Systroplax angusta (Rathbun, 1914)

CASTRO, PETER & NG, PETER K. L., 2010, Revision of the family Euryplacidae Stimpson, 1871 (Crustacea: Decapoda: Brachyura: Goneplacoidea), Zootaxa 2375 (1), pp. 1-130 : 93-98

Systroplax angusta (Rathbun, 1914)

( Figs. 37A–G ; 38G–J )

Carcinoplax angusta Rathbun, 1914b: 142 [ Philippines]. — Tesch 1918: 154 [in list]. — Estampador 1937: 533 [in list]; 1959: 89 [in list] [ Philippines]. — Serène 1968: 90 [in list]. ― Sakai 1969: 269 [in list], fig. 15b [holotype]. — Zarenkov 1972: 241, fig. 4–4, 6–4 [South China Sea]. — Serène & Lohavanijaya 1973: 62 [in list], 64 [in key]. — Guinot 1989: 309 [discussion], fig. 39, pl. 12, figs. A–E [holotype]. — Castro 2007: 623, 624 [discussion]. — Yang et al. 2008: 770 [in list] [ China].

Carcinoplax angustata [sic] — Chen 1998: 266 [in key], 273, 310 [in list], fig. 6 [South China Sea].

“ Carcinoplax ” angusta — Ng et al. 2008: 79 [in list].

(?) Carcinoplax cf. angusta — Naderloo & Sari 2007: 344, 335, 347 [Persian Gulf].

Type material. Female holotype, 23.7 mm × 26.9 mm; 1 male paratype, 10 mm × 10.8 mm (USNM 46166).

Type locality. Philippines, Verde Island Passage between Luzon and Mindoro, Albatross, stn. 5376, 13°42.8’S, 121°51.5’E, 165 m.

Material examined. South China Sea. Dampier, stn. DD19, 05°26.0’S, 111°13.5’E, 210– 201 m, 1963: 1 female ( NHM) .

Philippines. Verde Island Passage. Albatross, stn. 5376, 13°42.8’S, 121°51.5’E, 165 m, 02.03.1909: female holotype ( USNM 46166 ) .

Solomon Is. SALOMON 2: stn. CP 2284, 8°38,4' S, 157°21,5' E, 195–197 m, 06.11.2004: 2 males, 10.7 mm × 11.8 mm, 8.3 mm × 8.8 mm ( MNHN-B30100 ). – No stn. no.: 1 female, 14.6 mm × 16.5 mm (MNHN- B30074) .

Vanuatu. MUSORSTOM 8: stn. CP 1117, 15°09.91’S, 166°53.37’E, 170–220 m, 09.10.1994: 1 male ( MNHN-B29404 ) . – Stn. CP 1123, 15°07.19’S, 166°55.20’E, 262–552 m, 09.10.1994: 2 males, 28.0 mm × 31.6 mm, 18.9 mm × 21.4 mm ( MNHN-B29402 ) .

BOA1: stn. CP22447, 15°07.20’S, 166°52.0’E, 10.09.2005: 1 male, 27.8 mm × 31.1 mm ( MNHN-B30135 ) .

Fiji. MUSORSTOM 10: stn. CAS 1321, 17°17.3’S, 177°47.7’E, 200 m, 07.08, 1998: 1 male, 32.3 mm × 37.2 mm, 1 female 27.8 mm × 31.4 mm ( MNHN-B29377 ) . – Stn. CP 1322, 17°17.1’S, 177°47.9’E, 210–282 m, 07.08, 1998: 1 male, 22.1 mm × 24.6 mm ( MNHN-B29378 ) . – Stn. CP 1328, 17°16.8’S, 177°50.4’E, 248– 277 m, 07.08.1998: 1 male ( MNHN-B29488 ) .

BORDAU 1: stn. CP 1403, 16°40’S, 179°36’E, 220–224 m, 25.02.1999: 2 males, 19.9 mm × 22.5 mm, 13.5 mm × 14.8 mm ( MNHN-B29470 ); 1 pre-adult male, cl 9.9 mm [rest of carapace damaged] ( MNHN-B29489 ) .

Diagnosis. Carapace suboctagonal, almost round, granular, with two short teeth on each anterolateral bor- der ( Fig. 37A ).

