Gonyosoma iadinum sp. nov.

urn:lsid:zoobank.org:act: 1353625F-8F05-4584-9FD5-773D692DED91

(Figures 3–4; Tables 1–2)

Chresonymy:

Gonyosoma prasinum (nec Gonyosoma prasinum Blyth, 1855)— Nguyen et al. (2009: 316, in part); David et al. (2023: 273– 274; in part);

Elaphe prasina — Stuart (1999: 58); Stuart & Heatwole (2008: 99, Fig. 4);

Rhynchophis prasinus — Wallach et al. (2014: 655, in part);

Gonyosoma cf. prasinum — David et al. (2022: 191); Poyarkov et al. (2023: 339, in part).

Holotype. ZMMU Re-18222 (field number NAP-15953; will be subsequently deposited in the VNMN under collection number VNMN R.2015.15953), adult male, from Chu Yang Sin National Park in Bong Krang Commune, Lak District, Dak Lak Province, Vietnam (12.39328°N, 108.34684°E; altitude 1,200 m asl) collected by N.A. Poyarkov on 13 April 2012.

Paratypes (n = 4, all from Vietnam). ZMMU Re-18223 (field number NAP-15413), adult male, from Ta Dung NP in Dak Plao Commune, Dak Glong District, Dak Nong Province (11.85566°N, 108.02951°E; altitude 1,780 m asl) collected by N.A. Poyarkov and A.M. Bragin on 24 January 2021; ZMMU Re-14426 (field number NAP-02866), adult female, from Bao Loc Forest Enterprise, in Loc Bao Commune, Bao Lam District, Lam Dong Province (11.83647°N, 107.639889°E; elevation 655 m a.s.l.) collected by N. A. Poyarkov on April 24, 2012; ZMMU Re- 11551, adult female, from Hon Ba NR in Cam Lam District, Khanh Hoa Province (ca. 12.082088°N, 108.955533°E; altitude 1,550 m asl) collected by V.V. Bobrov on April 27, 2003; VRTC DTU.411, subadult male, from Son Dien Commune, Di Linh District, Lam Dong Province (ca. 11.390085°N, 108.042776°E; altitude 920 m asl) collected by H. H. Nguyen in April 2023.

Referred materials (n = 4, all from Vietnam). VRTC DTU.596, subadult female, from Ba Na-Nui Chua NR in Hoa Vang District, Da Nang City (ca. 15.991583°N, 107.996635°E; altitude 1,200 m asl) collected by T. D. Nguyen in May 2024; DTU 597, adult female, from Kon Plong District, Kon Tum Province (ca. 14.737767°N, 108.282712°E; altitude 1,300 m asl) collected by T. D. Nguyen in July 2021; DTU 649, adult male, from Bach Ma NP in Phu Loc District, Thua Thien-Hue Province (16.195265°N, 107.848148°E; altitude 950 m a.s.l.) collected by T. A. Tran, L. S. Pham, and A. G. B. Vu on 15 June 2024.

Etymology. The specific name “ iadinum ” is a Latin adjective derived from a Latinised form of the classical Greek ἰαδίνως (iadinos), meaning “jade”. The specific name is given in reference to the characteristic greenish-yellow colouration of the iris of the new species and the general greenish body colouration resembling polished jade jewels. We suggest the following common names in English: “Jade Tree Snake”, in Vietnamese: “Thanh xà Trường SƠn”, and in the Russian language: “Nephritovyi poloz” (Нефритовый полоЗ).

Diagnosis. A medium-sized, slender-bodied species of the genus Gonyosoma distinguished by the following combination of diagnostic characteristics: (1) maximum snout-vent length 807 mm; (2) 19 dorsal scale rows at midbody, 7–11 upper dorsal rows feebly keeled; (3) cloacal plate undivided; (4) 198–210 ventral plates all with a strong lateral keel; (5) 92–112 paired subcaudal scales; (6) one loreal scale, one large preocular and two postoculars on each side; (7) 9 supralabials, 4 th –6 th entering orbit; (8) dorsum and sides bright green, grass green, or bluish green; (9) dorsal scales usually not edged with black in adult specimens, distinctly black-edged in young individuals; (10) interstitial skin with black and white reticulations; (11) colour of the iris varying from yellowish-green to bluish-green in life; (12) ventral surface greenish-white to pale green or pale yellow with white or cream lateral keels; (13) head colouration similar to body colouration, lacking black preocular or postocular streaks; (14) supralabials yellowish-green; (15) hemipenes short, spinous, reaching the 5 th –6 th subcaudal when everted.

