Description of a New Genus with Two New Species of Freshwater Crab (Decapoda: Brachyura: Gecarcinucidae) from the Southwestern Ghats, India

© 2021 Academia Sinica, Taiwan Open Access Description of a New Genus with Two New Species of Freshwater Crab (Decapoda: Brachyura: Gecarcinucidae) from the Southwestern Ghats, India Smrithy Raj1,2 , Appukuttannair Biju Kumar1, and Peter K.L. Ng3,* 1Department of Aquatic Biology and Fisheries, University of Kerala, Kariavattom, Thiruvananthapuram – 695581, Kerala, India. E-mail: [email protected] (Kumar) 2National Centre for Biological Sciences, Tata Institute of Fundamental Research, GKVK Campus, Bellary Road, Bangalore 560065, Karnataka, India. E-mail: [email protected] (Raj) 3Lee Kong Chian Natural History Museum, Faculty of Science, National University of Singapore, 2 Conservatory Drive, Singapore 117377, Republic of Singapore. *Correspondence: E-mail: [email protected] (Ng) Received 24 October 2020 / Accepted 29 January 2021 / Published 6 April 2021 Communicated by Benny K.K. Chan A new genus of freshwater crab, Rajathelphusa gen. nov., and two species, R. ala sp. nov. (type species) and R. muni sp. nov., are described from the Kerala part of the Western Ghats biodiversity hotspot, India. The new genus is superficially similar to Arcithelphusa and Cylindrotelphusa from the same hotspot, but can easily be distinguished by the following suite of carapace and gonopod characters: an inflated carapace with distinct epigastric and postorbital cristae; the third maxilliped exopod has a long flagellum; male sternites 2 and 3 separated by a shallow median groove while sternites 3 and 4 are completely fused, demarcated by shallow oblique grooves; a male first gonopod that is almost straight with a short distal conical distal segment; and a male second gonopod that is as long as the first gonopod with a long and slender distal segment. The two new species can be differentiated from each other by the lateral margins of male pleonal somite 6 and the structure of the male first gonopod. Key words: Taxonomy, New freshwater crab, Gecarcinucoidea, Kerala, Idukki, Arcithelphusa, Cylindrotelphusa. BACKGROUND In India, the Western Ghats biodiversity hotspot is well-known for its rich endemic flora and fauna (Myers 1988; Myers et al. 2000). The region is rich in ancient endemic clades of many animal groups, including cryptic lineages of frogs (Ramesh et al. 2020), arguably the highest global diversity of endemic fish, many with a complex evolutionary history (Britz et al. 2019), and a high proportion of ancient Gondwanan relicts (Karanth 2015; Sidharthan et al. 2020). Although the Western Ghats is an area of high conservation value (Das et al. 2006); anthropogenic impacts have been growing in the region (Molur et al. 2011), and there is an urgency to better document the fauna there. With regards to brachyuran crabs, 59 species of freshwater crabs (all Gecarcinucidae) have been reported from this region (see Pati et al. 2020). The fauna, however, remains poorly studied as many parts of the Western Ghats have not been surveyed for freshwater crabs (Rajesh et al. 2017). The southern part of the Western Ghats, which is in the state of Kerala, is home to 37 freshwater crab species in 14 genera, 18 of which are endemic to the state (Pati and Sharma 2013; Pati and Sudha Devi 2015a b; Kumar et al. 2017; Rajesh et al. 2017; Pati et al. 2017 2019 2020). Citation: Raj S, Kumar AB, Ng PKL. 2021. Description of a new genus and two new species of freshwater crab (Decapoda: Brachyura: Gecarcinucidae) from the southwestern Ghats, India. Zool Stud 60:14. doi:10.6620/ZS.2021.60-14. Zoological Studies 60:14 (2021) doi:10.6620/ZS.2021.60-14 1 © 2021 Academia Sinica, Taiwan A survey in the southern Western Ghats recently obtained specimens of a terrestrial crab from burrows and nearby streams in the Rajakumari and Marayoor regions of Idukki district, Kerala state, India. Detailed morphological studies showed that they belong to a new genus, here named Rajathelphusa, with two new species, R. ala and R. muni. The new genus and species are