Carcinoplax mistio, Ng & Mitra, 2019

Ng, Peter K. L. & Mitra, Santanu, 2019, Carcinoplax mistio, a new species of goneplacid crab from the Indian Ocean (Decapoda: Brachyura: Goneplacoidea), Nauplius (e 2019004) 27, pp. 1-15 : 2-11

publication ID	https://doi.org/ 10.1590/2358-2936e2019004
publication LSID	lsid:zoobank.org:pub:0302A867-CBEC-4A09-9870-CB9A39FF25FE
DOI	https://doi.org/10.5281/zenodo.10878466
persistent identifier	https://treatment.plazi.org/id/8F8DAFB6-8148-4F6A-BC5D-D853E681C5F6
taxon LSID	lsid:zoobank.org:act:8F8DAFB6-8148-4F6A-BC5D-D853E681C5F6
treatment provided by	Felipe
scientific name	Carcinoplax mistio
status	sp. nov.

Treatment

Carcinoplax mistio View in CoL n. sp.

( Figs. 1 View Figure 1 , 2 View Figure 2 , 6A, B View Figure 6 , 7A, G, H View Figure 7 , 8A–G View Figure 8 , 9A, B View Figure 9 )

Carcinoplax (purpurea)? View in CoL – Stephensen, 1946: 166, 208, fig. 44 (not Carcinoplax purpurea Rathbun, 1914 View in CoL ).

Carcinoplax purpurea View in CoL – Guinot, 1967: 276 (list); Titgen, 1982: 252 (list) (not Carcinoplax purpurea Rathbun, 1914 View in CoL ).

Carcinoplax sinica View in CoL – Guinot, 1989: 285, text-fig. 14A, B, pl. 5 figs. A, B, B1, C, C1, D, E, E1; Apel, 2001: 101; Naderloo and Sari, 2007: 449; Naderloo, 2017: 69, text-fig. 11.2d, e, fig. 12.1 (not Carcinoplax sinica Chen, 1984 View in CoL ).

Type material. Holotype: male (29.2 x 19.0 mm) ( ZSI Reg. No. C 7123/2), Fresargunj Fishing Harbour, trawl by-catch, District 24 Pargana (South), West Bengal, Bay of Bengal , India, coll. local fishermen, 24 February 2017 . Paratypes: 1 female (36.4 x 24.2 mm) ( ZSI Reg. No. C 7124/2), same data as holotype ; 1 female (36.7 x 27.5 mm) ( ZSI), same location as holotype, coll. local fishermen, 28 July 2018 .

Comparative material. Carcinoplax purpurea Rathbun, 1914: 1 male (31.7 x 24.3 mm), 2 females (larger 38.8 x 29.5 mm) ( ZRC 2001.0017 View Materials ), Tahsi , Ilan County, northeastern Taiwan, coll. K.-X. Li, 2000 ; 1 male (31.0 x 24.2 mm) ( ZRC 1999.0773 View Materials ), Tashi port, Ilan County, northeastern Taiwan, coll. P.K.L. Ng and K. Lim, May 1999 . — Carcinoplax sinica Chen, 1984: 1 male (31.7 x 20.9 mm) ( ZRC 2011.0607), station 6215, sand-mud substrate, Gulf of Tonkin, South China Sea, 48 m, China-Vietnam Cooperative Expedition of Comprehensive Oceanographic Investigation on Beibu Gulf (Gulf of Tonkin) 1959–1960, trawl, coll. 18 April 1960 ; 1 juvenile female (30.2 x 19.3 mm) ( ZRC 2011.0609 View Materials ), station 6234, muddy-sand substrate, Gulf of Tonkin, South China Sea, 30 m, China-Vietnam Cooperative Expedition of Comprehensive Oceanographic Investigation on Beibu Gulf (Gulf of Tonkin) 1959–1960, trawl, coll. 21 April 1960 ; 1 male (28.5 x 19.5 mm) ( ZRC 1984.5693 View Materials ), near Horsburg Lighthouse , about 241.4km off Singapore, South China Sea, coll. trawlers, H. Huat, 28 August 1983 ; 2 females (35.9 x 25.6 mm, 30.6 x 21.0 mm) ( ZRC 1984.6312 View Materials - 6313 View Materials ), about 48.3 km from Horsburg Lighthouse , South China Sea, off Singapore, coll. trawlers, H. Huat, 10 September 1983 ; 1 male (35.2 x 23.4 mm), 1 female (42.8 x 29.2 mm) ( ZRC 1984.7842 View Materials - 7843 View Materials ), Horsburg Lighthouse , South China Sea, near Singapore, coll. trawlers, H.Huat, 26 November1982 and 15 December 1982 ; 1 male (34.6 x 25.0 mm) ( ZRC 1984.6314 View Materials ), near Horsburg Lighthouse , about 241.4 km off Singapore, South China Sea, coll. trawlers, H. Huat, 28 August 1983 ; 1 female (44.0 x 29.3 mm) ( ZRC 2001.0136 View Materials ), Tungkang , Kaohsiung County, southwestern Taiwan, coll. L.-S. Huang, 4 August 1996 .

