Leptobrachium sylheticum, Al-Razi & Maria & Poyarkov, 2021

Al-Razi, Hassan, Maria, Marjan & Poyarkov, Nikolay A., 2021, Integrative taxonomic analysis reveals a new species of Leptobrachium Tschudi, 1838 (Anura, Megophryidae) from Bangladesh, Journal of Natural History 55 (1 - 2), pp. 85-114 : 96-108

publication ID	https://doi.org/ 10.1080/00222933.2021.1894365
persistent identifier	https://treatment.plazi.org/id/7D4A87CE-FFAA-FFE1-9BC5-FA1FFCCE0A69
treatment provided by	Plazi
scientific name	Leptobrachium sylheticum
status	sp. nov.

Treatment

Leptobrachium sylheticum sp. nov.

( Figures 3–7 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 ; Tables 3–4)

Chresonymy

Leptobrachium hasseltii View in CoL (partim) (?) – Pillai and Chanda 1979; Chanda 1994; Chanda 2002.

Leptobrachium smithi View in CoL (partim) – Sengupta et al. 2001; Pawar and Birand 2001; Das and Chanda 2004; Asmat 2005; Lalremsanga et al. 2007; Ahmed et al. 2009; Reza 2009; Mahony et al. 2009; Mathew and Sen 2010a; 2010b; Sengupta et al. 2010; Dutta et al. 2013; Hasan et al. 2014; Reza 2014; Ghose et al. 2017; Mondal et al. 2019; Hakim et al. 2020.

Leptobrachium rakhinensis View in CoL (partim) – Wogan 2012; Mahony et al. 2017.

Leptobrachium rakhinense View in CoL (partim) – Pawangkhanant et al. 2018; Frost 2020 (correction in gender of the species name).

Holotype. JnUZool-A0120, an adult male, collected from lowland mixed evergreen forest of Lawachara National Park , Moulvibazar District, Sylhet Division in the northeast of Bangladesh (24.325061° N, 91.786152° E; elevation 56 m a.s.l.) on 9 June 2020, by Hassan Al-Razi and Marjan Maria ( Figures 3–4 View Figure 3 View Figure 4 ). GoogleMaps

Paratypes. In total five specimens, all collected from lowland mixed evergreen forest of Lawachara National Park, Moulvibazar District , Sylhet Division on June 9–11, 2020, by Hassan Al-Razi and Marjan Maria ; including three adult males: JnUZool-A0220 (24.324223° N, 91.785312° E; elevation 63 m a.s.l.), JnUZool-A0320 (24.323016° N, 91.789474° E; elevation 58 m a.s.l.), JnUZool-A0420 (24.325462° N, 91.785765° E; elevation 55 m a.s.l.); and two adult females: JnUZool-A0520 (24.323016° N, 91.789474° E; elevation 58 m a.s.l.), and JnUZool-A0620 (24.325462° N, 91.785765° E; elevation 55 m a.s.l.) GoogleMaps .

Diagnosis. A member of the genus Leptobrachium based on head width being larger than tibia length; skin dorsally with a network of ridges; oval and large axillary glands present; extremities of digits rounded; breeding males lacking spines on fingers and breast; and bicoloured iris ( Yang et al. 2016). The new species can be distinguished from other congeners by the following combination of morphological characteristics: (1) medium-sized species, with adult SVL of 50.2–60.9 mm in males and 63.9–68.3 mm in females; (2) all tips of finger and toes rounded; (3) relative finger lengths: II<I<IV<III; relative toe lengths: V <I = II<III<IV; (4) toe webbing well developed and thick, toe webbing formula: I 1–1 II 1–2 III 1–3¼ IV 3½–1 V; (5) inner metatarsal tubercle large, welldeveloped; (6) iris bicoloured, black ventrally and fire-red dorsally in upper one-third, with bright-blue sclera; (7) dorsum uniform grey with few small black spots laterally, dark head markings indistinct; (8) belly and limbs ventrally dark-grey to black with irregular sparse whitish marbling on chest and anterior portion of belly and characteristic tiny orange-red dots on ventral surfaces of lower jaws and gular area; (9) dark marking covering the entire tympanum; (10) dark canthal stripe present, comparatively narrow, covering only the uppermost portion loreal region; (11) dark spots or blotches on body flanks absent or small, not forming a row; (12) limbs, including fingers and toes, dorsally with dark markings not forming complete transverse bars; tibia with 2–3 such dark markings; (13) dark blotch at groin absent, posterior sides of thighs blackish with white spots; (14) femoral gland in shape of small indistinct white blotch; axillary gland as large roundish blotch posterior of arm insertion; (15) males with single vocal sac, during breeding season lacking spinules on lips.

