Sibon nebulatus Dewynter et al. (2021)

Fouquet, Antoine, Arteaga, Alejandro, De Sousa, Átilas R. & Avila, Robson W., 2025, A new species of Sibon (Serpentes, Dipsadini) from French Guiana and amended diagnosis of Sibon nebulatus, Zootaxa 5646 (1), pp. 93-114 : 102-105

Sibon nebulatus Dewynter et al. (2021)

Proposed standard English name: black and white snail-eating snake

Type material. Holotype: MNHN-RA-2024.0004 (AF3606) ( Figs 4 , 6 ), adult male collected by Antoine Fouquet, Elodie Courtois, Benoit Villette and Mael Dewynter, on 07 January 2016 at Mont Itoupé , French Guiana (3.02647 -53.07983; 800 m a.s.l.).

Paratypes: MNHN-RA-2024.0005 (AF1433), adult male collected by Elodie Courtois on 16 December 2013 at RN2 PK80, French Guiana (4.38526 -52.30456; 90 m a.s.l.) . MNHN-RA-2024.0006 (RG86), adult female collected by Michel Blanc on 18 January 2013 at RN2 PK40, French Guiana (4.57989 -52.39734; 90 m a.s.l.) . MNHN- RA-2024.0007 (RG98), adult male collected by Michel Blanc on 15 October 2016 at RN2 PK36, French Guiana (4.60042 -52.40194; 90 m a.s.l.) . MNHN-RA-2024.0008 (RG9) adult male collected by Michel Blanc on 18 June 2001 at CD5 PK2, French Guiana (4.79283 -52.43410; 60 m a.s.l.) .

Diagnosis. Sibon nigralbus sp. nov. is placed in the genus Sibon based on phylogenetic evidence ( Fig. 1 ) and on having the penultimate supralabial conspicuously higher than all other supralabials. The species is diagnosed based on the following combination of characters: (1) 14–15/15/15 smooth dorsals with enlarged vertebral row (2× as wide as adjacent rows); (2) loreal and prefrontal in contact with orbit; (3) 7–8 supralabials, 4 th and 5 th, or 5–6, or 6–7 contacting orbit; (4) 8–9 infralabials, 1st–6th in contact with chinshields, first pair of infralabials in contact behind symphysial; (5) 194–200 ventrals in males, 199 in the single female; (6) 99–114 divided subcaudals in males, 101 in the single female; (7) dorsal background color gray with 47–67 black bands (1–3 dorsal black scales per band separated by 2–3 gray dorsal scales) boarded by light gray speckles ( Fig. 5 ), ventral surfaces cream white 53–64 black bands interrupted medially and with smaller black speckles, dorsal aspect of head black with white speckles dorsally and white blotches laterally, throat with cream white background with black blotches and spots, iris light gray variegated with black reticulations; (8) 360–447 mm SVL in males, 493 mm in the single female; (9) 152–175 mm TL in males, 202 mm in the single female.

Comparisons. Sibon nigralbus sp. nov. is compared to other species of Sibon forming the S. nebulatus clade. Sibon nigralbus sp. nov. co-occurs in French Guiana with S. nebulatus (in parenthesis), from which it differs primarily on the basis of its light gray band with white flecks between black bands (vs dark gray with ochre/orange flecks and stripes), by having more ventral scales (194–200 vs 181–185 in males); more subcaudals (99–114 vs 73–100), a smaller head (2.1–2.9% vs 3.0–3.8% of SVL) and larger eyes (28–32 % vs 21–25% of HL).