Etymology. The name is derived from the Latin “mistio ” for “a mixture”;alluding to the mix of diagnostic characters in this species which are shared with C. purpurea and C. sinica . The name is used as a noun.

Diagnosis. Carapace transversely hexagonal, 1.33– 1.54 times wider than long; dorsal surface gently convex, smooth, lateral surfaces with densely packed low, rounded granules; epigastric region low but visible; postorbital regions not clearly demarcated; frontal margin lamellar, truncate, bilobed with small median notch; anterolateral margin with first tooth very low not spiniform, second tooth long, acute, sharp, curving gently obliquely anteriorly; posterolateral margin gradually converging towards gently sinuous posterior carapace margin; posterior margin of epistome with prominent but low triangular median projection; merus of third maxilliped with anteroexternal margin auriculiform; dorsal margin of cheliped palm rounded, smooth; carpus of male cheliped with low rounded tooth on distal inner angle; merus elongate with low rounded tooth on distal third of dorsal margin; ambulatory legs (P2–P5) slender, long; thoracic sternum surface covered with numerous small, rounded, densely packed granules; sternopleonal cavity extending to about two-thirds of sternite 4, reaching imaginary line connecting proximal part of coxae of chelipeds; male pleon triangular, transversely broad, telson broadly triangular, somite 6 transversely rectangular, much wider than long; G1 relatively slender, distal two-thirds almost straight; distal part slightly flared, laterally flattened, subtruncate; G2much longer than G1, distal segment flagelliform, about three-quarters length of basal segment; vulvae ovate, large, level with surface of sternum.

Description of male holotype. Carapace ( Figs. 1A View Figure 1 , 6A View Figure 6 ) transversely hexagonal, 1.54 times wider than long; dorsal surface gently convex, smooth, without setae, lateral surfaces with densely packed low, rounded granules; regions poorly defined; epigastric region low but visible, separated by shallow Y-shaped groove; postorbital regions not clearly demarcated; gastrocardiac shallow but visible; external orbital angle low, broadly triangular, surface granular, not extending to frontal margin. Frontal margin ( Figs. 1A View Figure 1 , 6A View Figure 6 ) lamellar, truncate, straight, slightly deflexed downwards, with deep transverse submarginal groove; bilobed with shallow but distinct median notch; front separated from supraorbital margin by small but distinct notch. Anterolateral margin ( Figs. 1A View Figure 1 , 6A View Figure 6 ) convex, first tooth very low not spiniform; second tooth long, acute, sharp, curving gently obliquely anteriorly; anterolateral margin separated from posterolateral margin by shallow concavity, junction with posterolateral margin gently convex; posterolateral margin gradually converging towards gently sinuous posterior carapace margin. Orbit short, ovate, much shorter than front margin ( Figs. 1B View Figure 1 , 6A View Figure 6 ); ocular peduncle stout, very short; cornea large, round.Supraorbital margin ( Figs. 1A View Figure 1 , 6A View Figure 6 ) gently concave, lined with very low, small rounded granules, with shallow median groove mostly obscured by granules, lateral end of margin confluent with external orbital tooth. Suborbital margin ( Fig. 1B View Figure 1 ) gently concave, entire, distinctly granulated, with low, rounded inner tooth near base of eye. Suborbital, subhepatic, anterior half of pterygostomial regions ( Fig. 1B View Figure 1 ) covered with small, low rounded densely packed granules. Basal antennular article ( Fig. 1B View Figure 1 ) subrectangular; article 3 rectangular, distal margin touching frontal margin; article 4 shorter than article 3; flagellum long, folding transversely. Basal antennular article ( Fig. 1B View Figure 1 ) subrectangular; article 3 rectangular, distal margin touching frontal margin; article 4 just shorter than article 3; flagellum long, folding transversely. Epistome ( Fig. 1B View Figure 1 ) longitudinally narrow; posterior margin of epistome with prominent but low triangular median projection, with distinct median fissure, separated from gently concave lateral margin by fissure. Endostomial ridge sharp, distinct on anterior part, posterior part low. Third maxillipeds ( Fig. 1C View Figure 1 ) almost completely closing buccal cavern when closed; merus quadrate, anteroexternal margin auriculiform, median part gently depressed; ischium rectangular, with deep submedian oblique sulcus, inner (mesial) margin lined with dense stiff setae; exopod relatively stout with prominent subdistal triangular tooth on inner (mesial) margin, with long flagellum.