Description of the holotype. Adult male (Fig. 3), specimen in a good state of preservation. The body is slender, distinctly compressed laterally, firm and muscular (SVL 607 mm, TaL 218 mm, TL 825 mm, ratio TaL/TL 0.264) (Fig. 3A–B). Head much elongated, triangular, slightly narrowing anteriorly, rather thick posteriorly, flattened anteriorly, moderately distinct from the relatively thick neck (HL 24.2 mm; HW 13.3 mm; ratio HW/HL 0.55; Fig. 3C–E); snout long, rather thick and flattened, rounded from dorsal view (Fig. 3C), truncated from lateral view, not projecting beyond the lower jaw (Fig. 3E); no canthus rostralis; nostril lateral, large, vertically ovoid, piercing in the middle of the nasal (Fig. 3E); eye large, with a round pupil (ED 4.4 mm; EN 5.9 mm; ratio ED/EN 0.74); tail long, slender, cylindrical, tapering and prehensile (Fig. 3B). Rostral developed, wider than high, visible from above but barely separating internasals. Nasal divided into two scales; two internasals, wider than high, bordered by two large prefrontals posteriorly (Fig. 3C); frontal single, enlarged, pentagonal, narrowed posteriorly; parietals longer than wide, in contact with each other (Fig. 3C); one loreal small, subrectangular, elongate; supralabials 9/9, first and second in contact with the nasal, second and third entering loreal, fourth and sixth entering orbit, six to eighth largest (Fig. 3E); infralabials 9/9, first pair in broad contact with each other, first to fifth in contact with anterior pair of chin shields; anterior and posterior pairs of chin shields elongate, second pair meeting in midline, six largest (Fig. 3D); preocular 1/1, large; postoculars 2/2, lower ones smaller, bordering anterior temporals; anterior temporals 1/1, elongate; posterior temporals 2/2 (Fig. 3E).

Dorsal scales in 19-19-15 rows, upper dorsal rows feebly keeled (KAD 9, KMD 11, KPD 11). Two preventrals and 200 ventrals; cloacal plate undivided; 112 subcaudals, all divided. Hemipenes are rather thick, short, spinous, and unforked, reaching the 5 th –6 th subcaudal (Fig. 3F).

Colour of the holotype in life. Dorsal surface bright green with brownish-yellow tip of tail; upper lips yellowish-green; anterior ventral surface greenish-yellow and posterior ventral surface light green; tip of tail pale brownish-yellow on its ventral surface; ventrals outside the lateral keel usually yellowish-white; iris yellowish-green.

Variation (for detailed information, see Table 1, Fig. 4). The longest known specimen is 1,056 mm long (SVL 807 mm, TaL 249 mm; adult male, ZMMU Re-18223); the longest known female is 1,030 mm long (SVL 780 mm, TaL 250 mm; DTU 409). Ratio TaL/TL: 0.234 –0.264 (not showing any sexual dimorphism).

Body scalation. 19-19-15 DSR; 198–210 VEN, not showing any sexual dimorphism; 92–112 SC, not showing any sexual dimorphism; 295–312 VEN+SC (310–312 in males, 295–303 in females).

Head scalation. SL 9; IL: 9 (rarely 10); temporal 1+2 (rarely 2+3).

Colouration and pattern. The dorsum and side of the body are uniform grass green or pale emerald green (all colours turning to dark green or often bluish-black in preservative), paler on the lower sides on 1 st and 2 nd dorsal scale rows; the interstitial dorsal skin is black and white, forming reticulations when the snake inflates the anterior part of its body. The tail exhibits the same uniform green colour as the body, albeit with a brown tip in some specimens. The head is of the same shades of green as the dorsal surface uniform; the rostral and region of the nasal and loreal are slightly paler; the anterior supralabials are yellowish green or pale green, paler than the dorsal surface of the head, either entirely or only on their lower half; other supralabials behind the eye are cream green, yellowish-green or very pale bluish-green; and there is no postocular or preocular streak, but the temporal area is somewhat faintly darker green. The chin is yellowish-green, turning to pale green on the throat. The iris varies from yellowish-green to bluish-green or greenish-blue in life; the pupil is black. The venter is pale green, darker on the posterior part of the venter; the outer part of the ventral plates has a broad pale stripe that extends from the neck to the base of the tail, cream or yellowish-cream anteriorly and more creamish-green posteriorly; the keels of the ventral plates are white or cream; and the tips of the ventrals are green, as are the lower sides of the body. The ventral surface of the tail is green anteriorly and more reddish-olive posteriorly. The pattern and colouration of juveniles are slightly different from those in adults. The dorsum is green, perhaps slightly darker than in adults; many scales are edged with black, forming faint, narrow transverse crossbands; the anterior part of the body is distinctly speckled with black.