Remarks. Carcinoplax angusta Rathbun, 1914 , was described from two specimens from the Philippines, the female holotype (23.8 mm × 27.6 mm, USNM 46166) and a pre-adult male (10 mm × 10.8 mm, same catalogue number). Most of the characters that separate this species from Carcinoplax sensu stricto were nevertheless clearly listed by Rathbun (1914b: 142) and discussed by Castro (2007: 624), who formally placed the species “in a yet undescribed genus in the family Euryplacidae ” ( Castro 2007: 623). These characters, plus the very slender G1 of relatively small specimens, were illustrated by Zarenkov (1972: figs. 4–4, 6–4, as C. angusta ), and particularly Chen (1998: fig. 6, as C. angustata [sic]), who also showed the short G2.

In large specimens (e.g. male, 22.1 mm × 24.6 mm, MNHN-B29378; male, 27.8 mm × 31.1 mm, MNHN- B30135) the second anterolateral tooth is hardly noticeable ( Fig. 38I, J ).

Distribution. Southwestern Pacific from the South China Sea to Fiji. A record from the Persian Gulf ( Naderloo & Sari 2007; as Carcinoplax angusta ) is highly questionable. Depth: 165–282 m (also from a station at depths of 226-552 m).

Genus Trissoplax n. gen.

Heteroplax Stimpson, 1858: 94 (40); 1907: 94 [diagnosis] (part). — Balss 1922a: 136 [list of species] (part). — Campbell 1969: 117 (part). — Guinot 1969b: 511 [discussion] (part). — Serène & Lohavanijaya 1973: 71 [discussion] (part). — Ng et al. 2008: 78, 79 [discussion] (part).

Diagnosis. Carapace ( Figs. 39D–F ; 40A–E ; 42A ) hexagonal, transversely elongated, wider than long, convex; anterolateral borders nearly straight, only slightly arched in small individuals, progressively arched with increasing carapace size in T. dentata ; dorsal surface smooth without clear indication of regions (larger individuals of T. dentata with short sulcus on each branchial region extending laterally from third anterolateral tooth, short elongated boss on each branchial region near second anterolateral tooth); front wide, straight with median notch, transverse sulcus along margin (continuing as a short vertical groove in large individuals of T. dentata ). Three anterolateral teeth (first triangular, obtuse; second acute, dorsally salient [particularly in T. dentata ]; third small, obtuse) posterior to triangular, anteriorly-oriented outer orbital angle (variations in Tirmizi & Ghani 1988: fig. 3A–G’; 1996: fig. 30A–G’, as E. haswelli ). Orbits moderately long, about as long as front ( Fig. 41B ), sinuous to slightly sinuous, one notch on inner third portion of thin to thick supraorbital border (second notch on outer portion more noticeable in larger individuals of T. dentata ); triangular inner suborbital tooth, short median lobe, outer notch on thick suborbital border ( Fig. 41A, B ) (suborbital border sinuous, granular, without inner suborbital tooth or median lobe in T. tuberosa n. sp.; Fig. 42E ); eye peduncles moderately long, nearly half of frontal width, much longer than large, slightly elongated corneas ( Figs. 41A, B ; 42E ). Basal antennal article immobile (slightly mobile in small specimens), with small disto-lateral process so that orbital hiatus is closed excluding antennal flagellum from orbit ( Fig. 41A, B ; Campbell 1969: fig. 3E, as E. affinis ; fig. 5D, as E. haswelli ; Yang & Sun 1979: fig. 3–1, as Eucrate costata ). Anteroexternal margin of third maxilliped merus angular ( Fig. 41A ). Cheliped fingers moderately stout, slightly longer than swollen propodus, light in colour ( Figs. 41E ; 42E ); carpus with tooth on inner margin; dense tomentum consisting of plumose setae on anterior margin of carpus ( Figs. 40A, C, E ; 41E ) (clumps of plumose setae in T. tuberosa n. sp.; smallest individuals of T. dentata ). Dorsal margins of ambulatory legs (P2–P5) meri, carpi, propodi unarmed, dactyli slender, smooth, setose; P5 propodus long, slender, subcylindrical, fringed with many long setae ( Campbell 1969: fig. 3I, as E. affinis ; fig. 5E, as E. haswelli ). Thoracic sternum ( Fig. 41C, F, G ) wide, thoracic suture 2/3 complete, convex ( Figs. 41C ; 42C ); 3/4 deep, short, interrupted; 4/5, 6/7, 7/8 interrupted, 5/6 complete ( Fig. 41G ); median groove on thoracic sternites 7, 8. Sterno-abdominal cavity of male deep, only reaching median portion of sternite 4. Press-button of male abdominal-locking mechanism as small tubercle near thoracic suture 4/5 (small tubercle present in pre-adult females). Male abdomen narrow, slender, transversely narrow (T-shaped), lateral margins of somites 4–6 abruptly narrowing from somite 3 to transversely narrow, pointed telson ( Figs. 41D ; 42C ); somite 3 reaching inner margins of P5 coxae, somite 2 transversely slightly shorter or as long as somite 3; no portions of thoracic 8 exposed by closed abdomen or small portion of each side exposed ( T. tuberosa n. sp.). G1 long, slender, slightly sinuous to nearly straight, acuminate apex, with small denticles ( Figs. 41F ; 43A, B, D, E ); G2 less than one-third of G1, straight, apex with 2 processes: one long, tip acute; second much shorter, tip acute ( Fig. 43C, F ). Male genital opening (gonopore) coxal; coxo-sternal disposition of long penis ( Fig. 41F ), protected by concave posterior portion of thoracic sternite 7. Female abdomen relatively narrow, not covering outer portions of thoracic sternum, with 6 freely-mobile somites ( Serène & Lohavanijaya 1973: fig. 179, as Heteroplax dentatus , fig. 184, as H. transversus ; Tirmizi & Ghani 1988: fig. 3H; 1996: fig. 30H, as E. haswelli ). Vulva ovoid, extending transversely across anterior portion of sternite 6 close to median axis of thorax; short, transverse, triangular sternal vulvar cover on posterior margin ( Fig. 41G ).