described and compared with allied genera. MATERIALS AND METHODS Methods of measurement and terminology follow Ng (1988) and Davie et al. (2015). The sizes of the specimens (in millimetres) refer to the maximum carapace width and length, respectively. The samples are deposited in the Western Ghats Regional Centre, Zoological Survey of India (ZSI/WGRC), Calicut, Kerala and museum collections of the Department of Aquatic Biology and Fisheries, University of Kerala (DABFUK), India. The following abbreviations are used: G1 and G2 = male first and second gonopods, respectively; P2–P5 = pereopods 2–5 (first to fourth ambulatory legs, respectively). RESULTS TAXONOMY Superfamily Gecarcinucoidea, Rathbun, 1904 Family Gecarcinucidae Rathbun, 1904 Rajathelphusa gen. nov. urn:lsid:zoobank.org:act:0006A325-FA77-42A5-9290111BAEC7CDFE Diagnosis: Carapace high, inflated in frontal view, dorsal surface convex; epigastric cristae low but distinct, not cristate, medially separated by shallow inverted Y-shaped furrow, slightly anterior of postorbital cristae; postorbital cristae distinct, raised, not sharp, confluent with epigastric cristae, joining lateral margin as granules and interrupted striae; cervical grooves distinct, not broad, joining prominent H-shaped and branchial grooves; epibranchial tooth small, separated from external orbital tooth by small cleft, positioned above external orbital tooth in frontal view; orbits subovate, oblique in frontal view; posterior margin of epistome with distinct median triangular tooth, separated from lateral concave lobe by distinct gape; third maxilliped exopod with long flagellum that extends across width of merus; ambulatory legs slender, not elongate, merus without subdistal tooth or spine; anterior male thoracic sternum transversely narrow; sternites 1 and 2 completely fused to form triangular plate with convex lateral margins; separated from sternite 3 by shallow, median groove, otherwise fused; sternites 3 and 4 fused, demarcated by shallow oblique grooves, suture between sternites 4/5, 5/6 and 6/7 medially interrupted, separated by narrow space; suture between sternites 7/8 complete, with longitudinal groove on sternites 7 and 8, sternite 8 not visible when pleon closed; male sternopleonal cavity deep, narrow, reaching to imaginary line joining median points of cheliped coxae, distal margin on sternite 4 cristate; male pleonal-locking tubercle low, round, positioned on distal third of sternite 5; male pleon triangular, somite 6 subquadrate, telson triangular with concave lateral margins; G1 almost straight, groove for G2 ventral in position; terminal and subterminal segments clearly demarcated, terminal segment relatively short, conical, surface lined with rows of long and short setae which do not obscure surface, gradually tapering to truncate tip; G2 long, as long as G1; with long, slender distal segment, about half length of basal segment; vulvae positioned just adjacent to and pressing into the suture with sternite 5, opening membranous without sternal vulvar cover. Type species: Rajathelphusa ala sp. nov. by present designation. Etymology: The genus name is derived from the Rajakumari, the type locality of the type species. The word ‘Raja’ in vernacular Malayalam language means King or Prince, and ‘Kumari’ or girl is feminine gender (as opposed to ‘Kumar’ or boy who is masculine), with ‘Rajakumari’ meaning princess. The name is arbitrarily combined with the generic name ‘Thelphusa’. The gender of the genus is feminine. Remarks: Rajathelphusa gen. nov. has a suite of characters that differentiates it from all the other gecarcinucid genera (see Bahir and Yeo 2007). In its high and inflated carapace, Rajathelphusa is superficially most similar to Arcithelphusa Pati & Sudha Devi, 2015, and Cylindrotelphusa Alcock, 1909. Two species of Arcithelphusa are known: A. cochleariformis Pati & Sudha Devi, 2015 (type species) and A. tumpikkai Pati, Sujila & Sudha Devi, 2019, both from the Wayanad district in northern Kerala. Four species of Cylindrotelphusa have been reported from Trivandrum, Ernakulam, Thrissur, Idukki, Kollam, Kozhikode