Comparisons. Morphologically, Gonyosoma iadinum sp. nov. is most closely resembling G. coeruleum and G. prasinum. The main diagnostic characters separating Gonyosoma iadinum sp. nov. from these two species are summarised in Table 2 and illustrated in Figs. 5–6. The main characters that separate it from G. coeruleum are the cloacal plate undivided (vs. divided) and hemipenes short, spinous, reaching the 5 th –6 th subcaudals (vs. long, weakly spinous, reaching the 13 th subcaudal). The main characters that separate the new species from G. prasinum are the upper dorsal rows feebly keeled (vs. smooth) and a slightly higher total number of VEN+SC in males (310–312 [avg. 311.0] vs. 298–308 [avg. 303.7]), and a greater maximum SVL in males (807 mm vs. 650 mm), although smaller than the maximum SVL observed in females (780 mm vs. 1050 mm). Furthermore, Gonyosoma iadinum sp. nov. is geographically well separated from G. prasinum, which is distributed in northeast India, Bhutan, Nepal, and northern and northwestern Myanmar. Additionally, G. coeruleum occupies an intermediate range, effectively separating the distributions of both species (Fig. 1).

Gonyosoma iadinum sp. nov. differs from G. boulengeri (Mocquard) by lacking a nasal appendage (vs. rostral distinct from the nasal appendage) and no dark stripe behind the eye (vs. an indistinct dark stripe behind the eye) (see Fig. 7A); it differs from G. frenatum (Gray) by having present one loreal (vs. loreal full with the prefrontal) and no black streak along the side of the head (vs. a black streak along the side of the head above the supralabials) (see Fig. 7B); it differs from G. hainanense by lacking a nasal appendage (vs. rostral distinct from the nasal appendage) and one loreal (vs. two loreals) (see Fig. 7C); it differs from G. jansenii (Bleeker) by having 19 midbody dorsal scale rows (vs. 23–25) and dorsal surface uniform bright green with brownish-yellow tip of tail (vs. olive or yellowishbrown, entirely black posteriorly and on the tail) (see Fig. 7D); it differs from G. margaritatum Peters by having dorsal surface uniform bright green with brownish-yellow tip of tail (vs. black, each scale with a yellowish green spot, or green with black borders to the scales, hinder part of body and tail with bright orange rings) and no black streak on each side of the head (vs. a black streak on each side of the head behind the eye) (see Fig. 7E); it differs from G. oxycephalum (Boie) by having 19 midbody dorsal scale rows (vs. 23–27), different colors of tail (green with brownish-yellow tip vs. the whole tail light chestnut or buff-red or yellowish brown), and no blackish stripe along the side of the head (vs. an indistinct blackish stripe along the side of the head immediately above the supralabials) (see Fig. 7F).

Distribution and Natural history notes. Gonyosoma iadinum sp. nov. inhabits a broad altitudinal range, from 655 to 1,780 m asl, and is found across diverse forest communities, where it is active usually during daylight hours.

In Vietnam, Gonyosoma iadinum sp. nov. is currently known from the montane evergreen forests of the Kon Tum-Gia Lai Plateau in the Truong Son (Annamite) Mountains of central Vietnam, as well as the Langbian Plateau in southern Vietnam. Verified records include Chu Yang Sin NP) in Dak Lak Province (our data), where it was observed at an elevation of 1,200 m asl at 21:00, sleeping on a tangle of Gramineae grasses and Brainea insignis ferns overhanging a path along a river terrace at the boundary of a dry pine forest dominated by Pinus kesiya; Hon Ba NR in Khanh Hoa Province (our data); and Ta Dung NP in Dak Nong Province (our data, David et al. 2022), where it was found at 1,780 m asl at 22:00, resting on lower branches in a foggy, evergreen, small-leaved polydominant forest at the summit of the narrow Ta Dung Mountain ridge. Additional confirmed records include Bidoup-Nui Ba NP and Di Linh District in Lam Dong Province; Kon Chu Rang NR in Gia Lai Province (David et al. 2022); Kon Plong District in Kon Tum Province (David et al. 2022; our data); Ba Na-Nui Chua NR in Da Nang City (David et al. 2022; our data); Bach Ma NP in Thua Thien-Hue Province (our data); and Song Thanh NP and Nam Giang District in Quang Nam Province (our data), where the species was found at elevations ranging from 655 to 1,550 m asl. Records from Quang Ngai, Binh Dinh, Phu Yen, Ninh Thuan, and Binh Thuan provinces in central and southern Vietnam are anticipated. In Bach Ma NP, Thua Thien-Hue Province, an individual was observed at 22:00 resting on a 2.5 m-high tree over a slow-flowing stream. In Ba Na-Nui Chua NR, Da Nang City, another specimen was encountered at 10:00 while it was actively moving across a road.

