Neorhynchoplax patnahi sp. nov.

(Figs. 1–5)

Neorhynchoplax nasalis — Nesemann et al. 2007: 217, 223, pl. 68 figs. 6–10, 235 (not Rhynchoplax nasalis Kemp, 1917). Neorhyncoplax sp.— Nesemann et al. 2011: 20, 24, 26.

Material examined. Holotype: 3 (6.73 × 6.24 mm) (ZSI-GPRC IV-3021), Ganga River, Patna, Adalat Ghat, 25°37’34.4”N 85°9’33.7”E, India, coll. H. Nesemann & G. Sharma, 16 January 2011. Paratypes: 1 3 (6.89 × 6.48 mm), 1 Ƥ (6.68 × 6.50 mm) (ZSI-GPRC IV-3022), 12 3 (largest 7.35 × 7.55 mm), 14 ƤƤ (largest 6.47 × 6.57 mm), same data as holotype; 3 3, 3ƤƤ (ZSI-GPRC IV-3023), Ganga River, Danapur, Takiya par Ghat, 25°38’7.05”N, 85°3’59.4”E, coll. H. Nesemann & G. Sharma, 12–16 January 2011, 1 3 (ZSI-GPRC), Gandhi Ghat, Patna, Bihar, 25°37’19.38”N, 85°10’15.10”E, coll. H. Nesemann et al., 31 January 2000; 3 3, 7 ƤƤ (ZSI- GPRC), Krishna Ghat, Patna, Bihar, 25°37’19.65”N, 85°10’08.82”E, coll. H. Nesemann et al., 1 February 2000; 2 3 (ZSI-GPRC IV-3024), university guest-house at Patna, Bihar, 25°37’19.57”N, 85°10’2.18”E, coll. H. Nesemann et al., 13 March 2001; 4 3 (ZSI-GPRC IV-3025), Mahendru Ghat, Patna, Bihar, 25°37’21.13”N, 85°9’04.98”E, coll. H. Nesemann et al., 1 December 2001; 3 3 (ZSI-GPRC IV-3026), Mahendru Ghat, Bihar, 25°37’21.13”N, 85°9’4.98”E, coll. H. Nesemann, January–February 2002; 4 3 (ZSI-GPRC IV-3027), Mahendru Ghat, Patna, Bihar, 25°37’21.13”N, 85°9’4.98”E, coll. H. Nesemann, January–February 2002; 1Ƥ (5.45 × 5.37 mm) (ZSI-GPRC IV-3028), Old Royal Palace site, Bihar, 25°37’6.05”N, 85°11’22.78”E, coll. H. Nesemann, 1–3 April 2002; 9 ƤƤ (ZSI-GPRC IV-3029), Royal Old Palace, Alam Ganj, Patna, 25°37’6.05”N, 85°11’22.78”E, Bihar, India, coll. coll. H. Nesemann et al., 30 March 2002; 9 ƤƤ (ZSI-GPRC IV-3030), Royal Old Palace site, Alam Ganj, Patna, Bihar, 25°37’6.05”N, 85°11’22.78”E, coll. H. Nesemann et al., 1–3 April 2002. All localities in India.

Diagnosis. Carapace wider than long, width to length ratio 1.02–1.19 (Figs. 2 A, 3A); rostrum trilobate; median lobe triangular, lateral lobes about half or less than half length of median lobe (Fig. 2 A, 3A); lateral margin of carapace with prominent anteriorly curved, spiniform tooth at junction of antero-, posterolateral margins; each anterolateral margin with 2 low but distinct teeth on anterior half; posterior half with very low tooth, sometimes absent (Fig. 2 A, 3A); cheliped merus with strong subdistal tooth on distal third of ventral margin, dorsal margin crenulated or uneven, unarmed (Figs. 2 A, 3C); ambulatory legs meri with prominent distal spine on dorsal margin; dactylus falciform, all with prominent subdistal curved spine, that of P2 without additional spines, those of P3-5 usually with additional 4 or 5 smaller spines, not always distinct (Fig. 3 E–I); male abdomen-pleotelson with somites 3–6 fused, immovable; pleotelson triangular with rounded tip, lateral margins almost straight (Fig. 5 A); female abdomen with somites 2 5 completely fused, subovate in non-ovigerous specimen, rounded in ovigerous specimen; G1 gently curved outwards, tip slightly hooked upwards (Fig. 5 B, C).