Type species. Heteroplax dentata Stimpson, 1858 (by present designation, gender feminine).

Etymology. From the Greek trissos for “threesome”, in reference to the three conspicuous teeth (the outer orbital tooth and the first two anterolateral teeth) on each side of the carapace, which is characteristic of the new genus, and plax, Greek for “plate” or “tablet”, which is derived from the generic name Heteroplax to denote that the type species of the new genus was formerly included in Heteroplax sensu lato.

Remarks. Stimpson (1858: 94 (40); 1907: 94) described two species of Heteroplax from or near Hong Kong, H. transversa and H. dentata , but his descriptions were brief, no illustrations were given, and the type material of both species is presumably lost (see Evans 1967; Deiss & Manning 1981; Manning 1993; Manning & Reed 2006). A specimen from the type locality that closely follows Stimpson’s description of H. transversa has been selected as its neotype and the genus Heteroplax Stimpson 1858 , is being re-described herein, with H. transversa as its only species, H. nagasakiensis Sakai, 1934 , being a junior synonym (see above).

Trissoplax n. gen. is described herein for the second of Stimpson’s (1858) Hong Kong species, H. dentata . Stimpson compared his two new species, describing H. dentata as having a “carapax narrow” in contrast to the “much broader carapax” of H. transversa . Indeed, the Hong Kong male specimen herein selected as the neotype of H. dentata Stimpson, 1858 (16.4 mm × 21.5 mm; QM W27400), the type species of Trissoplax n. gen. (see below), has a much narrower carapace than H. transversa . Whereas the carapace-length to carapacewidth ratio of H. transversa given by Stimpson is 1:1.5 (1:1.6 ratio for the neotype and other Hong Kong specimens; see above), the ratio given by Stimpson for his H. dentata is 1:1.27 (1: 1.3 for the neotype and additional Hong Kong specimens listed here; 1:1.3–1.4 for specimens from other locations). A second species of Trissoplax n. gen. being described as new, T. tuberosa n. sp., also has a narrow carapace (1: 1.3 in the holotype, see below).

