Vanni gracilis, Z, 2025

Vanni gracilis sp. nov.

[Thattekkad forest crab]

( Figs. 16 View FIGURE 16 , 17 View FIGURE 17 )

Not Vanni travancorica View in CoL – Pati & Sureshan 2022: 187, pl. 5 fig. 5.

Type material examined. Holotype: male ( CW 23.8 mm, CL 17.5 mm, CH 10.4 mm, FW 7.5 mm) ( ZSI-WGRC IR/INV/9192A), India, Kerala State, Ernakulam District, Thattekkad Bird Sanctuary, Knachery , 10.12275° N, 76.74864° E, elev. 256 m a.s.l., coll. Md. Jafer Palot, 21 September 2016 GoogleMaps . – Paratypes: 2 males ( CW 21.2–25.3 mm, CL 15.4–18.0 mm, CH 9.2–10.5 mm, FW 7.3–8.2 mm), female ( CW 20.2 mm, CL 14.7 mm, CH 9.0 mm, FW 6.7 mm) ( ZSI-WGRC IR/INV/9192B), same collection data as for holotype GoogleMaps ; 9 males ( CW 13.9–21.2 mm, CL 10.7–15.9 mm, CH 5.6–9.2 mm, FW 5.0– 7.4 mm) , 7 females ( CW 12.1–20.9 mm, CL 9.8–15.3 mm, CH 5.4–9.2 mm, FW 4.6–7.0 mm) ( ZSI-WGRC IR/INV/9192C), same collection data as for holotype GoogleMaps .