Description of male. Carapace rounded, slightly wider than long, width to length ratio 1.02–1.19; dorsal surface gently convex to relatively flat, surrounded by continuous rim, regions well demarcated by prominent grooves, H-shaped gastric groove continuous with cervical groove, cervical groove branching on anterior half of carapace, confluent with anterolateral rim (Figs. 2 A, 3A). Rostrum trilobate; median lobe triangular with gently convex lateral margins, tip rounded; lateral lobes triangular, tip rounded, about half or less than half length of median lobe (Figs. 2 A, 3A). Lateral margin of carapace with prominent anteriorly curved spiniform tooth at junction of antero-, posterolateral margins; anterolateral margin with 2 low but distinct teeth on anterior half; posterior half with very low tooth, sometimes absent (Figs. 2 A, 3A). Posterior carapace margin almost straight (Figs. 2 A, 3A).

Orbits not visible; eyes well developed, mobile, visible dorsally (Figs. 2 A–C, 3A). Antennular fossae separated by septum, antennules folding obliquely (Fig. 2 B, C). Epistome distinct, broad; posterior margin triangular, lateral margins gently concave, not projecting anteriorly, not visible from dorsal view (Fig. 2 B, C). Suborbital tooth distinct, clearly visible from dorsal view (Fig. 2 A, 3A). Third maxilliped narrow, median part of buccal cavity not covered, exposing mandibles; merus longitudinally ovate, longer than ischium, anteroexternal angle subauriculiform; ischium with inner distal angle strongly produced, without visible sulcus; palp as long as merus, all articles subqual in length; exopod long, slender, reaching to just before distal margin of merus, with distinct flagellum (Figs. 2 B, 3B).

Thoracic sternum relatively wide; sternites 1, 2 completely fused, separated from sternite 3 by distinct convex suture, lodged inside buccal cavity, distal end of sternite 1 with 2 small spines; sternites 3, 4 fused, demarcated by relatively shallow but distinct median sutures, lateral sutures poorly marked; pterygostome pronounced, joining anterolateral extension of sternite 4, clearly separating Milne Edward’s openings from base of chelipeds (Fig. 2 B). Milne Edward’s openings relatively broad, margins lined with dense setae (Fig. 2 B). Sternoabdominal cavity deep, reaching to middle of sternite 4 (Fig. 2 B).

Chelipeds symmetrical, relatively stouter in males; with relatively long merus, ventral margin with strong subdistal tooth on distal third, dorsal margin crenulated or uneven, never armed with teeth; carpus cup-shaped, inner angle with distinct low tooth, ventral margin with 2 small tubercles; chela with rounded palm, almost smooth, outer surface gently convex, fingers as long as palm; cutting edges of fingers with 5 or 6 small tuberculiform teeth on proximal half, crenulations on distal half (Figs. 2 A, 3C, D).

Ambulatory legs slender, long, second longest; all meri with prominent distal spine on dorsal margin; dactylus falciform, all with prominent subdistal curved spine, that of P2 without additional spines, those of P3–5 usually with additional 4 or 5 smaller spines, decreasing is size from subdistal spine, posterior ones may be strongly reduced to absent (Figs. 2 A, 3E–I).

Male abdomen-pleotelson relatively short; somite 1 broad, subrectangular, reaching to base of P5, visible from dorsal view, anterodistal angles produced to form 2 dentiform processes; somite 2 very short, slightly wider than half width of somite 1; somites 3–6 fused, immovable, lateral margins of posterior half convex, gradually becoming concave towards anterior part before becoming convex again anteriorly, suture separating somites 5, 6 very shallow, barely visible, surface of sternite 4 gently concave; anterodistal margin of somite 6 dilated, rounded; pleotelson triangular with rounded tip, lateral margins almost straight, base not as wide as distal margin of somite 6 (Fig. 5 A).

G1 gently curved outwards, tip slightly hooked upwards, inner distal margin lined with long setae (Fig. 5 B, C). G2 very short, with subspatuliform tip.