and Pathanamthitta districts in Kerala: C. breviphallus Pati, Rajesh, Raj, Sheeja, Kumar & Sureshan, 2017, C. granulata (Pillai, 1951), C. longiphallus Pati, Rajesh, Raj, Sheeja, Kumar & Sureshan, 2017, and C. steniops (Alcock, 1909) (type species). Rajathelphusa can easily be distinguished from Arcithelphusa by the front being gently deflexed (Figs. page 2 of 13Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan 3B, 4C, 7C) (vs. front strongly deflexed); with distinct postorbital cristae (Figs. 3A, 4B, 7B) (vs. indistinct postorbital cristae); presence of a long flagellum on the exopod of third maxilliped (Fig. 9A) (vs. short or no flagellum); a G1 with the terminal segment short and conical (Fig. 9B–D, F–H) (vs. curved and elongate); and a G2 with a long distal segment (Fig. 9E, I) (vs. G2 with short distal segment) (cf. Pati and Sudha Devi 2015a: fig. 2A, B, G 3B, D, E, I, Pati et al. 2019: fig. 2A, B, D, E, 3B, D, H, K, N, P, 4A, B, D, E). Rajathelphusa can be distinguished from Cylindrotelphusa by the anterolateral margins being weakly serrated to smooth (Figs. 2A, 3A, 4B, 7B) (vs. distinctly serrated); male telson triangular (Figs. 3D, 4E, 7E) (vs. distinctly elongate); and the G2 with a long distal segment (Fig. 9E, I) (vs. G2 with short distal segment) (cf. Pati et al. 2017: fig. 11a, b, h, i, 12d, 13a, b, h, i, 14d). These morphological differences are substantial and require the establishment of a new genus. Distribution: The genus is known only from Rajakumari and Marayoor regions of Idukki district, the southern Western Ghats, Kerala (Fig. 1). Rajathelphusa ala sp. nov. (Figs. 2–5, 9A–E) urn:lsid:zoobank.org:act:78B5F375-FF49-4A67-A4796706357C4B2B Material examined: Holotype: male (27.0 × 20.9 mm) (ZSI/WGR/IR.INV./15361), in shallow to deep burrows near Rajakumari, Idukki district, Kerala, 9.966911°N, 77.129715°E, coll. S. Raj, 29 August 2016. Paratypes: 2 males (29.9 × 23.1 mm, 29.5 × 22.1 mm) (DABFUK/AR-BR-100, 101), 12 females (25.1 × 18.9 mm, 26.8 × 20.8 mm, 27.3 × 21.9 mm, 29.2 × 22.2 mm, 30.0 × 21.9 mm, 30.3 × 22.7 mm, 30.4 × 23.2 mm, 30.4 × 23.6 mm, 30.9 × 23.9 mm, 31.0 × 24.0 mm, 31.0 × 24.4 mm, 34.9 × 26.3 mm) (DABFUK/ AR-102-113), 3 males (24.2 × 20.8 mm, 26.3 × 20.7 mm, 30.7 × 23.5 mm), 2 females (28.2 × 22.5 mm, 32.0 × 25.3 mm) (DABFUK), same data as holotype. Diagnosis: Carapace frontal region relatively narrow (Figs. 2A, 3A, B, 4A–C); external orbital tooth with outer margin gently sinuous to almost straight (Figs. 3A, B, 4A–C); male pleonal somite 6 subquadrate with gently convex lateral margins, telson with concave lateral margins (Figs. 3D, 4E); G1 terminal segment relatively short, conical, straight, directed anteriorly, surface lined with rows of long and short setae which do not obscure surface, gradually tapering to truncate tip, ca 0.16 times length of subterminal segment (Figs. 3F, 9B–D). Description of male holotype: Carapace broader than long, width ca. 1.3 times length; high, inflated in frontal view, dorsal surface convex (Figs. 2A, 3A, B, 4A–C). Frontal region relatively narrow, surface smooth or with low, flattened granules; lateral parts of anterolateral and posterolateral regions with prominent short oblique striae; suborbital with scattered low granules and striae; pterygostomial regions with distinct low granules of different sizes on margins, glabrous, separated from other regions by low ridge; sub-branchial regions with numerous short striae (Figs. 3B, 4C). Epigastric cristae low but distinct, not cristate, medially separated by shallow inverted Y-shaped furrow, slightly anterior of postorbital cristae; postorbital cristae distinct, raised, not sharp, confluent with epigastric cristae, joining lateral margin as granules and interrupted striae (Figs. 3A, 4A, B). Cervical grooves distinct, not broad, joining prominent H-shaped and branchial grooves (Figs. 3A, 4A, B). External orbital tooth low, tip slightly posterior to orbit, outer margin gently sinuous to almost straight, 3 times longer than inner margin; epibranchial tooth small, but separated from external orbital