Other material examined. India– Kerala State: 7 males ( CW 12.7–21.5 mm, CL 10.2–15.6 mm, CH 6.0– 9.7 mm, FW 4.9–7.4 mm), 10 females ( CW 11.4–25.1 mm, CL 9.4–18.0 mm, CH 4.7–11.0 mm, FW 4.2–8.3 mm) ( ZSI-WGRC IR/INV/9159), Ernakulam District , Thattekkad Bird Sanctuary, Ovumkal, 10.11517° N, 76.70175° E, elev. 46 m a.s.l., coll. K.G. Emiliyamma, 27 October 2015 GoogleMaps ; 3 males ( CW 12.9–17.5 mm, CL 10.3–13.2 mm, CH 5.7–7.7 mm, FW 5.2–6.1 mm), 3 females ( CW 15.7–23.5 mm, CL 12.1–16.7 mm, CH 6.9–9.8 mm, FW 5.7–8.1 mm) ( ZSI-WGRC IR/INV/9151), Ernakulam District , Thattekkad Bird Sanctuary, Kolumba, 10.11464° N, 76.70936° E, elev. 58 m a.s.l., coll. Md. Jafer Palot, 27 February 2014 GoogleMaps ; 5 males ( CW 14.7 –20.0 mm, CL 11.3–14.9 mm, CH 6.4–9.0 mm, FW 5.4–6.9 mm), female ( CW 24.5 mm, CL 17.4 mm, CH 10.7 mm, FW 8.0 mm) ( ZSI-WGRC IR/INV/9157), same collection data as for preceding, coll. P.M. Sureshan, 21 April 2015 GoogleMaps ; 3 males ( CW 19.3–19.4 mm, CL 14.2– 14.5 mm, CH 8.2–9.1 mm, FW 6.8–7.3 mm), 2 females ( CW 16.4–16.5 mm, CL 12.6–12.7 mm, CH 7.1–7.6 mm, FW 6.1–6.3 mm) ( ZSI-WGRC IR/INV/9187), same collection data as for preceding, coll. P.M. Sureshan, 18 June 2016 GoogleMaps ; 6 males ( CW 9.8–23.2 mm, CL 8.0– 16.7 mm, CH 4.2–10.3 mm, FW 3.9–8.0 mm), 8 females ( CW 12.2–23.8 mm, CL 9.4–16.9 mm, CH 5.1–10.3 mm, FW 4.6–8.3 mm) ( ZSI-WGRC IR/INV/9189), same collection data as for preceding, coll. Md. Jafer Palot, 20 September 2016 GoogleMaps ; female ( CW 20.1 mm, CL 15.2 mm, CH 9.0 mm, FW 7.0 mm) ( ZSI-WGRC IR/INV/9874), same collection data as for preceding, coll. K. Rajmohana, 5 February 2017 GoogleMaps ; 8 males ( CW 13.8–23.3 mm, CL 11.0–17.0 mm, CH 6.1–10.3 mm, FW 5.2–7.9 mm), female ( CW 18.3 mm, CL 13.8 mm, CH 8.3 mm, FW 6.9 mm) ( ZSI-WGRC IR/INV/9148), Ernakulam District , Thattekkad Bird Sanctuary, Knachery, 10.12275° N, 76.74864° E, elev. 256 m a.s.l., coll. K.G. Emiliyamma, 29 October 2015 GoogleMaps ; 3 males ( CW 15.5–17.5 mm, CL 11.6–13.3 mm, CH 6.7–7.6 mm, FW 6.1–6.7 mm), 3 females ( CW 12.7–18.6 mm, CL 9.8–14.0 mm, CH 5.0– 8.8 mm, FW 5.0– 7.1 mm) ( ZSI-WGRC IR/INV/9163), Ernakulam District , Thattekkad Bird Sanctuary , Urulanthanni, Pullasserykkuthu, 10.12594° N, 76.75106° E, elev. 81 m a.s.l., coll. K.G. Emiliyamma, 29 October 2015 GoogleMaps ; male ( CW 19.9 mm, CL 14.6 mm, CH 8.9 mm, FW 7.0 mm), female ( CW 11.0 mm, CL 8.9 mm, CH 4.3 mm, FW 4.2 mm) ( ZSI-WGRC IR/INV/9175), Ernakulam District , Thattekkad Bird Sanctuary, Urulanthanni, 10.131° N, 76.750° E, elev. 120 m a.s.l., coll. Md. Jafer Palot, 25 February 2014 GoogleMaps ; 2 females ( CW 18.4–24.1 mm, CL 14.1– 17.3 mm, CH 8.8–11.3 mm, FW 6.6–8.0 mm) ( ZSI-WGRC IR/INV/9155), same collection data as for preceding, coll. P.M. Sureshan, 22 April 2015 GoogleMaps ; 2 males ( CW 11.4–19.1 mm, CL 9.2–14.3 mm, CH 5.0– 8.2 mm, FW 4.5–7.1 mm) ( ZSI-WGRC IR/INV/9878), same collection data as for preceding, coll. K. Rajmohana, 4 February 2017 GoogleMaps ; 3 males ( CW 15.7–17.8 mm, CL 12.2–13.4 mm, CH 6.8–8.0 mm, FW 5.9–6.5 mm), 4 females ( CW 9.6–15.2 mm, CL 7.7–12.1 mm, CH 4.2–6.8 mm, FW 3.7–5.8 mm) ( ZSI-WGRC IR/INV/9173), Ernakulam District , Thattekkad Bird Sanctuary, between Urulanthanni-Thampakachodu, 10.1305° N, 76.7495° E, elev. 120 m a.s.l., coll. K.G. Emiliyamma, 6 January 2015 GoogleMaps ; 2 males ( CW 9.5–18.1 mm, CL 7.6–14.0 mm, CH 4.7–8.2 mm, FW 3.8–6.2 mm), 2 females ( CW 15.5–16.1 mm, CL 12.1–12.6 mm, CH 6.8–7.1 mm, FW 5.5–5.7 mm) ( ZSI-WGRC IR/INV/9188), Ernakulam District , Thattekkad Bird Sanctuary, Kallippara, 10.13579° N, 76.72643° E, elev. 85 m a.s.l., coll. Md. Jafer Palot, 22 September 2016 GoogleMaps ; 7 males ( CW 9.7–20.8 mm, CL 7.7–15.8 mm, CH 4.3–9.7 mm, FW 3.9–7.5 mm), female ( CW 15.2 mm, CL 11.6 mm, CH 6.7 mm, FW 5.5 mm) ( ZSI-WGRC IR/INV/9171), Ernakulam District , Thattekkad Bird Sanctuary, Koottampara, 10.1385° N, 76.73822° E, elev. 68 m a.s.l., coll. K.G. Emiliyamma, 28 October 2015 GoogleMaps ; 2 males ( CW 19.6–20.9 mm, CL 14.6–15.3 mm, CH 7.4–9.2 mm, FW 7.2– 7.1 mm), 4 females ( CW 15.3–18.5 mm, CL 11.7–13.7 mm, CH 6.7–8.7 mm, FW 5.8–6.4 mm) ( ZSI-WGRC IR/INV/9185), same collection data as for preceding, coll. P.M. Sureshan, 19 June 2016 GoogleMaps .