Description of holotype. Adult male in a good state of preservation, with SVL 50.2 mm; habitus robust, limbs slender, head very large, head and body oval-shaped ( Figure 3 View Figure 3 (a,b)). Head. Head large, broad and flattened, much wider (HW to SVL ratio 42.1%) than long (HL

to SVL ratio 36.3%), HW/HL ratio 115.9%; snout gently rounded with snout tip slightly blunt in dorsal view ( Figure 3 View Figure 3 (a)), gently sloping in profile ( Figure 3 View Figure 3 (e)), slightly projecting beyond lower jaw; nostrils round, oriented dorsally, located on the canthus rostralis, closer to tip of snout than to eye (EN to SVL ratio 7.0%); canthus rostralis distinct, rounded; loreal region deeply concave; internarial distance (IND to SVL ratio 7.0%) less than twice the interorbital distance (IO to SVL ratio 12.0%), IND/IO ratio 58.3%; eyes very large, bulging, distinctly projecting from sides of head in lateral and dorsal views ( Figure 3 View Figure 3 (a,e)), eye diameter greater (ED to SVL ratio 12.5%) than snout length (SL to SVL ratio 11.5%); interorbital distance slightly greater than upper eyelid width (UEW to SVL ratio 9.3%); pineal ocellus absent; tympanum distinct, not depressed, rounded in shape, tympanum diameter (TD to SVL ratio 7.0%) over a half of eye diameter (TD/ED ratio 55.7%), notably greater than the distance between tympanum and eye (TD/E-T ratio 170.7%) ( Figure 3 View Figure 3 (e)); vomerine teeth absent; tongue large, broad and lacking posterior notch; vocal sac single, gular. Limbs. Forelimbs slender, comparatively long; fingers slender ( Figure 3 View Figure 3 (c)), all fingers free of webbing; all finger tips rounded and slightly swollen; relative finger lengths: II<I<IV<III; dermal fringes on fingers absent; two prominent large bulging palmar tubercles, not contacting each other medially, much larger than finger tips, inner palmar tubercle rounded in shape and slightly smaller than the ovate outer palmar tubercle; callous tissue forming smooth low ridges on ventral surfaces of all fingers; subarticular tubercles not discernable ( Figure 3 View Figure 3 (c)); nuptial pads absent. Hindlimbs comparatively short, slender; heels not contacting when legs held at right angles to body; shank distinctly longer (ShL to SVL ratio 37.9%) than foot (FL to SVL ratio 33.0%), FL/ShL ratio 87.1%; tibiotarsal articulation of adpressed limb not reaching to the level of tympanum. Toes slightly dorsoventrally flattened ( Figure 3 View Figure 3 (d)), all with thick lateral dermal fringes reaching to the toe tips; toe webbing well-developed and thick, toe webbing formula: I 1–1 II 1–2 III 1–3¼ IV 3½–1 V. Tips of all toes rounded, notably swollen; relative toe lengths: V <I = II<III<IV. Subarticular tubercles not discernable, replaced by smooth low callous ridges ( Figure 3 View Figure 3 (d)); inner metatarsal tubercle distinct, bean-shaped, comparatively large (IML to SVL ratio 4.6%), slightly less than the distance between tip of toe I and the tubercle; outer metatarsal tubercle absent.