tooth by small cleft, positioned above external orbital tooth in frontal view (Figs. 3A, B, 4A–C). Anterolateral margins distinctly convex, uneven, striae given impression of margin being gently serrated (Figs. 3A, 4A, B). Posterolateral margins gently concave, converging to almost straight or gently sinuous posterior carapace margin (Figs. 3A, 4A, B). Orbits subovate, oblique in frontal view; eyes filling up most of orbital space, eye peduncle moderately long, stout; cornea large, pigmented (Figs. 3A, B, 4A–C). Supraorbital margin sinuous, complete; suborbital margin concave, lined with small rounded granules (Figs. 3A, B, 4A–C). Antennae short, not reaching cornea of eyes; antennules folding transversely in longitudinally narrow fossa (Figs. 3B, 4C). Posterior margin of epistome with distinct median triangular tooth, separated from lateral concave lobe by distinct gape (Figs. 3B, 4C). Mandibular palp distinctly 2-segmented; terminal article bilobed. Third maxilliped ischium subrectangular, with distinct median sulcus; merus subquadrate with margins raised, median part depressed; exopod slender, not reaching midlength of merus, flagellum long, reaching about entire width of merus (Figs. 3C, 4D, 9A). Chelipeds distinctly asymmetrical (Figs. 3A, G, H, 4A, G). Dorsal margin of merus with strong striae, appears unevenly serrated, ventral margin with low granules (Fig. 2). Outer surfaces of carpus rugose, distal angle with prominent sharp tooth with basal tubercle (Figs. 2A, 3A, 4A). Outer surface of chelae unevenly granulated, granules scattered; fingers of major chela curved, with median proximal gape when fingers closed, cutting margin with small teeth and denticles; fingers of minor chela straighter, similar to those of major chela except more slender, without gape (Figs. 3G, H, 4G). page 3 of 13Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan Fig. 1. Map of India showing type localities of Rajathelphusa ala gen. nov., sp. nov. and R. muni gen. nov., sp. nov. (Map: Josin Tharian). N page 4 of 13Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan Fig. 2. Colour in life. Rajathelphusa ala gen. nov., sp. nov., holotype male (27.0 × 20.9 mm) (ZSI/WGR/IR.INV./15361) Live specimen. A, overall dorsal view; B, ventral view of cephalothorax. Ambulatory legs slender, not elongate; P3 longest, P5 shortest (Figs. 2, 3A, 4A). Merus with sharp dorsal and ventral margins but not cristate, margins unevenly serrate but low; carpus with low median crest; propodus subrectangular, with longitudinal median crest, ventral margin serrate; dactylus elongate, gently curved, lined with sharp spines (Figs. 2, 3A, 4A). Anterior thoracic sternum transversely narrow; sternites 1 and 2 completely fused to form triangular plate with convex lateral margins; separated from sternite 3 by shallow, median groove, otherwise fused; sternites 3 and 4 fused, demarcated by shallow oblique grooves; dense short setae on grooves between sternites 2 and 4. Suture between sternites 4/5, 5/6 and 6/7 medially interrupted, separated by narrow space; suture between sternites 7/8 complete, with longitudinal groove on sternites 7 and 8; sternite 8 completely covered by closed pleon (Figs. 3E, F, 4F). Sternopleonal cavity deep, narrow, reaching to imaginary line joining median points of cheliped coxae, distal margin on sternite 4 cristate (Figs. 3E, 4F). Male pleonal-locking tubercle low, round, positioned on distal third of sternite 5 (Fig. 3F). Penis on condyle of coxa of fourth ambulatory leg. Pleon triangular, all somites and telson free; somites 1 and 2 longitudinally narrow, subequal in width, both less wide than somite 3; somite 3 page 5 of 13 Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan Fig. 3. Rajathelphusa ala gen. nov., sp. nov., holotype male (27.0 × 20.9 mm) (ZSI/WGR/IR.INV./15361). Preserved. A, overall view; B, frontal view of cephalothorax; C, right third maxilliped; D, pleon; E, anterior thoracic sternum and pleon; G, outer view of right chela; H, outer view of left chela. page 6 of 13 Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan