Machaerus oxyacanthus ( Monod, 1956 )

CASTRO, PETER & NG, PETER K. L., 2010, Revision of the family Euryplacidae Stimpson, 1871 (Crustacea: Decapoda: Brachyura: Goneplacoidea), Zootaxa 2375 (1), pp. 1-130 : 78-82

Machaerus oxyacanthus ( Monod, 1956)

( Figs. 30A–G ; 32C–F )

? Pilumnoplax sp. Cano 1889a: 91 [unknown location].? Pilumnoplax incerta Cano, 1889b: 228 , pl. 7, fig. 14 [unknown location]. Pilumnoplax atlantica — Balss 1922b: 76 [ Liberia, Ivory Coast, Equatorial Guinea]. — Monod 1933: 533, fig. 19A, 20,

21E [ Mauritania]. — Capart 1951: 166, fig. 63 [ Gabon, Cabinda, Congo-RC]. [not Machaerus atlanticus ( Miers,

1881)] Pilumnoplax oxyacantha Monod, 1956: 340 [in key], 346, figs. 450–455 [ Senegal, Ghana]. — Forest & Guinot 1966: 85

[ Guinea, Sierra Leone, Liberia, (?) Equatorial Guinea]. — Guinot 1969b: 517, 518 [discussion]; 1969c: 688 [discussion].

“ Pilumnoplax ” oxyacantha — Guinot 1969b: 507, 508 [discussion], figs. 38, 42, 54, 55 [ Senegal].

[ Pilumnoplax ] oxyacantha — Guinot 1971: 1081 [in list].

Machaerus oxyacantha — Manning & Holthuis 1981: 163 [references] [ Ivory Coast, Ghana, Benin, Nigeria].

Machaerus oxyacanthus — Ng et al. 2008: 78 [in list].

Type material. Unknown status [given as “ MP ” by Monod (1956: 346) but not in MNHN] .

Type locality. Senegal, unknown location .

Material examined. Guinea. Calypso , stn. 9, 18– 30 m, 18.05.1956: 5 males , 2 pre-adult females, 7 females ( MNHN-B10243 ) .

Sierra Leone. Calypso , stn. 11, 34– 30 m, 19.05.1956: 1 male , 19.0 mm × 30.3 mm, 1 male, 2 pre-adult females, 3 females ( MNHN-B10242 ) .

Gulf of Guinea. Identified as “ Frevillea rosaea ”: 1 male, 26.1 mm × 16.9 mm ( NHM 1850.32 ) .

Congo-RC. Pointe Noir, 25 m, mud, A. Crosnier coll., 05.1963: 2 males, 11.6 mm × 27.2 mm, 22.5 mm × 35.8 mm ( MNHN-B10231 ) .

Diagnosis. Anterolateral teeth acute, different from triangular outer orbital teeth, second tooth much longer, more elevated than first ( Fig. 30A, B ).

Remarks. Differences between M. oxyacanthus and its only (and apparently sympatric) congener, M. atlanticus , are given in the Remarks for the latter. Unusual for a euryplacid, M. oxyacanthus has been collected in brackish water ( Manning & Holthuis 1981: 163).

On the basis of the description and a rather simple figure ( Cano, 1889b: 228, pl. 7, fig. 14), Pilumnoplax incerta Cano, 1889 , is likely to be a senior synonym of M. oxyacanthus . This suggestion had already been made by Ahyong (2008: 54) and Ng et al. (2008: 84). The species is certainly not a Pilumnoplax Stimpson, 1858 (a junior synonym of Eucrate De Haan, 1835 ) as understood here, or a Neopilumnoplax Serène, 1969 (see Ahyong 2008; Tavares & Melo in press). Cano’s (1889b) species is only known from the small (“ 6 mm × 8mm ”) holotype, which was obtained from an unknown location, and its whereabouts are uncertain. As Cano’s (1889a, b) material was from all over the world, with the bulk from the Atlantic, it is possible that the specimen of P. incerta was obtained from somewhere in the western Atlantic where M. oxyacanthus is found. For now, we defer from synonymising the two species until more information can be obtained.

Distribution. Atlantic coast of Africa from Senegal to Angola ( Manning & Holthuis 1981). Depth: 7–73 m ( Manning & Holthuis 1981).

Genus Nancyplax Lemaitre, García-Gómez, von Sternberg & Campos, 2001

Nancyplax Lemaitre et al. 2001: 952 . — Karasawa & Kato, 2003a: 151 [in list]; 2003b: 139 [in list]. — Ng & Castro 2007: 44 [in list]. — Ng et al. 2008: 78 [in list]. — De Grave et al. 2009: 33 [in list].