Skin. Upper lip smooth lacking cornified spinules or spines ( Figure 3 View Figure 3 (e)). Skin dorsally shagreened covered with numerous tiny equal-sized and densely scattered granules; prominent dermal ridges on dorsum or dorsal surfaces of limbs absent; upper eyelid dorsally with a weak superciliary dermal ridge; granules getting larger on body flanks and belly; supratympanic ridge prominent, thick, running from posterior edge of eye towards mouth corner, sharply curving ventrally above the tympanum; limbs ventrally smooth, shagreened dorsally, lacking dermal ridges; axillary gland very prominent, large (4–5 mm in diameter), oval-shaped with irregular borders, flat, located postero-ventrally at medial border of axilla behind arm insertion; femoral gland flat, small (1–2 mm in diameter), ovate, located on distal half of posteroventral surface of thigh closer to knee than to vent.

Measurements of holotype (all in mm). SVL 50.2; HL 18.2; HW 21.1; ED 6.3; TD 3.5; EN 3.5; E-T 2.1; SL 5.8; NS 2.3; ICD 9.4; IBE 16.7; IO 6.0; IND 3.5; UEW 4.7; MAE 12.0; MPE 5.7; MN 15.9; TL 22.3; ShL 19.0; FL 16.6; HAL 11.8; FLL 17.5; FAL 27.0; Ta-Toe4 27.1; IML 2.3; F1 L 4.1; F2 L 3.9; F3 L 7.5; F4 L 4.6; Toe1 L 3.2; Toe2 L 3.2; Toe3 L 4.9; Toe4 L 7.5; Toe5 L 2.5.

Colouration of holotype in life. Dorsally uniform slate-grey with an indistinct dark blotch in the interorbital region with unclear edges ( Figure 4 View Figure 4 (a,b)); on lateral surfaces of dorsum, body flanks and sacral area few small sharply-edged dark-grey blotches of irregular shape ( Figure 4 View Figure 4 (a)); a distinct black canthal stripe running below the canthus rostralis from anterior corner of eye to nostril with a pair of roundish black spots around the nostrils. Loreal region and upper jaw dark-grey, two elongated black stripes at snout tip edging a lighter greyish medial stripe ( Figure 4 View Figure 4 (d)). Two black spots on upper jaw, anterior reaching the anterior corner of the eye, posterior shorter, located ventrally from the posterior eye corner but not reaching it ( Figure 4 View Figure 4 (d)). A thick black supratympanic stripe running below the supratympanic ridge from the posterior third of upper eyelid to axilla, covering the tympanum ( Figure 4 View Figure 4 (d)). Ventral surfaces blackish to dark-grey with irregular sparse whitish marbling on the chest and the anterior portion of the belly; lateral surfaces of the belly and chin with numerous tiny white dots, which are also present on ventral surfaces of limbs ( Figure 4 View Figure 4 (c)). The gular area scattered with numerous tiny orange-red dots getting denser laterally, at the ventral surfaces of lower jaws ( Figure 4 View Figure 4 (c)). Limbs dorsally dark-grey with several irregular black blotches, not forming complete transverse bars, edged with beige: thighs and shanks have two to three such incomplete dark bars ( Figure 4 View Figure 4 (a)); ventrally limbs with irregular black and white pattern; posterior surfaces of thighs black with white blotches; vent blackish ( Figure 4 View Figure 4 (c)). Femoral and axillary glands off-white. Iris bicoloured, black in lower twothirds, fire-red in upper one-third; scleral arc bright light blue ( Figure 4 View Figure 4 ).

Colouration of holotype in preservative. After storage in ethanol for 6 months ( Figure 3 View Figure 3 ) the colouration pattern did not change significantly; ventral colouration faded to greyishbrown, red spots on chin, reddish colouration of iris and bluish colouration of sclera faded completely; blotches on belly, groin and posterior surfaces of thighs are still distinct.