Diagnosis. Medium sized crabs (maximum CW 25.3 mm). Carapace relatively broad (CW/CL = ca. 1.2–1.4), relatively low ( CH /CW = ca. 0.4–0.5) ( Fig. 16A, B View FIGURE 16 ); external orbital angle triangular, with short outer margin, ca. 2 times length of inner margin ( Fig. 16A View FIGURE 16 ); epibranchial tooth indiscernible ( Fig. 16A View FIGURE 16 ); male sternopleonal cavity reaching to imaginary line joining anterior part of cheliped coxae ( Fig. 16C, G View FIGURE 16 ); male pleonal somite 6 subquadrate, slightly broad, proximal width ca. 1.2 times medial length, as long as telson, with strongly concave lateral margins ( Fig. 16C, H View FIGURE 16 ); male telson relatively less narrow, medial length ca. 1.1 times proximal width ( Fig. 16C, H View FIGURE 16 ); G1 relatively slenderer ( Fig. 17A, B View FIGURE 17 ); G1 ultimate article relatively slenderer, gently curved outwards at ca. 10° from longitudinal axis of G1, relatively longer, ca. 0.3 times length of penultimate article ( Fig. 17A, B View FIGURE 17 ); G1 penultimate article with distal portion relatively less narrow than basal portion, outer margin sinuous, basally strongly concave ( Fig. 17A, B View FIGURE 17 ); G2 slightly longer than G1, ca. 1.1 times G1 length, ultimate article long, ca. 0.4 times length of penultimate article ( Fig. 17C View FIGURE 17 ); vulvae subovate, relatively large, occupying ca. 0.6 times length of s6, located relatively close to s5/s6 ( Fig. 16J View FIGURE 16 ).

Description of male holotype. Medium sized crab (CW 23.8 mm). Carapace transversely subovate, broader than long (CW/CL = ca. 1.3), low ( CH /CW = ca. 0.4); dorsal surface glabrous, generally smooth except for epigastric and postorbital cristae and lateral surfaces; anterolateral surface low in frontal view; anterolateral margins gently convex, subcristate, shorter than posterolateral margins; posterolateral margins converging posteriorly, strongly concave medially; front broad (FW/CW = ca. 0.3), deflexed anteriorly, trapezoidal, anterior margin cristate, smooth, gently concave medially in dorsal view; frontal medial triangle incomplete, with dorsal margin only, lateral margins indiscernible; epigastric cristae well-developed, rugose, slightly anterior to postorbital cristae; postorbital cristae well-developed, sharp, reaching lateral margins of carapace; external orbital angle triangular, with short outer margin, ca. 2 times length of inner margin; epibranchial tooth low, indiscernible; postorbital region concave; branchial regions low; cervical grooves shallow, broad, discontinuous, not reaching level of postorbital cristae; mesogastric groove deep, narrow, short, bifurcated posteriorly; H-shaped groove distinct; subhepatic region rugose, glabrous; suborbital region smooth, glabrous; pterygostomial region smooth, glabrous; supraorbital margin cristate, smooth; suborbital margin concave, cristate with low granules, discontinuous with supraorbital margin; epistome posterior margin with well-developed, triangular medial tooth and gently sinuous lateral lobes ( Fig. 16A–C View FIGURE 16 ). Eyes large; eyestalk short, moderately stout; cornea moderately large, pigmented ( Fig. 16B View FIGURE 16 ).

Antennules long, folded in longitudinally broad fossae; antennae shorter than eyestalk ( Fig. 16B View FIGURE 16 ). Mandibular palp with 2 articles; terminal article bilobed, anterior lobe narrow, long, posterior lobe broad, short, ca. 0.5 times length of anterior lobe ( Fig. 16D View FIGURE 16 ). Maxillipeds 1, 2 each with long flagellum on exopod ( Fig. 16D View FIGURE 16 ). Maxilliped 3 cover most of buccal cavity, when closed; ischium subrectangular, longer than broad, longitudinal medial groove indiscernible; merus subrectangular, broader than long, anterolateral corner rounded; exopod slender, distally narrow, reaching proximal third length of ischium, with well-developed flagellum, exceeding merus width ( Fig. 16B, C, E View FIGURE 16 ).

Chelipeds slender, smooth, glabrous, unequal, left chela larger ( Fig. 16A, C, F View FIGURE 16 ). Major chela with 2 distinct, blunt teeth on dactylus and 3 distinct, blunt, basally fused teeth on fixed finger, remaining teeth on fingers small, low, distinct gape when fingers closed; dactylus curved, moderately stout, longer than upper margin of palm, smooth; fixed finger stout, forming concave margin with ventral margin of palm; palm smooth, inflated, longer than high; carpus smooth, gently inflated, with prominent, broad, acute inner distal tooth and low, blunt basal tooth; merus smooth except for rugose or granular margins, lacking subterminal spine ( Fig. 16A, C, F View FIGURE 16 ).