Stimpson (1858: 94 (40); 1907: 94, 95) mentioned three additional characters in contrasting between the two species. One was the presence in his H. dentata of “four unequal teeth” (including the outer orbital tooth) in contrast to only two teeth that were described posterior to the outer orbital tooth in T. transversa , the third always being very small so that Stimpson possibly overlooked it. A third difference was the presence of “a tuft of pubescence” in H. dentata , which he did not mention for H. transversa , a character that is found in the neotype and all the remaining specimens of T. dentata that were examined except the pre-adults. A final difference was in the length of the eye peduncles, being proportionately longer in H. transversa than in H. dentata . Alcock (1900), Balss (1922b), and Guinot (1969) commented on the obvious affinities between Stimpson’s H. dentata and Eucrate . In fact, Alcock (1900: 299, 301) erroneously regarded H. dentata as a subspecies of Eucrate crenata , although large specimens of both species may be confused (see Remarks for H. dentata below). Eucrate is clearly different from both Heteroplax and Trissoplax n. gen. ( Table 1) but the confusion unfortunately developed as a result of authors only relying on the short description with no illustrations and, most probably, the absence of specimens of Stimpson’s H. dentata . Specimens from other localities were described as new species, thus adding to the confusion (see Remarks for H. dentata below).

The most obvious difference between Trissoplax n. gen. and Eucrate is that in Eucrate the carapace is trapezoidal and with relatively short orbits and eye peduncles (e.g. Fig. 3B ), whereas in Trissoplax n. gen. (as in Heteroplax ) the carapace is slightly more elongated and with longer orbits and eye peduncles (e.g. Fig. 41B ) (see Table 1). The anterolateral borders of the carapace of T. dentata nevertheless become arched with increasing size and as such they may be superficially similar to large individuals of Eucrate species. These large specimens of Eucrate can still be easily distinguished from T. dentata in having proportionately much longer eye peduncles and orbits, a higher and more conspicuous inner suborbital tooth, and thinner orbital margins than in Eucrate . Both Eucrate and Trissoplax n. gen., however, share the presence of a conspicuous setous patches along the anterior margin of the cheliped carpus, which is absent in Heteroplax .

Species included.

Trissoplax dentata ( Stimpson, 1858)

= Eucrate affinis Haswell, 1882

= Pseudorhombila sulcatifrons var. australiensis Miers, 1884

= Eucrate haswelli Campbell 1969

= Eucrate costata Yang & Sun 1979

Trissoplax tuberosa n. sp.

The genus is restricted to the Indo-West Pacific region.

Figures

FIGURE 3. Eucrate crenata (De Haan, 1835); A–F, male neotype of Pilumnoplax sulcatifrons Stimpson, 1858 (34.0 mm × 49.1 mm) (QM W27449), Hong Kong, New Territories, Tolo Channel; G, female (22.1 mm × 27.1 mm) (ZRC 2002.0493), China, Qingdao. A, overall view; B, frontal view showing orbits and antennae; C, anterior portion of male thoracic sternum and abdomen; D, third maxillipeds, antennules, antennae and orbits; E, outer surface of right chela; F, posterior portion of male thoracic sternum and abdomen; G, female thoracic sternum and vulvae. Abbreviations: a1–a6 = male abdominal somites 1–6, respectively; cx5 = coxa of fifth pereopod (P5); s3–s7 = thoracic sternites 3 to 7, respectively; t = telson; v = vulva.

FIGURE 37. Systroplax angusta (Rathbun, 1914); A–C, G, female (27.8 mm × 31.4 mm) (MNHN-B29377), Fiji; D–F, male (32.3 mm × 37.2 mm) (MNHN-B29377), Fiji. A, overall view; B, frontal view showing orbits and antennae; C, third maxillipeds; D, outer surface of left chela; E, anterior portion of male thoracic sternum and abdomen; F, posterior portion of male thoracic sternum and abdomen; female sternum and vulvae; G, female sternum and vulvae. Abbreviations: a1–a3 = male abdominal somites 1, 2, 3, respectively; cx5 = coxa of fifth pereopod (P5); es7 = episternite 7; mg = median groove; s4–s7 = thoracic sternites 4 to 8, respectively; v = vulva.

FIGURE 38. Gonopods of Platyozius, Psopheticoides, and Systroplax; carapace of Systroplax. A–C, Platyozius laevis (Borradaile, 1902), male (6.1 mm × 7.8 mm) (ZRC 2007.0707), Maldives, Laamu Atoll; D–F, Psopheticoides sanguineus Sakai, 1969, male (21.9 mm × 27.8 mm) (ZRC 2008.1357), Philippines, Bohol, Balicasag I.; G–J, Systroplax angusta (Rathbun, 1914); G, G1 (after Guinot 1989: fig. 39D); H, male (32.3 mm × 37.2 mm) (MNHN-B29377), Fiji; I, J, variation in anterolateral borders of carapace (after Guinot 1989: fig. 39A, C). A, B, D, E, right G1, dorsal view; C, F, H, right G2, dorsal view. Scale bars: A, D, 1.0 mm; B, E, 0.5 mm; C, F, H, 0.2 mm; G, 0.1 mm.