Morphological variation. Variation of the type series in morphometric characters is presented in Table 3. Generally, external morphology and colouration of the paratypes matched to those described for the holotype. Males have much smaller body size (mean SVL 55.4 ± 4.8; n = 6) than the two female paratypes (mean SVL 66.1 ± 3.1; n = 2); SDI index equals 1.19. Paratype female JnUZool-A0520 showed colouration in preservative ( Figure 5 View Figure 5 ) and in life ( Figure 6 View Figure 6 (b)) generally similar to that of male holotype. Clutch size for two females varies from 1950 to 2430 (n = 2); with an average egg diameter 1.66 ± 0.05 mm (n = 15). Eggs covered with gelatinous capsules, bicoloured with greyish animate pole and yellowish yolk ( Figure 5 View Figure 5 (c)).

Advertisement call. The following description is based on the calls of three male individuals recorded at 31°C (see Table 4). The call contained 3–8 (mean 5.8; n = 10), highly pulsed notes and lasted 0.67– 2.04 s (mean 1.57 s; n = 10), repeated at a variable interval (2.69– 20.78 s, mean 8.11 s). Within a call, notes were evenly spaced and each note contained 10–24 (mean 15.64) pulses repeated at varying rates across the call (64.36– 208.33 pulses/s). The relative amplitude was gradually rising over the first half and then gradually declining across the second half of each note. The dominant frequency of calls varied from 1.4 to 1.9 kHz (mean 1.6 kHz), and spanned approximately 1 kHz ( Figure 7 View Figure 7 ); harmonics were not discernable. There was no frequency modulation within each note. Temporal and spectral properties of the call did not obviously vary in the three specimens recorded. To the human ear, the call sounds like a rapid barking ‘ wahk-wahk-wahk-wahk ’, slightly resembling a call of a duck.

Distribution. Geographic distribution of Leptobrachium sp. in Bangladesh and Northeast India is well-documented in a number of works (e.g. Dutta et al. 2013; Hasan et al. 2014; Ghose et al. 2017; Mondal et al. 2019); the known localities of Leptobrachium from this region are summarised in Figure 1 View Figure 1 (b). However, not all of these populations can be confidently attributed to Leptobrachium sylheticum sp. nov. In the present paper we reliably assign to the new species the populations of Leptobrachium from the Sylhet Division of Bangladesh ( Figure 1 View Figure 1 (b), localities 29, 30, 32, 37) due to geographic proximity to the type locality, and a population from Tripura State of India ( Figure 1 View Figure 1 (b), locality 28) based on the results of our phylogenetic analysis (see Figure 2 View Figure 2 , sample 70). Geographically, the southwestern portion of Tripura belongs to the same hilly area as the type locality of the new species, so the genetic proximity of these two populations is not surprising.

definitions see materials and methods.

The status of the remaining Leptobrachium populations from India and Bangladesh, as well as the actual extent of distribution of Leptobrachium sylheticum sp. nov. still remains unclear and requires further studies. The populations from Assam and Meghalaya in India appear to be at least partially associated with subtropical forests highlands of Khasi Hills ( Figure 1 View Figure 1 (b), localities 1–22), while the populations from Chittagong Hills in southeastern Bangladesh ( Figure 1 View Figure 1 (b), localities 31, 33–36) and from Mizoram State of India ( Figure 1 View Figure 1 (b), localities 23–26) belong to the Arakan (Rakhine) mountain system, part of the Mizoram- Manipur-Kachin rainforest ecoregion. The taxonomic status of these populations still has to be clarified via the integrative taxonomic approach, and in the present paper we refrain from assigning them to any of the currently recognised species of Leptobrachium (see Discussion).