Chelipeds ( Fig. 1A, F–H View Figure 1 ) unequal, left chela larger; fingers relatively slender, distinctly shorter than palm; surfaces covered with low, rounded granules, appears rugose in parts; inner surface smooth, with slightly swollen longitudinal median part; pollex of major chela smooth on outer surface, cutting edge with low, rounded teeth; dactylus smooth on outer surface, cutting edge with low, rounded teeth; fingers of minor chela similar to those on major chela except teeth more distinct; carpus subtriangular with low rounded tooth on distal inner angle, outer (lateral) angle with sharp tubercle; merus elongate, trigonal in cross-section, surface with small, rounded granules, dorsal margin with low rounded tooth on distal third.

Ambulatory legs (P2–P5) ( Figs. 1A View Figure 1 , 7G,H View Figure 7 ) slender, long; P3 longest; P2–P5 merus subcylindrical, laterally flattened, outer surface almost smooth or with very low flattened granules, generally glabrous or with scattered setae, ventral margin smooth, dorsal margin with low granules but not serrate; P2–P5 carpus elongate, outer surface and dorsal margin with low granules and setae; P2–P4 propodus long, distinctly laterally flattened, with longitudinal median sulcus (very shallow on P5), lateral margins of distal third with relatively dense setae which partially obscure margin; P2–P4 dactylus elongate, falciform, entire surface except for sharp tip covered with pubescence; P5 shortest, when folded reaching second anterolateral tooth, merus gently curved dorsal margin with very low granules, appearing almost smooth, propodus longitudinally ovate with shallow median sulcus, lateral margins with numerous long setae, dactylus shorter than those on P2–P4, with setae relatively longer; dactylo-propodal lock not developed.

Thoracic sternum ( Fig. 1D View Figure 1 ) relatively wide, surface covered with numerous small, rounded, densely packed granules; sternites 1, 2 completely fused, triangular, lateral margins gently convex, separated from sternite 3 by shallow, gently convex suture, partially obscured by low granules; sternites 3, 4 fused with only lateral part of suture clearly visible, median parts indicated by shallow depressions, partially obscured by low granules; sutures 4/5, 5/6, 7/8 medially interrupted, suture 6/7 complete; distinct median longitudinal groove extending across most of sternites 7, 8. Posterior edge of episternite 7 partially overlapping small anterior part of P5 coxa. Sternopleonal cavity ( Fig. 1D View Figure 1 ) deep, extending to about two-thirds of sternite 4, reaching imaginary line connecting median part of coxae of chelipeds; with longitudinal groove on sternite 4; part of sternite 8 ( Fig. 1E View Figure 1 ) exposed between somites 2 and 4 when completely completely closed. Press-button male pleonal locking mechanism present as short peg-like tubercle on anterior margin of sternite 5, just adjacent to sternite 4. Opening for penis coxal, at anterior edge of condyle of P5 coxa; penis short, tubular with distal half wider than proximal part.

