Carcinoplax mistio Ng & Mitra 2019

Carcinoplax mistio Ng & Mitra, 2019 Figs 1, 2, 3 Carcinoplax (purpurea)? – Stephensen 1946: 166, 208, fig. 44 (not Carcinoplax purpurea Rathbun, 1914). Carcinoplax purpurea – Guinot 1967: 276 (list); Titgen 1982: 252 (list) (not Carcinoplax purpurea Rathbun, 1914). Carcinoplax sinica – Guinot 1989: 285, fig. 14 A, B, pl. 5 figs A, B, B 1, C, C 1, D, E, E 1; Apel 2001: 101; Naderloo and Sari 2007: 449; Naderloo 2017: 69, text-fig. 11.2 d, e, fig. 12.1 (not Carcinoplax sinica Chen, 1984) [= Carcinoplax haswelli Miers, 1884)]. Carcinoplax mistio Ng & Mitra, 2019: e 2019004, figs 1, 2, 6 A, 7 A, G, H, 8 A – G, 9 A, B. Carcinoplax haswelli – Sureandiran et al. 2024: figs 2–7 (not Carcinoplax haswelli Miers, 1884). Material examined. Holotype. India • ♂ (29.2 × 19.0 mm); northern Bay of Bengal, Fresargunj Fishing Harbour; 24 Feb. 2017; coll. local fishermen by trawl; ZSIK C 7123/2. Paratypes. India • 1 ♀ (36.4 × 24.2 mm); same collection data as for holotype; ZSIK C 7124/2 • 1 ♀ (36.7 × 27.5 mm); northern Bay of Bengal, Fresargunj Fishing Harbour; 28 Jul. 2018; ZSIK. Other material. India • 4 ♂♂ (35.1 × 30.0 mm, 29.2 × 22.3 mm, 25.1 × 18.2 mm, 23.2 × 29.2 mm), 5 ♀♀ (37.1 × 31.0 mm, 36.2 × 30.1 mm, 32.1 × 25.2 mm, 31.0 × 25.2 mm, 26.1 × 24.0 mm); southern Bay of Bengal, eastern Tamil Nadu, Pazhayar Fishing Port; 11 ° 21 ' N, 79 ° 50 ' E; depth 50–100 m; 2016–2020; coll. M. Prema & S. Ravichandran; CASAU CR-1031 • 1 ♂ (29.7 × 19.6 mm), 1 ♀ (43.2 × 29.2 mm); same collection data as for preceding; 2016–2020; CASAU CR-1032 • 2 ♂♂ (37.6 × 25.8 mm, 32.0 × 21.3 mm), 7 ♀♀ (37.9 × 24.5 mm, 37.0 × 25.1 mm, 34.0 × 22.9 mm, 32.3 × 21.6 mm, 31.2 × 21.1 mm, 29.5 × 19.6 mm, 27.6 × 25.6 mm,); same collection data as for preceding; 18 Mar. 2018; CASAU CR-1033 • 1 ♂ (26.4 × 18.0 mm), 1 juv. ♀ (19.7 × 13.4 mm); same collection data as for preceding; Mar. 2018; CASAU CR-1034 • 4 ♂♂ (36.0 × 23.8 mm, 33.0 × 22.7 mm, 32.5 × 22.2 mm, 31.5 × 21.4 mm), 1 ovig. ♀ (39.0 × 26.6 mm), 1 ♀ (36.1 × 23.4 mm); same collection data as for preceding; 2020–2021; CASAU CR-1035 • 3 ♂♂ (29.2 × 20.8 mm, 29.2 × 20.5 mm, 27.6 × 19.4 mm); same collection data as for preceding; 12 Jan. 2022; CASAU CR-1036 • 1 ♂ (33. × 23.4 mm); same collection data as for preceding; CASAU CR-1037 • 2 ♀♀ (37.3 × 24.6 mm, 35.9 × 23.7 mm); same collection data as for preceding; 26 Mar. 2023; CASAU CR-1038 • 2 ♂♂ (26.2 × 18.1 mm, 24.8 × 16.4 mm), 2 ♀♀ (32.2 × 22.0 mm, 29.5 × 19.4 mm); same collection data as for preceding; 11 Feb. 2024; CASAU CR-1039 • 7 ♀♀ (37.9 × 26.0 mm, 36.2 × 24.3 mm, 34.7 × 22.5 mm, 33.5 × 24.0 mm, 30.5 × 20.5 mm, 29.2 × 20.1 mm, 25.1 × 17.1 mm); same collection data as for preceding; 11 Feb. 2024; CASAU CR-1040. Diagnosis. Modified from Ng and Mitra (2019). Carapace broad, dorsal surface gently convex; antero-lateral surfaces generally with small, rounded, densely packed granules, sometimes appearing almost smooth; post-orbital region with small, rounded granules; second anterolateral teeth relatively short in larger specimens, slightly sharp in smaller specimens; gastro-cardiac groove shallow but visible (Figs 1, 2 A, B, 3 A). Chelipeds unequal, male carpal spine more rounded, that on female more elongate (Figs 1, 2 G – I, J – L, 3 B – D). Ambulatory legs long, slender; articles laterally