Ecology and natural history. The Lawachara National Park (LNP) is located in the Moulvibazar District of Sylhet Division in the northeast of Bangladesh. This is a 1250 ha lowland mixed evergreen forest, located on gently sloping hills at an average elevation range is 10–80 m above the sea level. There are several small streams and ponds present inside the forest (NACOM 2003). The area of LNP lies on the western cusp of the Indo- Burma Biodiversity Hotspot, where mixture of biogeographic elements of both Gondwanan and Laurasian origin was recorded ( Reza 2010). LNP has a tropical monsoon climate, with annual mean diurnal temperature ranging from 27°C (June–September) to 16°C (January) ( Mollah 2004). The average annual rainfall is approximately 3000 mm, of which 80% falls from June to September. October to March is the dry season with the first rain usually falling in April. The territory of LNP includes areas of mature forest, degraded forest, tea plantations, paddy fields and village habitats. ‘Mature forest’ represents a dense secondary forest which was used for tree plantation in the 1920s–1960s, and is predominantly comprised of such tree species as jarul ( Lagerstroemia speciosa (L.) Pers.), loha ( Xylia xylocarpa Roxb. Taub. ), kadam ( Neolamarckia cadamba (Roxb.) Bosser ), garjan ( Dipterocarpus turbinatus C.F.Gaertn. ), and teak ( Tectona grandis L.f.) ( Rahman et al. 2013a). As the LNP is a key biodiversity conservation area in northeastern Bangladesh ( Hakim et al. 2020), a road and a railway line going through the forest represents a constant threat for its biota including the herpetofauna ( Rahman et al. 2013b; Al-Razi et al. 2020b). Further efforts must be made to protect these remaining forest patches in northeastern Bangladesh, which may still cradle undiscovered new species, as documented in this study.

Specimens of Leptobrachium sylheticum sp. nov. were recorded along slow-flowing streams in lowland mixed evergreen forests at elevations from 50 to 70 m a.s.l. (see Figure 6 View Figure 6 (a)). As other Leptobrachium , the new species is mostly a litter dweller. The frogs were observed to hide themselves under the litter and close their eyes when disturbed. Generally the specimens of the new species were recorded in the forest floors. During the breeding season males were observed calling from the slopes of hillocks near the streams. Males of Leptobrachium sylheticum sp. nov. usually start calling after sunset (ca 1900 h) and continue till midnight (2400 h).

Amplexus was recorded on 9 June 2020; it is inguinal as it is typical for other Megophryidae representatives ( Duellman and Trueb 1994): the male tightly holds the waist of the female by its forelimbs ( Figure 6 View Figure 6 (b)). We observed amplexus pairs and calling males from March to October. Amplexus takes place on the land; afterwards the pair comes down to the transparent slowly-flowing water of the stream. They usually lay their eggs near the steep slope of the hillock where the water current is comparatively low. Eggs are laid in the water and are covered with gelatine capsules.

Amphibian species recorded sympatrically with the new species at the type locality include: Microhyla berdmorei (Blyth) (Microhylidae) , Fejervarya sp. (Dicroglossidae) , Hydrophylax leptoglossa (Cope) and Clinotarsus alticola (Boulenger) (Ranidae) , and Raorchestes rezakhani Al-Razi, Maria & Muzaffar (Rhacophoridae) . At the type locality the new species was recorded in the same biotopes as Microhyla berdmorei , Hydrophylax leptoglossa and Clinotarsus alticola , and these species shared same streams for reproduction.

Etymology. The specific name is a Latinised toponymic adjective in neutral gender derived from ‘Sylhet’ – a name of the region in the northeastern part of Bangladesh, from where the new species is described. The name ‘Sylhet’ is an anglicisation of ‘Shilhot’, which seem to come from the Sanskrit words of ‘ śilā ’ (meaning ‘stone’) and ‘ hatta ’ (meaning ‘market’). ¨ ¨

Comparisons. According to the results of our phylogenetic analyses, Leptobrachium sylheticum sp. nov. belongs to L. smithi species complex (subclade L1) of the Sundaland – western Indo-Burma clade (subgenus Leptobrachium ). It is unlikely that the new species is conspecific to one of the numerous island taxa of Leptobrachium known from Sumatra, Java, Bali, Borneo, and the Philippines, and they can be safely omitted from comparisons for simplicity. Below we compare the new species to the mainland members of the subgenus Leptobrachium , as well as to the remaining species of Leptobrachium from mainland Southeast Asia which belong to the subgenus Vibrissaphora .