In Laos (based on David et al. 2022, 2023), Gonyosoma iadinum sp. nov. is definitively recorded from central and southern regions of the country. Verified occurrences include Khammouan Province (Hin Nam No National Protected Area) in central Laos and Champasak Province (Dong Hua Sao National Biodiversity Conservation Area, Bolaven Plateau, and an additional nearby locality on the same plateau) in southern Laos. Additional records from Salavan and Xekong provinces in southern Laos are anticipated. Stuart (1999) documented this species in wet evergreen forests of the Annamite Mountain Range. Furthermore, a specimen reported by Stuart & Heatwole (2008) from Champasak Province was found on a trail within a disturbed, humid evergreen forest at an elevation of 1,000 m asl.

Conservation status. Gonyosoma iadinum sp. nov. is currently confirmed from 11 localities, including six national parks and three nature reserves in central and southern Vietnam. This species is arboreal and primarily diurnal, typically inhabiting closed-canopy evergreen forests. Despite its relatively wide distribution, it remains uncommon throughout its range, possibly due to its elusive nature within forested biotopes. The primary threats to this species in Vietnam include habitat loss and degradation. Additionally, it is occasionally misidentified and killed due to its superficial resemblance to green pit vipers (Trimeresurus spp.). Road mortality is another significant concern, particularly in areas with high vehicular traffic related to travel and tourism, such as Bach Ma NP (Thua Thien-Hue Province), Chu Yang Sin NP (Dak Lak Province), Ta Dung NP (Dak Nong Province), Bidoup-Nui Ba NP (Lam Dong Province), and Hon Ba NR (Khanh Hoa Province) (T.V. Nguyen, pers. obs.). Although key aspects of its distribution, population trends, reproductive behaviour, and ecology remain poorly understood, based on the available data, we propose that Gonyosoma iadinum sp. nov. be classified as Least Concern (LC) according to the IUCN Red List criteria (IUCN 2019)

Discussion

Historically, Gonyosoma prasinum was considered a widely distributed species, occurring in India, Bhutan, Nepal, China, Myanmar, Vietnam, Laos, Thailand, and Peninsular Malaysia (e.g., Boulenger, 1894; Smith, 1943; Yang & Rao, 2008; Nguyen et al. 2009; Wallach et al. 2014; Chan-ard et al. 2015; Wangyal et al. 2020; David et al. 2022). Several studies have attempted to reassess the taxonomy and distribution of the Gonyosoma prasinum species complex. Recently, Liu et al. (2021), using both morphological and molecular data, and David et al. (2022), based on morphological characteristics alone, concluded that G. prasinum sensu stricto (characterised by an undivided cloacal plate) should be restricted to northeast India, Bhutan, northern and northwestern Myanmar, and possibly eastern Nepal. Meanwhile, populations from central and eastern Myanmar, southern China, northern Laos, northern Vietnam, Thailand, and Peninsular Malaysia should be assigned to G. coeruleum (mainly characterised by a divided cloacal plate).

David et al. (2022) further suggested that the population identified as Gonyosoma cf. prasinum from central and southern Vietnam, as well as central and southern Laos (characterised by a divided cloacal plate), required further taxonomic investigation. In this study, we present that these Gonyosoma cf. prasinum populations from central and southern Vietnam are morphologically distinct from G. prasinum s. str., G. coeruleum, and other members of the genus Gonyosoma. Based on these findings, we formally describe this population as a new species, Gonyosoma iadinum sp. nov. Notably, we observed intraspecific variation in eye colouration, with individuals exhibiting either blue or yellow-green eyes. Consequently, we suggest that eye coloUr should not be considered a reliable diagnostic characteristic within the G. prasinum species complex. Furthermore, we confidently assign specimens previously identified as Gonyosoma cf. prasinum from central and southern Laos characterised by an undivided cloacal plate and non-bright blue eyes, as reported by Stuart (1999), Stuart & Heatwole (2008), and David et al. (2022), to Gonyosoma iadinum sp. nov. As a result, we propose the removal of Gonyosoma prasinum from the snake fauna of Vietnam and Laos.