Pleon ( Figs.1D, E View Figure 1 , 7A View Figure 7 ) triangular, transversely broad, all somites, telson free; telson broadly triangular, lateral margins almost straight to gently sinuous; somite 6 transversely rectangular, much wider than long, lateral margins gently convex, gently converging towards telson; somites 3–5 wide, trapezoidal, somite 3 widest, lateral parts tapering, edges overlapping part of P5 coxae; somites 1, 2longitudinally narrow, reaching to P5 coxae.

G1 ( Fig. 8A–C View Figure 8 ) relatively slender, distal two-thirds with mesial margin gently concave; tip slightly flared, laterally flattened, subtruncate; distal surfaces with numerous short spines. G2 ( Fig. 8D View Figure 8 ) slender, much longer than G1, distal segment long, flagelliform, about three-quarters length of basal segment, tip weakly bifurcated.

Female. The carapace of the female paratypes are generally smoother on all their surfaces, with the second anterolateral tooth relatively shorter ( Figs. 2A View Figure 2 , 6B View Figure 6 ). The carapaces are also relatively higher with the dorsal surface relatively but distinctly more inflated ( Fig. 2B View Figure 2 ). The inner carpal tooth on the chelipeds are also proportionately longer and the inner surface of the chela more inflated ( Fig. 2A, D, E View Figure 2 ) compared to the male. The chelae of the females are similar in form to the holotype male except that the fingers and palm are relatively shorter ( Fig.2F, G View Figure 2 ). The pleons are ovate with all the somites and telson free ( Fig. 9A View Figure 9 ). The vulvae are ovate, large, level with the surface of the sternite, covered with stiff membrane, without opercular cover, and positioned on the anterior two-thirds of somite 6 but not reaching the suture with sternite 5 ( Fig. 9B View Figure 9 ).

Colour in life. Carapace whitish; chelipeds and ambulatory legs with the upper parts pale red; ventral surfaces white.

Remarks. The specimens from the Persian Gulf reported as C. sinica by Guinot (1989) and Naderloo (2017) are here also identified with the new species from the Bay of Bengal in India. They agree very well in the diagnostic features of the carapace and ambulatory legs with C. mistio n. sp.

Carcinoplax mistio n. sp. is superficially most similar to C. sinica , in the transversely hexagonal carapace and prominent second anterolateral tooth, which is spiniform and gently curved. It can, however, be reliably distinguished by the structure of the G1. In C. mistio n. sp., the mesial margin of the distal two-thirds is gently concave and the tip is elongate ( Fig. 8E, F View Figure 8 ) (versus distal two-thirds of the G1 is straight and the tip is rounded and short in C. sinica ; Fig. 8L, N View Figure 8 ). When specimens of similar sizes are compared, the male pleon of C. mistio n. sp. (holotype (29.2 x 19.0 mm, ZSI Reg. No. C7123/2) is proportionately broader (somite 6 and telson 1.93 and 1.11 times wider than long, respectively) ( Fig. 7A View Figure 7 ) (versus pleon of male 28.5 x 19.5 mm ( ZRC 1984.5693), relatively narrower in C. sinica , with the width to length ratio for somite 6 and telson 1.85 and 0.91 times, respectively; Fig. 7D View Figure 7 ). Larger specimens of C. sinica , however, have relatively wider male pleons, although in these specimens, the lateral margins of somite 6 are convex and those of the telson distinctly concave ( Fig. 7E, F View Figure 7 ) (versus lateral margins of somite 6 almost straight and those of the telson slightly concave in C. mistio n. sp.; Fig. 7A View Figure 7 ). The rounded tooth on the inner angle of the male carpus of the male cheliped is also proportionately shorter in C. mistio n. sp. ( Fig. 1F View Figure 1 ) compared to that of C. sinica , which is distinctly longer ( Fig. 4E View Figure 4 ).