Iris colouration in life is known as an important diagnostic character for species identification of the genus Leptobrachium ( Matsui et al. 1999; Hamidy and Matsui 2010; Stuart et al. 2011, 2012; Wogan 2012; Yang et al. 2016; Pawangkhanant et al. 2018). By having a bicoloured iris with upper one-third fire-red, lower one-third black with brightblue sclera, Leptobrachium sylheticum sp. nov. can be easily distinguished from the following congeners which have uniform black or dark-brown iris ( L. abbotti (Cochran) ; L. chapaense (Bourret) ; L. gunungense Malkmus ; L. hasseltii ; L. ingeri Hamidy, Matsui, Nishikawa & Belabut ; L. kanowitense Hamidy, Matsui, Nishikawa & Belabut ; L. kantonishikawai Hamidy & Matsui ; L. lumadorum Brown, Siler, Diesmos, & Alcala ; L. mangyanorum Brown, Siler, Diesmos, & Alcala ; L. montanum ; L. nigrops Berry & Hendrickson ; and L. tagbanorum Brown, Siler, Diesmos, & Alcala ). A bicoloured iris with upper one-third fire-red, lower one-third black with bright-blue sclera distinguishes the new species from the congeners with bicoloured black-and-white or black-and-blue iris ( L. ailaonicum (Yang, Chen & Ma) ; L. banae Lathrop, Murphy, Orlov & Ho ; L. boringii (Liu) ; L. buchardi Ohler, Teynié & David ; L. guangxiense Fei, Mo, Ye & Jiang ; L. hainanense Ye & Fei ; L. huashen Fei & Ye ; L. leishanense (Liu & Hu) ; L. leucops Stuart, Rowley, Tran, Le & Hoang ; L. liui (Pope) ; L. masatakasatoi Matsui ; L. ngoclinhense (Orlov) ; L. promustache (Rao, Wilkinson & Zhang) ; L. tenasserimense , L. tengchongense Yang, Wang & Chan ; L. xanthops Stuart, Phimmachak, Seateun & Sivongxay ; and L. xanthospilum Lathrop, Murphy, Orlov & Ho ). By having a bicoloured iris with upper one-third fire-red, lower one-third black, Leptobrachium sylheticum sp. nov. can be also distinguished from those species which have a uniform blue iris ( L. bompu and L. waysepuntiense ).

Three species of the subgenus Vibrissaphora from southern Vietnam have uniformly black or dark-brown iris with reddish or orange colouration of the scleral ring: L. lunatum , L. mouhoti Stuart, Sok & Neang , and L. pullum (Smith) , and thus can be easily diagnosed from the new species having bicoloured fire-red and black iris.

Leptobrachium hendricksoni Taylor from peninsular Malaysia, southern Thailand, Borneo, and Sumatra demonstrates uniform red or bicoloured red-and-black iris, and thus can be confused with the new species. However, Leptobrachium sylheticum sp. nov. can be easily distinguished from L. hendricksoni by having contrasting blackish belly with rare white marbling pattern (vs. creamy belly with black speckles), by females being slightly larger than males, SDI 1.19 (vs. females much larger than males, SDI 1.38), by generally larger body size with SVL 50.2–60.9 mm in males, 63.9–68.3 mm in females (vs. SVL 45.0–46.0 mm in males, 63.0 mm in female), by a narrow dark canthal stripe (vs. broad dark canthal stripe completely covering narial area), by finger length formula II<I<IV<III (vs. II<IV = I<III), and by small dark blotches on lateral sides of dorsum (vs. lavender brown with no distinct markings).

Comparisons of the new species with other members of L. smithi species complex (subclade L1), namely with L. smithi , L. rakhinense , and L. tenasserimense , appear to be the most pertinent. Leptobrachium sylheticum sp. nov. can be easily distinguished from L. tenasserimense from Tenasserim Mountains in peninsular Thailand by having a bicoloured red and black iris (vs. black and turquoise bicoloured iris), by having finger length formula II<I<IV<III (vs. II<IV<I<III), by having generally larger females, SVL 63.9–68.3 mm, n = 2 (vs. female SVL 54.7–58.6 mm, n = 2), distinct head markings absent (vs. present), by dark colouration of supratympanic stripe completely covering tympanum (vs. not covering or partially covering tympanum), by uniform grey dorsum with few small black spots laterally (vs. dorsum grey with darker blotches outlined with dark-brown), by having dark-grey to black belly with white irregular spots (vs. belly whitish with contrasting confluent black blotches), by having reddish dots on ventral surfaces of jaws (vs. absent), and by 2–3 incomplete dark transverse bars on shanks and thighs (vs. 4–5 complete and distinct dark transverse bars on shanks and thighs).