trapezoidal, reaching to coxae of fourth ambulatory legs; somites 4 and 5 trapezoidal; somite 6 subquadrate with gently convex lateral margins, slightly wider than long; telson triangular with concave lateral margins, slightly wider than long (Figs. 3D, 4E). G1 almost straight, groove for G2 ventral in position; terminal and subterminal segments clearly demarcated by distinct membranous suture; subterminal segment with proximal part broader, gradually tapering to more slender section along distal half, outer margin gently sinuous; terminal segment relatively short, conical, straight, directed anteriorly, surface lined with rows of long and short setae which do not obscure surface, gradually tapering to truncate tip, ca 0.16 times length of subterminal segment (Figs. 3F, 9B–D). G2 long, as long as G1; with long, slender distal segment, about half length of basal segment (Fig. 9E). Females: The adult paratype female specimens (largest 34.9 × 26.3 mm, DABFUK/AR-BR-113) resemble the holotype in most non-sexual characters, with the major chela less swollen compared to similarlysized males, although the surface is relatively more rugose (Fig. 5A). Its pleon is ovate, with all the somites and telson free, covering most of the thoracic sternum Fig. 4. Rajathelphusa ala gen. nov., sp. nov., paratype male (26.3 × 20.7 mm) (DABFUK). A, overall view; B, dorsal view of carapace; C, frontal view of cephalothorax; D, left third maxilliped; E, pleon (setae denuded); F, buccal cavity and anterior thoracic sternum; G, outer view of chelae. page 7 of 13Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan Fig. 5. Colour in life. Rajathelphusa ala gen. nov., sp. nov., paratype female (34.9 × 26.3 mm) (DABFUK/AR-BR-113). A, overall dorsal view; B, ventral view of cephalothorax and pleon; C, sternopleonal cavity and vulvae. page 8 of 13Zoological Studies 60:14 (2021) © 2021 Academia Sinica, Taiwan except the lateral edges and distal parts of sternite 3 when closed (Fig. 5B). The vulvae, on somite 6 are large, subovate, positioned just adjacent to and pressing into the suture with sternite 5, the opening being membranous without any sternal vulvar cover (Fig. 5C). Variation: The exopod of the third maxilliped usually has a long flagellum (Fig. 9A), but in a number of specimens, this structure is missing; almost certainly the result of it being broken off. In most of these cases, only one of the third maxillipeds lacks the flagellum, the other being present. There is no variation in the length of the flagellum when present. The distal segment of the G2 varies in shape, from being straight to slightly curved in the specimens examined. Colour: The carapace is dark brown in life, with the chelipeds and ambulatory legs light brown (Figs. 2, 5, 6D). Etymology: The name of the species, which lives in deep burrows, alludes to the deep rock shelters tribal people in the region used to live in, which they name as ‘alas” (ala, singular). The name is used as a noun in apposition. Distribution: The species is only known from Rajakumari (9.973453°N, 77.168647°E), a village about 1000 m above mean sea level, Idukki district of Kerala. Remarks: For differences with R. muni sp. nov., see remarks for that species. Ecology: The species lives in deep to shallow burrows along the sides of the streams and also away from water (Fig. 6A–C). The burrows, which are always away from the stream itself, are relatively deep. Rajathelphusa muni sp. nov. (Figs. 7, 8, 9F–H) urn:lsid:zoobank.org:act:BCEADCAB-2A47-42A3-AF555223AE837E68 Material examined: Holotype: male (30.2 × 22.6 mm) (DABFUK/AR-BR-115), in deep burrows near to Kovilkadavu, Idukki district, Kerala, 10.261413°N, 77.183799°E, coll. S. Raj, 20 August 2016. Paratype: 1 male (carapace detached, 26.3 × 20.0 mm) (DABFUK/AR/-BR-114), same data as holotype. Diagnosis: Carapace frontal region relatively wider (Fig. 7A, B); external orbital tooth with outer margin Fig. 6. Habitat of Rajathelphusa ala gen. nov., sp. nov., Cardamom planation area; Rajakumari, Idukki, Kerala, India. Specimen not preserved. page 9 of 13 Zoological Studies 60:14 (2021)