flattened, smooth; margins lined with setae (Fig. 1). G 1 relatively slender, laterally flattened, tip elongate, tapering, lined with numerous short spines (Fig. 3 G – K). G 2 longer than G 1, distal segment long, curved, tip weakly bifurcated (Ng and Mitra 2019: fig. 8 D). Habitat. The present specimens of C. mistio were collected from 50–100 m depth, off the coastal waters of Tamil Nadu state, Bay of Bengal, India. The three type specimens were obtained from West Bengal, also from a fishing port but without depth data (Ng and Mitra 2019). Coloration in life. Carapace orange, cheliped fingers and upper surface of ambulatory legs white (Fig. 1 A – C), merus of ambulatory leg generally orange (Fig. 1), and sternopleonal surfaces pale white. Distribution. Northern Indian Ocean: Bay of Bengal (West Bengal and off Tamil Nadu coast, India; Ng and Mitra 2019; present study); north-western Arabian Sea (Gujarat, India; Sureandiran et al. 2024); and Persian Gulf (Guinot 1989; Naderloo 2017). Remarks. The present specimens of C. mistio agree well with the type account (Ng and Mitra 2019). The large series of specimens, however, allowed us to document size-related morphological variation. The largest specimens of C. mistio collected in this study have a carapace width of 37.6 mm (male, CASAU CR-1033) and 43.2 mm (female, CASAU CR-1032), respectively; both are larger than the type specimens and are the largest known specimens of the species. In the types as well as in the smaller males (e. g., 26.4 × 18.0 mm, CASAU CR-1034; 24.8 × 16.4 mm, CASAU CR-1039) and most of the larger specimens of the present collection, the second anterolateral tooth of the carapace is prominent, being sharp and curved (Figs 1 B, 2 B). In the largest males (e. g., 35.1 × 30.0 mm, CASAU CR-1031; 33.3 × 23.4 mm, CASAU CR-1037), this tooth is relatively lower (Figs 1 A, 2 A) and comparable to the condition in C. purpurea. In C. purpurea, however, the second anterolateral tooth is even lower and more like a rounded tubercle (cf. Ng and Mitra 2019: fig. 6 C, D). As such, the form of the second anterolateral tooth is not a reliable diagnostic character for C. mistio at all body sizes, being sometimes size dependent, though it is usually sharp and longer. The cheliped of the largest males is elongate, with the merus and fingers extremely long (Fig. 1 A, B, 2 G – I), a condition like that of C. longimanus (see Guinot 1989). In the smaller holotype male of C. mistio as well as in smaller males, the chelipeds are relatively shorter (Fig. 2 J – L). Sexual dimorphism is apparent as all females have relatively shorter cheliped fingers (Figs 1 C, 3 B – D). The cheliped carpal spine of male C. mistio specimens examined, including the holotype male, is relatively more rounded and relatively shorter (Figs 1 A, B, 2 G, H, J, L) (versus the cheliped carpal spine being relatively less rounded, more elongate and curved in most of the females; Figs 1 C, 3 B, D). In the holotype male of C. mistio, the carpal spine is relatively short and rounded (Ng and Mitra 2019: fig. 1 A, F) and as such, its length is a sexually dimorphic character (Ng and Mitra 2019: fig. 2 A, D, F) that is not size dependent. This is similar to the condition of the cheliped carpal spine that was reported for C. haswelli (as C. sinica, cf. Ng and Mitra 2019: fig. 4 E). The lateral