The specimens from the Persian Gulf referred to C. sinica by Guinot (1989: 285), Castro (2007: 640) and Naderloo (2017: 69) should be referred to C. mistio n. sp. as well.Male specimens from the Persian Gulf have a relatively short rounded tooth on the distal inner angle of the carpus of cheliped (cf. Guinot, 1989: pl. 5 figs. A) as in the type of C. mistio n. sp. ( Fig. 1A, F View Figure 1 ), and their G1 structures are similar in form ( Fig. 8A–C, E, F View Figure 8 ).

It is unfortunate that the distal parts of both G1s of the holotype male of C. mistio n. sp. are damaged.In the more intact left G1, only the tip is broken off ( Fig.8B,C View Figure 8 ), but it is clear from the base that the intact structure would be more elongate and tapering, like that figured for the male from Iran (cf. Fig. 8E, F View Figure 8 ) rather than the more rounded tip observed for C. sinica ( Fig. 8L, N View Figure 8 ).

Carcinoplax sinica View in CoL was described from a large series of specimens from the Gulf of Tonkin (= Beibu Wan) in theSouthChinaSea, andtheauthorshaveexaminedtwo of the non-type specimens ( ZRC 2011.0607 and ZRC 2011.609), which had been listed by Chen (1984:190). It has since been reliably reported from Taiwan, mainland China, Philippines and Vietnam ( Serène and Lohavanijaya, 1973; Serène and Vadon, 1981; Dai et al., 1986; Guinot, 1989; Dai and Yang, 1991; Hsueh and Huang, 2002; Castro, 2007; Ng et al., 2017) (sometimes incorrectly as C. purpurea View in CoL ).

Although their carapaces are quite different, C.mistio View in CoL n. sp. shares similarly structured male pleons and G1s with C. purpurea View in CoL . Compared to C. purpurea View in CoL , however, C. mistio View in CoL n. sp. has the carapace more transversely hexagonal, with the second anterolateral tooth elongate, gently curved anteriorly and sharp ( Fig. 6A, B View Figure 6 ) (versus carapace more quadrate with the posterolateral margins convex and subparallel, the second anterolateral tooth is low and rounded, and if spine present, it is short, straight and directed anteriorly in C. purpurea View in CoL ; Fig. 6C, D View Figure 6 ); the dorsal surface of the male carapace is relatively low ( Figs. 1A, B View Figure 1 , 6A View Figure 6 ) (versus carapace appears inflated with the dorsal carapace surface prominently convex in C. purpurea View in CoL ; Figs. 3C View Figure 3 , 6C, D View Figure 6 ); and the P2–P4 merus is proportionately more elongate and slender ( Fig. 7G View Figure 7 ) (versus the P2–P4 merus shorter and stouter in C. purpurea View in CoL ; Fig.7I View Figure 7 ). Carcinoplax purpurea View in CoL is known for certain only from the west Pacific, with recordsfromJapan, Taiwan,mainlandChina, Philippines and South China Sea ( Rathbun, 1914; Sakai, 1976; Dai et al., 1986; Guinot, 1989; Dai and Yang, 1991; Hsueh and Huang, 2002; Castro, 2007; Ng et al., 2017).

The series of specimens of C. purpurea and C. sinica indicate that the carpal spine of adult female specimens is more elongate ( Fig. 9C, E View Figure 9 ) compared to comparably sized males ( Figs. 3A, E View Figure 3 , 4A, E View Figure 4 , 5 View Figure 5 ); and the inner surface of the chela is also relatively more inflated. The same pattern is observed for C. mistio n. sp. ( Figs. 1A, F View Figure 1 , 2A, D, E View Figure 2 ).

The discovery of the present new Carcinoplax species is not surprising; the first goneplacid being C.fasciata , which had been confused with C. specularis (see Ng and Kumar, 2016). Recent studies have shown that an increasing number of brachyuran species which have previously believed to occur in both Indian and west Pacific waters are actually separate taxa, even if they are relatively shallow water taxa (e.g., see Castro and Ng, 2010; Ng and Castro, 2013, 2016; Ng and Kumar, 2016; Ng and Richer de Forges, 2015; Ng et al., 2018). As reviewed in Trivedi et al. (2018), many Indian taxa will need to be compared with their Pacific and/or Southeast Asian counterparts, ostensibly the same species, to ascertain their actual identities.