Females. Females agree with males in most non-sexual characters. The chelae are relatively more slender. The number of teeth on the ambulatory dactyli appears to vary a bit more in the females: P2 has 1 or 2 spines and P3–5 with 2–5 spines. One ovigerous female (5.45 × 5.37 mm) (ZSI- GPRC IV-3028) was atypical compared to the other specimens in that it had proportionately shorter ambulatory legs and the median rostral lobe is relatively lower. It, however, agrees in all other aspects with the other specimens and is here regarded as conspecific. In the non-ovigerous female, the female abdomen is poorly calcified and subovate, with somite 1 subrectangular and free; somites 2–5 completely fused with only the lateral dents demarcating the somites; and the pleotelson is broadly triangular with gently concave lateral margins (Fig. 5 D). In the ovigerous female, the entire abdomen is distended in all directions and the membranes are fully expanded to contain the eggs (Fig. 4). This condition is almost identical to what has been reported for N. mangalis by Ng & Chuang (1996). The vulvae, which are positioned submedially on thoracic sternite 6, are relatively small (diameter less than that of each egg), rimmed with a simple tubercle medially (Fig. 4 B). The eyed eggs are spherical, diameter 0.27–0.30 mm (mean = 0.28, n = 15 from three females: 4.4 × 3.9 mm, 4.7 × 4.1 mm, 5.0 × 4.2 mm) (ZSI-GPRC IV-3029–3030), attached on the endopods of the pleopods, with exopods growing inwards, partially covering dome-shaped abdomen.

Etymology. The species is named after the Hindi term for a “creature that lives in Patna”. The name “ patnahi ” is used as a noun in apposition.

Remarks. Neorhynchoplax patnahi sp. nov. belongs to a group of species in which the lateral margin of the carapace is armed with a distinct recurved, spiniform tooth just above the first ambulatory leg (P2). In its carapace shape and leg proportions, N. patnahi sp. nov. is most similar to the marine N. alcocki (Kemp, 1917) from Goa, Cochin and Travancore in southern India (Fig. 6A; Kemp 1917: 256; Chopra & Das 1930: 414). It differs markedly from it, however, in the form of the male abdomen and ambulatory legs (Table 1). Neorhynchoplax patnahi sp. nov. is also superficially similar to another marine Indian species, N. woodmasoni, which was described from the Andaman Islands and near Kolkata (= Calcutta) (Fig. 6 B; Alcock 1895: 388) but differs in the shape of the carapace and legs (Table 1).

N. patnahi N. alcocki N. woodmasoni

Carapace Wider than long (Figs. 2 A, 3A) Wider than long (Fig. 6A) Longer than wide (Fig. 6 B) Ecology. The specimens of N. patnahi sp. nov. were collected from hard natural and man-made substrates (rocks, bricks, etc.) and fresh sand or soft mud in the side of the river to depths of about 0.5 m. They are confined to shorelines of the main river channel with a mixture of stones and mollusc shells on fine sand and silt substrates. The crabs are rheophilic in preferring the main current and wind-exposed shallow zones with strong wave action where no mud is deposited. They are closely associated with the gastropod molluscs Scaphula celox Benson, 1836, Novaculina gangetica Benson, 1830, Mekongia crassa (Benson, 1836), and Bellamya bengalensis (Lamarck, 1822) as well as dense colonies of freshwater bryozoans of the family Plumatellidae. The generally larger hymenosomatid Hymenicoides carteri dominates in habitats that are exposed to strong water movement and high riverine waves. In contrast, the slightly smaller N. patnahi sp. nov. is more abundant in habitats that are sheltered from water flow, like among bryozoan colonies and algal mats. In the same habitat are also two species of true freshwater crabs of the family Gecarcinucidae: Maydelliathelphusa lugubris (Wood-Mason, 1871) (as a Barythelphusa) and Acanthopotamon martensi (Wood-Mason, 1875) (as a Parathelphusa) (Nesemann et al. 2011).