Ambulatory legs generally smooth, slender, long, P3 longest; merus long, lacking subdistal spine; carpus setose; propodus and dactylus setose, with distinct, long, sharp chitinous spines on margins; dactylus (P3, P4) recurved, longer than propodus ( Fig. 16A, C View FIGURE 16 ).

Thoracic sternites smooth, glabrous; s1 and s2 completely fused; s2/s3 visible as very shallow groove, not reaching edge of sternum; s3/s4 indiscernible; s4/s5, s5/s6, s6/s7 shallow, narrow, medially interrupted; s7/s8 shallow, narrow, medially interrupted by longitudinal groove of s7, lacking transverse ridge; s8 completely covered by pleon, narrowed medially, longitudinal medial groove distinct ( Fig. 16C, G View FIGURE 16 ). Pleonal locking mechanism with prominent tubercle on medial part of s5 ( Fig. 16G View FIGURE 16 ). Sternopleonal cavity deep, short, reaching to imaginary line joining anterior part of cheliped coxae ( Fig. 16C, G View FIGURE 16 ).

Pleon narrow, T-shaped, with strongly concave lateral margins; pleonal somites 1, 2 subrectangular, narrower than pleonal somite 3; pleonal somite 3 trapezoidal, broadest, with convex lateral margins; pleonal somites 4 and 5 trapezoidal, with straight and gently concave lateral margins, respectively; pleonal somite 6 subquadrate, slightly broader than long (proximal width ca. 1.2 times medial length), distinctly longer than preceding pleonal somites, subequal in length to telson, with strongly concave lateral margins ( Fig. 16C, H View FIGURE 16 ). Telson bell-shaped, slightly longer than broad (medial length ca. 1.1 times proximal width), with straight lateral margins, apex broad, rounded ( Fig. 16C, H View FIGURE 16 ).

G1 slender, short, tip reaching beyond s6/s7 up to distal third of s 6 in situ; ultimate article conical, slender, short, ca. 0.3 times length of penultimate article, gently curved outwards at ca. 10° from longitudinal axis of G1, tip blunt; penultimate article sinuous, slender, slightly broad at base, inner margin sinuous, outer margin sinuous, basally strongly concave; groove for G2 marginal ( Figs. 16G View FIGURE 16 , 17A, B View FIGURE 17 ). G2 slightly longer than G1, ca. 1.1 times G1 length; ultimate article long, ca. 0.4 times length of penultimate article, with acute tip; penultimate article stouter at proximal third ( Fig. 17C View FIGURE 17 ).

Colour in life. The dorsal surface of the carapace is characterised by a mostly orange colour with dark purplish-brown blotches posteriorly; the chelipeds are entirely orange; and the ambulatory legs are light purplish-brown (https://www.inaturalist.org/observations/163961294).

Etymology. The species epithet is from the Latin word “ gracilis ”, meaning slender, which alludes to the fact that the slender male first gonopod of the species has the slenderest ultimate article among the known congeners. The species name is treated here as a Latin adjective in the nominative singular.

Remarks. The male paratypes of V. gracilis sp. nov., especially the adults, closely resemble the holotype in all diagnostic characters. The subadult male and female paratypes have a relatively less broad carapace (CW/CL = ca. 1.2–1.3). The female paratypes exhibit most of the non-sexual character states, similar to the holotype.

In the adult females of V. gracilis sp. nov., the pleon and telson together form a broadly ovate outline and cover the thoracic sternum except for the lateral edges, when closed ( Fig. 16I View FIGURE 16 ). Their pleonal somite 1 is the shortest; pleonal somites 2–5 are progressively longer; and pleonal somite 6 is the longest, conspicuously broader than long, subequal in length to the telson, with gently convex lateral margins ( Fig. 16I View FIGURE 16 ). Their female telson is broadly triangular, conspicuously broader than long, with almost straight lateral margins and broad apex ( Fig. 16I View FIGURE 16 ). The vulvae are widely located from each other (VD/SW = ca. 0.2) on the s6; each vulva opens ventrally and is subovate, large, ca. 0.6 times the length of the s6, located some distance from the s5/s6, not touching the s5/s6, surrounded by a low rim, and covered by a soft membranous operculum ( Fig. 16J View FIGURE 16 ).