margins of pleonal somite 6 of large males is gently convex, gradually converging towards the telson, which is similar to that of the holotype of C. mistio (Fig. 2 E, F; see Ng and Mitra 2019: fig. 7 A). In the large male of C. mistio (33.3 × 23.4 mm, CASAU CR-1037), pleonal somite 6 is proportionately broader, width-to-length ratios 2.1 (versus pleonal somite 6 width-to-length ratios in two smaller males (26.2 × 18.1 mm, 24.8 × 16.4 mm, CASAU CR-1040) being 1.96 and 1.98, respectively). The pleon of large females in the present collection is similar to that reported for the paratype C. mistio (36.4 × 24.2 mm, ZSIK C 7124 / 2) (cf. Ng and Mitra 2019: fig. 9 A), but in a smaller specimen (26.1 × 24.0 mm, CASAU CR-1031), the pleon is relatively wider than in the paratype. Among the 28 female specimens studied, only one was ovigerous (39.0 × 26.6 mm, CASAU CR-1035). In juvenile females (e. g., 19.7 × 13.4 mm, CASAU CR-1034), the pleon is not expanded, lacking setae on pleopods, and the operculum of the vulva is poorly developed. The proportions of the male telson vary regardless of size with the width-to-length ratios of three males (33.3 × 23.4 mm, CASAU CR-1037; 26.2 × 18.1 mm, 24.8 × 16.4 mm, CASAU CR-1040) are 0.76, 0.88 and 0.67, respectively. Overall, the male telson is slightly broader with the lateral margins being more concave (Fig. 2 E, F) than in C. haswelli (cf. Ng and Mitra 2019: fig. 7 D – F). The mesial margin of the distal two-thirds of the G 1 is gently concave in large specimens of C. mistio (Fig. 3 G) and almost straight in smaller ones (Fig. 3 I), but the tip is always elongate and tapering (Fig. 3 G – J). Ng and Mitra (2019) observed that the G 1 s of the holotype (29.2 × 19.0 mm, ZSIK C 7123 / 2) were distally damaged. Sureandiran et al. (2024) reported “ Carcinoplax haswelli ” based on one male specimen from Gujarat in western India, but all their figures of the G 1 and the carapace (see Sureandiran et al. 2024: figs 2–7), actually correspond to C. mistio. The genetic comparisons for seven species of Carcinoplax, including C. mistio, are interesting (Fig. 4). The intraspecific divergences of COI (657 bp) and 16 S rRNA (552 bp) genes for four morphologically distinct species of Carcinoplax are less than 1.5 %: C. haswelli (COI 0.2 %), C. mistio (COI 0 %, 16 S 0.2 %), C. purpurea (COI 0.5–1.1 %, 16 S 0 %), and C. longimanus (COI 0.2–0.8 %, 16 S 0.0 –0.4 %) (Table 2). As for the interspecific divergences of the three species under study here (Table 2), that between C. haswelli and C. mistio is high (COI 10.3–10.5 %, 16 S 3.5 –3.7 %), as is that between C. haswelli and C. purpurea (COI 9.9–10.5 %, 16 S 3.5 %) (Table 2, Fig. 4), corroborating their status as separate species. The genetic divergence between C. mistio and C. purpurea, however, was unexpectedly low (COI 0.3–0.8 %, 16 S 0.0 –0.2 %) and within the range normally considered for conspecificity (Fig. 4) when compared with the other four species of Carcinoplax (COI 12.4–21.1 %, 16 S 6.5 –12.1 %) (Table 2, Fig. 4). The morphological differences between C. mistio and C. purpurea, however, are substantial. In C. mistio, the carapace is proportionally wider, appearing more rectangular in shape with the posterolateral margins distinctly converging posteriorly (Figs 1 A – C, 2 A, B, 3 A; see Ng and Mitra 2019: figs 1