The River Ganga originates from the Gomukh glacier in the Himalayas at an altitude of 4100 m and after running for over 2700 km it discharges into the Bay of Bengal. Patna is located on the right bank of the river 1020 km from the river mouth. The river is sluggish at Patna, with a gradient of only 6 cm /km. The river receives some untreated sewage from the non-industrial city of Patna (population about 1.5 million). The riverbed consists mainly of fine silt and clay. The maximum depth during the dry season between February and March is about 7 8 m when the width is about 800 m to 1 km. The water level rises up to 10 m during June–September. The river sustains a highly diversified fauna in and around Patna. The average physico-chemical characteristics of the river water in the dry season are as follow: temperature 22 o C; pH 8.3; total suspended solids 35 mg /l; total dissolved solids 228 mg / l; total alkalinity 182 mg /l; dissolved oxygen 7.34 mg /l; biochemical oxygen demand 1.68 mg /l; chemical oxygen demand 5.6 mg /l; total hardness (as CaCO3) 151 mg /l; calcium 34.8mg /l; magnesium 15.55mg /l; iron 0.93 mg /l; chloride 13.4mg /l; nitrate 0.33mg /l; sulphate 14.04 mg /l; and phosphate 0.037 mg /l.

Due to shifting of the main current of the Ganga River at Patna since 2003 from the right bank of the city to some 2 km north of the original drainage, the entire bank is now filled with fine sediments. The hard substrates such as boulders are now covered with soft mud. As a result, the existing habitats of Neorhynchoplax patnahi sp. nov. downstream from Adalat Ghat to Bhadra Ghat has completely changed in the last five years.

Ovigerous females were observed in the month of March. They carry their eggs under the relatively small abdomen. The eggs are fully covered and enclosed by a prominent sac-like membrane inside the abdomen, like that reported for the mangrove species, N. mangalis (Ng, 1988) in Singapore (Ng & Chuang 1996). It is therefore very likely that N. patnahi sp. nov. also undergoes ovovivipary.

Neorhynchoplax Sakai, 1938

Neorhynchoplax alcocki (Kemp, 1917) [Rhynchoplax]

Neorhynchoplax aspinifera (Lucas, 1980) [Elamenopsis] Neorhynchoplax attenuipes (Chopra & Das, 1930) [Rhynchoplax] Neorhynchoplax bai Naruse, Mendoza & Ng, 2008

Neorhynchoplax bovis (Barnard, 1946) [Rhynchoplax]

Neorhynchoplax demeloi (Kemp, 1917) [Rhynchoplax] Neorhynchoplax dentata Ng, 1995

Neorhynchoplax elongata Rahayu & Ng, 2004

Neorhynchoplax euryrostris Davie & Richer de Forges, 1996 Neorhynchoplax exigua (Kemp, 1917) [Rhynchoplax]

Neorhynchoplax falcifera Naruse, Mendoza & Ng, 2008 Neorhynchoplax frontalis (Lucas & Davie, 1982) [Elamenopsis] Neorhynchoplax hirtirostris (Lucas & Davie, 1982) [Elamenopsis] Neorhynchoplax inachoides (Alcock, 1900) [Hymenicus] Neorhynchoplax inermis (Takeda & Miyake, 1971) [Rhynchoplax] Neorhynchoplax introversa (Kemp, 1917) [Rhyncoplax] Neorhynchoplax kempi (Chopra & Das, 1930) [Rhynchoplax] Neorhynchoplax mangalis (Ng, 1988) [Elamenopsis]

Neorhynchoplax minima (Lucas & Davie, 1982) [Elamenopsis] Neorhynchoplax nasalis (Kemp, 1917) [Rhynchoplax]

Neorhynchoplax octagonalis (Kemp, 1917) [Rhynchoplax] Neorhynchoplax okinawaensis (Nakasone & Takeda, 1994) [Elamenopsis] Neorhynchoplax pageti Pretzmann, 1975

Neorhynchoplax patnahi sp. nov.

Neorhynchoplax prima Ng & Chuang, 1996

Neorhynchoplax sinensis (Shen, 1932) [Rhynchoplax]

Neorhynchoplax thorsborneorum (Lucas & Davie, 1982) [Elamenopsis] Neorhynchoplax torrensica (Lucas, 1980) [Elamenopsis] Neorhynchoplax tuberculata (Chopra & Das, 1930) [Rhynchoplax] Neorhynchoplax woodmasoni (Alcock, 1900) [Hymenicus] Neorhynchoplax yaeyamaensis Naruse, Shokita & Kawahara, 2005