Carcinoplax mistio , a new species of goneplacid crab from the Indian Ocean ( Decapoda : Brachyura : Goneplacoidea )

Trawls from the Bay of Bengal obtained material of a new species of the goneplacid crab, Carcinoplax mistio, the sixth species of the genus known from India. The new species superficially resembles C. purpurea Rathbun, 1914, and C. sinica Chen, 1984, from the western Pacific. Records of C. sinica from the Persian Gulf are also here referred to the new species. Carcinoplax mistio n. sp. differs from its closest congeners mainly in the form of the carapace, anterolateral armature, proportions of the ambulatory legs and structure of the male pleon. KeywOrds taxonomy, Goneplacidae, Indo-Pacific region, Carcinoplax, Bay of Bengal, India

originally described from the South China Sea by Chen (1984) but has since been reported from the Philippines, Taiwan and Persian Gulf (Guinot, 1989;Hsueh and Huang, 2002;Castro, 2007;Naderloo, 2017;Ng et al., 2017).Close comparisons with material of C. sinica s. str.from the western Pacific (including topotypic material) showed that the Indian specimens belong to a different species, here named Carcinoplax mistio n. sp.

Material and MethOds
Specimens examined are deposited in the Zoological Survey of India (ZSI), Calcutta, India; and Zoological Reference Collection (ZRC) of the Lee Kong Chian Natural History Museum, National University of Singapore.Measurements (in millimetres) of the material examined are of the maximum carapace width (including teeth) and length, respectively.Male pleonal somite 6 is measured along its widest part (on the proximal margin) and the maximum length along the median line, while the male telson is measured along its longest and widest points.The following abbreviations are used: P2-P5 = first to fourth ambulatory legs, respectively; G1 and G2 = male first and second gonopods, respectively.The terminology used follows Ng et al. (2008) and Davie et al. (2015).Carcinoplax (purpurea) ? -Stephensen, 1946: 166, 208, fig. 44 (not Carcinoplax purpurea Rathbun, 1914).

New species of goneplacid crab from the Indian Ocean
Nauplius, 27: e2019004 Diagnosis.Carapace transversely hexagonal, 1.33-1.54times 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 New species of goneplacid crab from the Indian Ocean Nauplius, 27: e2019004 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; G2 much longer than G1, distal segment flagelliform, about three-quarters length of basal segment; vulvae ovate, large, level with surface of sternum.
Chelipeds (Fig. 1A, F-H) 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.

New species of goneplacid crab from the Indian Ocean
Nauplius, 27: e2019004 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.
Female.The carapace of the female paratypes are generally smoother on all their surfaces, with the second anterolateral tooth relatively shorter (Figs.2A, 6B).The carapaces are also relatively higher with the dorsal surface relatively but distinctly more inflated (Fig. 2B).The inner carpal tooth on the chelipeds are also proportionately longer and the inner surface of the chela more inflated (Fig. 2A, D, E) 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).The pleons are ovate with all the somites and telson free (Fig. 9A).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).

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) (versus distal two-thirds of the G1 is straight and the tip is rounded and short in C. sinica; Fig. 8L, N).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) (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).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) (versus lateral margins of somite 6 almost straight and those of the telson slightly concave in C. mistio n. sp.; Fig. 7A).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) compared to that of C. sinica, which is distinctly longer (Fig. 4E).
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), and their G1 structures are similar in form (Fig. 8A-C, E, F).
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), 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) rather than the more rounded tip observed for C. sinica (Fig. 8L, N).
Carcinoplax sinica was described from a large series of specimens from the Gulf of Tonkin (= Beibu Wan) in the South China Sea, and the authors have examined two of the non-type specimens (ZRC 2011.0607 andZRC 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; New species of goneplacid crab from the Indian Ocean Nauplius, 27: e2019004 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).
Although their carapaces are quite different, C. mistio n. sp. shares similarly structured male pleons and G1s with C. purpurea.Compared to C. purpurea, however, C. mistio n. sp. has the carapace more transversely hexagonal, with the second anterolateral tooth elongate, gently curved anteriorly and sharp (Fig. 6A, B) (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; Fig. 6C, D); the dorsal surface of the male carapace is relatively low (Figs.1A, B, 6A) (versus carapace appears inflated with the dorsal carapace surface prominently convex in C. purpurea; Figs.3C, 6C, D); and the P2-P4 merus is proportionately more elongate  and slender (Fig. 7G) (versus the P2-P4 merus shorter and stouter in C. purpurea; Fig. 7I).Carcinoplax purpurea is known for certain only from the west Pacific, with records from Japan, Taiwan, mainland China, 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) compared to comparably sized males (Figs.3A, E, 4A, E, 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, 2A, D, E).

New species of goneplacid crab from the Indian Ocean
Nauplius, 27: e2019004

New species of goneplacid crab from the Indian Ocean
Nauplius, 27: e2019004 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 andCastro, 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.

New species of goneplacid crab from the Indian Ocean
Nauplius, 27: e2019004