Vanni gracilis sp. nov. can be distinguished by the relatively slenderer G1 ( Fig. 17A, B View FIGURE 17 ) against the relatively stouter G1 of the known congeners ( Figs. 15A, B View FIGURE 15 , 18H, I View FIGURE 18 , 19A, B View FIGURE 19 , 20H, I View FIGURE 20 , 21A, B View FIGURE 21 ). Otherwise, the new species bears a strong resemblance to V. travancorica View in CoL in many aspects of carapace morphology and gonopod structures. The similarities between the two species led Pati & Sureshan (2022) to mistakenly identify V. gracilis sp. nov. as V. travancorica View in CoL . All the specimens reported in Pati & Sureshan (2022) have been re-examined, and the following key differences between V. gracilis sp. nov. and V. travancorica View in CoL were noted: the slightly broad male pleonal somite 6, with the proximal width ca. 1.2 times the medial length in V. gracilis sp. nov. ( Fig. 16C, H View FIGURE 16 ) (versus male pleonal somite 6 relatively narrower, appearing elongated, with the proximal width ca. 1.1 times the medial length in V. travancorica View in CoL ; Fig. 14C, E View FIGURE 14 ); the relatively less narrow male telson, with the medial length ca. 1.1 times the proximal width in V. gracilis sp. nov. ( Fig. 16C, H View FIGURE 16 ) (versus male telson relatively more narrow, with the medial length ca. 1.3 times the proximal width in V. travancorica View in CoL ; Fig. 14C, E View FIGURE 14 ); the relatively slenderer G1, with the relatively long G1 ultimate article, ca. 0.3 times the length of the penultimate article in V. gracilis sp. nov. ( Fig. 17A, B View FIGURE 17 ) (versus G1 relatively stouter, with the relatively short ultimate article, ca. 0.2 times the length of the penultimate article in V. travancorica View in CoL ; Fig. 15A, B View FIGURE 15 ); the relatively less narrow distal portion of the G1 penultimate article as compared to its basal portion in V. gracilis sp. nov. ( Fig. 17A, B View FIGURE 17 ) (versus G1 penultimate article with the distal portion conspicuously narrow than the basal portion in V. travancorica View in CoL ; Fig. 15A, B View FIGURE 15 ); the basally strongly concave outer margin of the G1 penultimate article in V. gracilis sp. nov. ( Fig. 17A, B View FIGURE 17 ) (versus G1 penultimate article with the basally straight outer margin in V. travancorica View in CoL ; Fig. 15A, B View FIGURE 15 ); and the vulvae located relatively close to s5/s 6 in V. gracilis sp. nov. ( Fig. 16J View FIGURE 16 ) (versus vulvae located clearly away from the s5/s 6 in V. travancorica View in CoL ; Fig. 15F View FIGURE 15 ).

Vanni gracilis sp. nov. can also be confused with V. deepta View in CoL , especially in the G1 structure, since their G1 ultimate articles are equally long (ca. 0.3 times the length of the penultimate article), and the G1 penultimate article has a basally concave outer margin ( Figs. 17A, B View FIGURE 17 , 18H, I View FIGURE 18 , 19A, B View FIGURE 19 ). Vanni gracilis sp. nov. is nevertheless differentiated from V. deepta View in CoL by the strongly concave lateral margins of the male pleonal somite 6 ( Fig. 16C, H View FIGURE 16 ) (versus male pleonal somite 6 with gently concave lateral margins; Fig. 18D, G View FIGURE 18 ) and the relatively slenderer ultimate and penultimate articles of the G1 ( Fig. 17A, B View FIGURE 17 ) (versus G1 ultimate and penultimate articles relatively stouter; Figs. 18H, I View FIGURE 18 , 19A, B View FIGURE 19 ).

Ecological notes. Vanni gracilis sp. nov. are found in degraded semi-evergreen forest patches of the Thattekkad Bird Sanctuary at lower elevations (46–256 m a.s.l.) of the southern Western Ghats. Crabs of this new species can be found underneath small boulders in moist areas and along stream banks. They are very abundant in the sanctuary. The new species was also found coexisting alongside either Karkata ghanarakta Pati, Rajesh, Raj, Sheeja, Kumar & Sureshan, 2017 , or Pilarta punctatissima Pati, Rajesh, Raj, Sheeja, Kumar & Sureshan, 2017 .

Geographical distribution. Vanni gracilis sp. nov. is currently known only from the Thattekkad Bird Sanctuary in the Ernakulam district of Kerala, southern India ( Fig. 1B View FIGURE 1 ). The sanctuary is situated in the foothills of the southern Western Ghats ( Fig. 1B View FIGURE 1 ).