Diagnosis. Carapace ( Fig. 31A ) quadrate, slightly wider than long, dorsal surface smooth (except granules on hepatic region) without clear indication of regions; anterolateral borders arched; front wide, with small median notch. One conspicuous, acute anterolateral tooth posterior to simple, unarmed outer orbital angle. Orbits short, shorter than front ( Fig. 31B, C ); 2 notches on thin supraorbital border; thin, sinuous suborbital border, inner suborbital tooth absent ( Fig. 31C ); eye peduncles short, nearly as long as corneas; large, spherical corneas ( Fig. 31B, C ). Basal antennal article immobile, disto-lateral process absent but orbital hiatus is closed excluding antennal flagellum from orbit ( Fig. 32B, C ; Lemaitre et al. 2001: fig. 2a). Anteroexternal margin of third maxilliped merus angular. Stridulating ridge on pterygostomian region ( Lemaitre et al. 2001: fig. 2a). Cheliped fingers moderately slender, slightly shorter than propodus, light in colour ( Fig. 31E ); carpus with acute tooth on inner margin; sparse setae on outer margin of propodus, carpus ( Lemaitre et al. 2001: fig. 3f); stridulating ridge on proximal margin of merus ( Lemaitre et al. 2001: figs. 2a, 3f). Dorsal margins of ambulatory legs (P2–P5) meri, carpi, propodi unarmed, dactyli slender, smooth, setose. P5 propodus, dactylus long, slender, fringed with long setae ( Lemaitre et al. 2001: fig. 4d). Thoracic sternum ( Fig. 31D, F, G ) wide; thoracic suture 2/3 complete, straight ( Fig. 31D ); 3/4 deep, short, interrupted; 4/5, 5/6, 6/7, 7/8 interrupted ( Fig. 31F, G ); median groove on thoracic sternites 7, 8. Sterno-abdominal cavity of male deep, reaching anterior margin of sternite 4 ( Fig. 31D, F ). Press-button of male abdominal-locking mechanism as large tubercle near thoracic suture 4/5 ( Fig. 31D ) (presence in pre-adult females unknown). Male abdomen proportionally wide, triangular (not T-shaped), telson wider than long ( Fig. 32G ); somite 3 only slightly transversely wider than somites 4–6, somite 3 not reaching inner margins of P5 coxae or episternite 7, small portion on each side of thoracic sternite 8 left exposed by closed abdomen (( Lemaitre et al. 2001: fig. 2b), somite 2 transversely shorter than somite 3 ( Lemaitre et al. 2001: fig. 2b). G1 long, slender, slightly sinuous, thin distal part, acuminate apex, with small, relatively few denticles ( Fig. 32H, I ); G2 less than one-third of G1, straight, apex with 2 processes, one much longer, larger ( Fig. 32J, K ). Male genital opening (gonopore) coxal ( Fig. 31F ); coxosternal disposition of long penis, protected by slightly convex posterior portion of thoracic sternite 7. Vulva relatively large, ovoid, transverse on median portion of sternite 6, extending across median half of sternite ( Fig. 31G ); covered by soft membrane, sternal vulvar cover absent.

Type species. Nancyplax vossi Lemaitre, García-Gómez, von Sternberg & Campos, 2001 (by original designation, gender feminine).

Diagnosis. One long, acute tooth on each anterolateral border of carapace ( Fig. 31A–C ). Outer orbital tooth absent ( Fig. 31B, C ).

Remarks. As in some of the other Western Atlantic and tropical Eastern Pacific genera of euryplacids, the male abdomen of Nancyplax is triangular in shape ( Fig. 32G ), not narrow and T-shaped (see Remarks for Euryplacidae ). The G2 departs from the typical euryplacid G2 by having an apex that consists of one large process and a much smaller one ( Fig. 32J ) in contrast to other euryplacids, where the two processes are unequal but not as dissimilar as in Nancyplax . The sterno-abdominal cavity, however, is deep and relatively narrow, and the G1 slender as in other euryplacids. With the exception of sternal suture 5/6, which is interrupted as in Systroplax n. gen. and Xenocrate whereas complete in the remaining euryplacid genera, the other defining characters of Nancyplax agree with those of the family.

Species included. Nancyplax vossi Lemaitre, García-Gómez, von Sternberg & Campos, 2001 The genus is restricted to the Western Atlantic region.

Figures

FIGURE 30. Machaerus oxyacanthus (Monod, 1956); male (19.0 mm × 30.3 mm) (MNHN-B10242), Sierra Leone. A, overall view; B, frontal view showing orbits and antennae; C, third maxillipeds; D, outer surface of left chela; E, male thoracic sternum and abdomen; F, male sterno-abdominal cavity, G1, and penis; G, posterior portion of male thoracic sternum and abdomen. Abbreviations: a1–a3 = male abdominal somites 1, 2, 3, respectively; cx5 = coxa of fifth pereopod (P5); es7 = episternite 7; G1 = first male pleopod; G2 = second male pleopod; g = male gonopore; p = penis; s4–s8 = thoracic sternites 4 to 8, respectively.

FIGURE 31. Nancyplax vossi Lemaitre, García-Gómez, von Sternberg & Campos, 2001; A–D, male paratype (12.1 mm × 17.2 mm) (USNM 308994), Suriname; E–G, female paratype (12.1 mm × 17.9 mm) (USNM 308994), Suriname. A, overall view; B, frontal view showing orbits and antennae; C, third maxillipeds; D, anterior portion of male thoracic sternum and sterno-abdominal cavity; E, outer surface of right chela; F, male sterno-abdominal cavity and penis; G, female thoracic sternum and vulvae. Abbreviations: cx5 = coxa of fifth pereopod (P5); es7 = episternite 7; g = male gonopore; mg = median groove; p = penis; pb = press-button of male abdominal-locking mechanism; s4–s8 = thoracic sternites 4 to 8, respectively; v = vulva.

FIGURE 32. A, B, Machaerus atlanticus (Miers, 1881) (after Guinot 1969b: figs. 36, 53, as “Pilumnoplax” atlantica); C–F, Machaerus oxyacanthus (Monod, 1956) (after Guinot 1969b: figs. 38, 54a, 54b, 55, as “Pilumnoplax” oxyacantha); G–K, Nancyplax vossi Lemaitre, García-Gómez, von Sternberg & Campos, 2001 (after Lemaitre et al. 2001: figs. 5b–e). A, C, posterior portion of male thoracic sternum, abdomen, and gonopore; B, D, E, H, I, G1; F, J, K, G2. Abbreviation: a1–a3 = male abdominal somites 1, 2, 3, respectively; g = male gonopore.