On the other hand, Gonyosoma prasinum has been widely reported from Hainan Island, China, although its morphology and molecular characteristics have not been thoroughly examined (Zhao et al. 1998; Shi et al. 2011). This population was recently tentatively assigned to Gonyosoma coeruleum (David et al. 2022). Interestingly, while Gonyosoma coeruleum has been reported from Yunnan, Sichuan, and Guizhou provinces of China, it appears to be absent from Guangxi Autonomous Region, which geographically lies between the Hainan and mainland G. coeruleum populations (see Fig. 1). Hainan Island was once connected to the central and southeastern regions of mainland Vietnam before shifting northward to its present location (Zhu 2016; Ali 2018), potentially facilitating long-distance migration of Gonyosoma populations (Sun et al. 2024). This discontinuous distribution pattern has been observed in various taxonomic groups, including plants, insects, fish, amphibians, reptiles, and mammals (Balakrishnan et al. 2003; Gong et al. 2009; Lin et al. 2010; Poyarkov et al. 2021, 2023; Hu et al. 2023; Sun et al. 2024; Li et al. 2024). Paleogeological records, fossils (Yao et al. 2009), and geological studies (Voris 2000; Liang et al. 2018) provide multiple lines of evidence supporting the “ Hainan Island shifting” hypothesis. Geological events, including plate movements, volcanic activity, and sea level changes, are believed to have led to the geographic isolation of Hainan Island from the Asian mainland approximately 2.5 million years ago (Mya), with tectonic activity beginning as early as the Eocene epoch (~30 Mya) (Voris 2000; Shi et al. 2006; Zhao et al. 2007; Liang et al. 2018). Shi et al. (2011: 228) documented a specimen (HNU-R0216, male) from Hainan Island with a divided cloacal plate, raising the possibility that this population is conspecific with Gonyosoma iadinum sp. nov. Additionally, Shi et al. (2011) reported the presence of Gonyosoma prasinum in Hubei Province, China, based on specimens housed at Wuhan University Museum. However, given the current understanding of the Gonyosoma prasinum species complex, this record is quite unusual. We suspect it may represent a misidentification of G. frenatum, which is also recorded from Hubei Province (Zhao et al. 1998). In summary, a comprehensive re-examination of Gonyosoma cf. prasinum specimens from Hainan Island, China, is necessary to clarify their taxonomic status and distribution.

The Kon Tum-Gia Lai Plateau and the Langbiang Plateau, both within the Truong Son (Annamite) Mountain Range, are rapidly emerging as herpetological diversity hotspots in Vietnam and the broader Indochina region, characterised by high herpetofaunal diversity and endemism (Poyarkov et al. 2021, 2023). In recent years, several new species of amphibians (e.g., Poyarkov et al. 2020, 2021; Gorin et al. 2024) and reptiles (e.g., Poyarkov et al. 2019a,b, 2022; Kliukin et al. 2023, 2024; Idiiatullina et al. 2024; Nguyen & Vogel 2024) have been described from these two plateaus. This underscores the urgent need for further studies, particularly in the montane forests of these regions, to complete biodiversity assessments, as both areas are experiencing significant habitat loss and deforestation.

Despite recent taxonomic studies on the genus Gonyosoma that integrate molecular and morphological data, our understanding of species diversity and distribution within the genus remains incomplete. This gap is particularly evident in widely distributed species with complex taxonomic histories, such as Gonyosoma frenatum, which occurs in India, China, and Vietnam, and G. oxycephalum, which is found across a vast range including India, Myanmar, Laos, Cambodia, Vietnam, Thailand, Malaysia (both Peninsular and Bornean), Singapore, Brunei, Indonesia, and the Philippines. Additional research is essential to clarify species boundaries and resolve taxonomic uncertainties within this genus.

The description of Gonyosoma iadinum sp. nov. increases the number of currently recognised species within the genus Gonyosoma to nine.Among these, five species are recorded from Vietnam, including G. coeruleum (suggested Vietnamese name: Thanh xà mắt xanh miền Bắc), G. boulengeri (suggested Vietnamese name: Thanh xà kỳ lân), G. frenatum (suggested Vietnamese name: Thanh xà không tấm má), G. oxycephalum (suggested Vietnamese name: Thanh xà đuôi đỏ), and G. iadinum sp. nov.