids, Linnaeoxantho has more xanthoid characters, but its general morphology is so unusual that Števčić (2005) established Linnaeoxanthinae for it within Xanthidae . However, he did not provide any reason for this action other than a brief, generalized description. This present study suggests that Linnaeoxantho can be classified in the Xanthidae on account of the third to fifth somites of the male abdomen being immovably fused and the stout, curved G1 which is twice as long as the G2. Ng et al. (2008) did not follow the monotypic establishment of Linnaeoxanthinae as proposed by Števčić (2005) to accommodate Linnaeoxantho . Instead they retained this genus within the Xanthinae , prior to a re-examination of the types and an elucidation of the relationships within this apparently polyphyletic and artificial grouping. Recently, Lai et al. (2011) presented a molecular phylogeny of the Xanthidae , supported by morphological characters, the most comprehensive analysis on the family thus far. They showed that the subfamily Xanthinae is indeed polyphyletic, and suggested that more subfamily-level taxa should be recognised. Now that the type material has been found and more specimens have been examined, Linnaeoxantho is here considered most similar to the monotypic Melybia Stimpson, 1871 , from the Caribbean (type species: Melybia thalamita Stimpson, 1871 ; see Stimpson, 1871: 144; A. Milne-Edwards, 1880: 275; Rathbun 1930: 562, pl. 230 figs. 1, 2; Williams 1984: 430, fig. 342) in the general shape of the carapace and pereiopods. In particular, both taxa have (1) a transversely subhexagonal carapace ( Figs. 1 , 2 ); (2) prominently quadridentate carapace anterolateral margins ( Figs. 1 , 2 ); (3) a relatively broad, bilobate and deflexed front ( Figs. 2 , 3B ); (4) a broad, oblique superior orbital margin, with large eyes and well-developed corneas ( Fig. 2 ); (5) narrow mxp3 which are widely separated from each other ( Figs. 3A, 3B ); (6) long, unequal chelipeds, with spinose meri and carpi, flattened chelae, with a double row of spiniform tubercles on the superior border of the palm, and fingers much shorter than the palm ( Figs. 1 , 2 , 3E, F ); and (7) moderately long and slender ambulatory legs, with anteriorly spinose meri, with a differential occurrence of the subdistal spine on the posterior margin of the ambulatory meri, present only on P2 and P 3 in Linnaeoxantho ( Figs. 2 , 4D, E ) (present in P2–P 4 in Melybia ) (see also Melybia forceps A. Milne-Edwards, 1880: 274 , pl. 49, figs. 1–1e, a junior synonym of M. thalamita ). A good series of specimens of M. thalamita were examined in the USNM for the present study and the resemblance between these two taxa is remarkable. Indeed, perhaps the only reason why this connection has not been made until now is due to the disparate distributions of these two taxa, and the relative rarity of L. acanthomerus . Interestingly, Števčić (2005: 88) was of the opinion that Melybia merited its own family, Melybiidae Števčić, 2005 , but he inexplicably classified it in the Portunoidea Rafinesque, 1815. However, his treatment of Melybiidae and its characters was cursory and unhelpful for comparative purposes. He also does not explain why he placed Melybiidae in the Portunoidea and not within Linnaeoxanthinae. Melybiidae was first used in a letter by Števčić to Martin & Davis (2001: 112), but as the name was not acompanied by any diagnosis or description. Melybiidae Števčić in Martin & Davis, 2001, must be regarded as a nomen nudum, and the first valid use of the name was in Števčić (2005).

This present study agrees with Ng et al. (2008) in assigning Melybia to the Xanthidae , prior to a systematic revision of this group. However, both Linnaeoxantho and Melybia are here considered related, and fall within or are allied to what is now understood as the Xanthidae (viz. Lai et al. 2011). Linnaeoxantho and Melybia share a suite of diagnostic characters (see diagnoses for subfamily and genus) that distinguish them from the Xanthinae s. str. (viz. Lai et al., 2011) and suggest that they can be placed in their own subfamily, for which the names Linnaeoxanthinae Števčić, 2005, and Melybiinae Števčić, 2005, are available. However, as the names appeared in the same paper ( Števčić 2005), the Code regards both as simultaneously published and neither has priority. In such a case, the First Reviser principle comes into effect. Therefore, Linnaeoxanthinae Števčić, 2005, is chosen here as having priority over Melybiinae Števčić, 2005, whenever these two taxa are regarded as synonyms.

Figures

FIGURE 1. Linnaeoxantho acanthomerus (Rathbun, 1911), young ♂, 7.1 × 5.5 mm (ZRC 2012.0160), Kumejima Is., Ryukyu Islands, Japan; live coloration.

FIGURE 2. Linnaeoxantho acanthomerus (Rathbun, 1911). A, holotype ♂, 12.9 × 8.5 mm (USNM 41353), B, paratype ♀, 15.0 × 10.1 mm (USNM 41353), Amirante Islands; C, ♀, 11.0 × 7.5 mm (UF 10556), Kiribati; D, 7.1 × 5.5 mm (ZRC 2012.0160), Kumejima. Scales: A–D = 3.0 mm.

FIGURE 3. Linnaeoxantho acanthomerus (Rathbun, 1911). A, C, E, F, holotype ♂, 12.9 × 8.5 mm (USNM 41353); B, D, paratype ♀, 15.0 × 10.1 mm (USNM 41353). A, ventral view; B, frontal view; C, male thoracic sternum; D, ♀ thoracic sternum; E, major chela; F, minor chela.

FIGURE 4. Linnaeoxantho acanthomerus (Rathbun, 1911), holotype ♂, 12.9 × 8.5 mm (USNM 41353). A, left mxp3; B, thoracic sternum and penis; C, abdomen and telson; D, right P3; E, right P5, coxa, basis-ischium, merus; F–H, left G1, F, entire external view, G, distal tip, external view, H, distal tip, internal view; I, left G2, external view. Scales: A–E = 2.0 mm; F, I = 1.0 mm; G, H = 0.5 mm.