The new species can be diagnosed from L. smithi (inhabiting Thai-Malay Peninsula, Tenasserim, western Thailand, Laos and eastern Myanmar) by having finger length formula II<I<IV<III (vs. IV<II<I<III), by females being slightly larger than males, SDI 1.19 (vs. females much larger than males, SDI 1.34), by a narrow dark canthal stripe (vs. broad dark canthal stripe completely covering narial area), by distinct head markings absent (vs. present), by uniform grey dorsum with few small black spots laterally (vs. dark grey, no distinct markings), by having dark-grey to black belly with white irregular spots (vs. belly whitish with dark speckles posteriorly), by having reddish dots on ventral surfaces of jaws (vs. absent), and by 2–3 incomplete dark transverse bars on shanks and thighs (vs. usually four complete and distinct dark transverse bars on shanks and thighs). Most populations of L. smithi have iris colouration similar to that of Leptobrachium sylheticum sp. nov., however some populations of L. smithi from the southern part of peninsular Thailand show bicoloured black-and-yellow iris and can be further diagnosed from the new species by this character.

Leptobrachium sylheticum sp. nov. is morphologically similar to L. rakhinense from Rakhine State in Myanmar but can be distinguished from it by the following combination of morphological attributes: by having finger length formula II<I<IV<III (vs. II<IV<I<III), by females being larger than males, SDI 1.19 (vs. females only little larger than males, SDI 1.10), by a narrow dark canthal stripe separated by a lighter area from two anterior dark stripes on snout (vs. broad dark canthal stripe completely covering narial area and merging with two anterior dark stripes on snout), by distinct head markings absent (vs. strong dark markings on head), by uniform grey dorsum with few small black spots laterally (vs. light grey to brown dorsum with distinct dark brownish blotches outlined with white), by having dark-grey to black belly with white irregular spots (vs. light belly with white speckles), by having reddish dots on ventral surfaces of jaws (vs. absent), and by 2–3 incomplete dark transverse bars on shanks and thighs (vs. 4–5 complete and distinct dark transverse bars on shanks and thighs). Furthermore, Leptobrachium sylheticum sp. nov. can be distinguished from L. rakhinense by having only a few small dark spots on body flanks not forming a distinct series and not edged with white or beige (vs. large dark lateral spots with light edging forming series on body flanks).

Not much is known about the bioacoustics of the Leptobrachium smithi complex members: only fragmentary data on the male advertisement calls of L. smithi from Thailand ( Matsui et al. 1999) and on the population of Leptobrachium from northeastern India ( Dutta et al. 2013) are available, the calls of L. rakhinense and L. tenasserimense were not studied. The advertisement call of Leptobrachium sylheticum sp. nov. differs structurally from the call of L. smithi recorded at 23–25°C by having 3–8 (usually 5–6) notes in each call (vs. 1–7, usually one note in L. smithi ), by having shorter internote interval 0.03– 0.24 s (vs. 0.28– 0.48 s in L. smithi ), and by having generally higher dominant frequency of 1.37–1.90 kHz, mean 1.60 kHz (vs. 0.75–1.60 kHz, mean 1.30 kHz in L. smithi ). The call of the new species is different from call parameters of the Northeast Indian population of Leptobrachium , as reported by Dutta et al. (2013), in having higher dominant frequency of 1.37–1.90 kHz, mean 1.60 kHz (vs. 1.28–1.42 kHz, mean 1.37 kHz), and in having shorter internote interval 0.03– 0.24 s (vs. 0.4 s).