FIGURE 39. Colours in life. A, Euryplax nitida Stimpson, 1871; male neotype (7.1 mm × 11.3 mm) (UF 15166), Gulf of Mexico; B, Euryplax nitida Stimpson, 1871; male (4.4 mm × 7.0 mm) (UF 15168), Gulf of Mexico; C, Psopheticoides sanguineus Sakai, 1969, male (22.2 mm × 26.7 mm) (MNHN-B29728), Philippines, Bohol, Panglao; D, Trissoplax dentata (Stimpson, 1858), male (6.2 mm × 8.3 mm) (NHM), Thailand, Andaman Sea; E, Trissoplax dentata (Stimpson, 1858), young male, not preserved, Singapore, Changi; F, Trissoplax dentata (Stimpson, 1858), female (8.7 mm × 11.5 mm) (ZRC 2000.2240), Singapore. Photo credits: A, B by Florida Museum of Natural History (UF); D by P. F. Clark (NHM).

FIGURE 40. Trissoplax dentata (Stimpson, 1858); A, male neotype of Heteroplax dentatus Stimpson, 1858 (16.4 mm × 21.5 mm) (QM W27400), Hong Kong, Tolo Harbour; B, male holotype of Eucrate haswelli Campbell, 1969 (15.0 mm × 19.5 mm) (AM P6991), Australia, Queensland, Port Denison; C, male (20.0 mm × 26.2 mm) (ZRC 2008.1236), Singapore, Changi Point; D, male lectotype of Eucrate affinis Haswell, 1882 (6.6 mm × 8.7 mm) (AM P2972), Australia, Queensland, Port Denison; E, male paratype of Eucrate costata Yang & Sun, 1979 (25.0 mm × 19.0 mm) (BMNH J79140), China, Dongshan, Fujian Province; F, pre-adult female holotype of Pseudorhombila sulcatifrons var. australiensis Miers, 1884 (8.4 mm × 6.0 mm) (NHM 1882.93), Australia, Queensland, Port Molle. A–C, E, F, overall views; D, dorsal view of carapace. Photo credits: D by R.T. Springthorpe © (AM), F by P. Crabb (NHM).

FIGURE 41. Trissoplax dentata (Stimpson, 1858); A–D, F, male neotype of Heteroplax dentatus Stimpson, 1858 (16.4 mm × 21.5 mm) (QM W27400), Hong Kong, Tolo Harbour; E, male (20.0 mm × 26.2 mm), Singapore, Changi Point; G, female (8.7 mm × 11.5 mm) (ZRC 2000.2240) (ZRC 2008.1236), Singapore. A, frontal view showing third maxillipeds; B, frontal view showing orbits and antennae; C, anterior portion of male thoracic sternum; D, posterior portion of male thoracic sternum and abdomen; E, outer surface of right chela; F, male sterno-abdominal cavity, G1 and penis; G, female thoracic sternum and vulvae. 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; mg = median groove; p = penis; s4–s7 = thoracic sternites 4 to 7, respectively; vc = sternal vulvar cover.

FIGURE 42. Trissoplax tuberosa n. sp.; male holotype (7.6 mm × 10.2 mm) (MNHN-B10340), Indonesia, Moluccas. A, overall view; B, ventral view of carapace; C, anterior portion of male thoracic sternum and abdomen; D, male sternoabdominal cavity and G1; E, frontal view showing orbits, antennae and outer surfaces of chelae. Abbreviations: G1 = first male pleopod; sc = sterno-abdominal cavity. Photo credits: A, B by H. H. Tan (ZRC).

FIGURE 43. Gonopods. A–C, Trissoplax dentata (Stimpson, 1859); male neotype of Heteroplax dentata Stimpson, 1859 (16.4 mm × 21.5 mm) (QM W27400); D–F, Trissoplax tuberosa n. sp., male holotype (7.6 mm × 10.2 mm) (MNHN-B10340), Indonesia, Moluccas. A, B, D, E, right G1, dorsal view; C, F, right G2, dorsal view. Scale bars: A, 1.0 mm; B, D, 0.5 mm; C, E, F, 0.2 mm.