Sibon nigralbus sp. nov. can be further differentiated from the other nominal Sibon from South America by a combination of meristic characters ( Table 2) and color pattern. From S. vieirai it differs by a higher number of ventrals (194–200 vs 84–105), presence of dorsal black bands (vs blackish blotches or incomplete bands) and a throat with black blotches and spots (vs black markings that form a checkerboard pattern). From S. leucomelas , the new species is differentiated by having higher number of subcaudals (99–114 vs 84–101) and a throat with black blotches and spots (vs throat entirely black). From S. ayerbeorum , the new species can be distinguished by its higher number of supra and infralabials (7–8 and 8–9 vs 6 and 6 respectively), higher number of ventrals (194–200 vs 136–155) and subcaudals (99–114 vs 78–93), and a dorsal coloration with black bands (vs irregular middorsal and ventrolateral dark-bordered ocelli). It can be differentiated from S. bevridgelyi by its higher number of ventrals (194–200 vs 175–193), higher number of subcaudals (99–114 vs 80–98), and a gray dorsal coloration with black bands (vs pale yellow with or without black bands and a black stippled disruptive pattern of irregular rusty to reddish brown blotches). From S. dunni , the new species differs by its higher number of supralabials (7–8 vs 6), higher number of ventrals (194–200 vs 136–145), and its gray dorsal coloration with black bands (vs light brownish-cream with a vertebral series of small, irregular, ovate, chocolate-brown spots). From S. marleyae , it differs by having a lower number of subcaudals (99–114 vs 109–143) and a gray dorsal coloration with black bands (vs olive to yellow with maroon bands). From S. hartwegi , it differs by its higher number of ventrals (194–200 vs 171–195) and subcaudals (99–114 vs 75–103).

Etymology. The specific epithet is the combination of the Latin words nigrum (meaning “black”) and albus (meaning “white”). It refers to the color pattern of the species which consists of a succession of white, gray, and black bands.

Description of holotype. Adult male, SVL 366 mm, tail length 170 mm (46.4% SVL); head length 10.4 mm (2.8% SVL) from tip of snout to angle of jaw; head width 8.6 mm (82.8% head length) taken at broadest point; nostril-orbit distance 2.0 mm; head distinct from neck; snout short, blunt in dorsal outline and rounded in profile; rostral 2.2 mm wide, broader than high; internasals 3.0 mm wide, broader than long; prefrontals 4.4 mm wide, broader than long, entering orbit; supraocular 3.4 mm long, longer than broad; frontal 3.3 mm long, with a pentagonal shape, in contact with prefrontals, supraoculars, and parietals; parietals 5.4 mm long, longer than broad; nasal divided, in contact with two supralabials, loreal, prefrontal, internasal, and rostral; loreal 1.3 mm long, longer than high, entering the orbit; eye diameter 3.3 mm; pupil semi-elliptical; no preocular; two postoculars; temporals 1+2; seven supralabials, 4th and 5th contacting orbit; symphysial precluded from contacting chinshields by first pair of infralabials; eight infralabials,1st–6th contacting chinshields; two pairs of chinshields longer than wide; dorsal scales in 15/15/15 rows, smooth, without apical pits; 197 ventrals; 114 divided subcaudals; cloacal plate entire.

Hemipenis. Based on the partially everted left hemipenis of the paratype MNHN-RA-2024.0005 (AF1433). The organ is slightly bilobed, capitate; on the sulcate side the capitulum occupies 1/2 of the hemipenis size, the capitulum is covered by spinulate calyces and small spines; the sulcus spermaticus is centrolineal and bifurcates just after the capitulum base; spines on edges of sulcus smaller than those on lateral and median region of sulcate side; truncus with enlarged asymmetrical spines at the sulcate and asulcate sides.

Distribution and ecology. Sibon nigralbus sp. nov. is only known from French Guiana, (where it was so far mistaken with S. nebulatus whereas specimens of S. nebulatus sensu stricto were considered to belong to a new species) but it is widespread throughout the territory at the exception of the upper Maroni basin and along the coastal strip of savannas where S. nebulatus occurs ( Dewynter et al. 2021). It is indeed a species found in close association with secondary and old growth forests. Therefore, it is very likely that this species also occurs in the forests of the neighboring Amapá state ( Brazil) and Suriname. A picture of a specimen from Marabá, Pará state ( Brazil), south of the Amazon river, is morphologically similar to S. nigralbus (S. sp. in Fig. 3 ). Several specimens have been observed feeding on snails in French Guiana. One specimen was observed at 05h30 am perched at about 6 m above the ground on a vine of the genus Bauhinia and disappeared in a cavity in the trunk.

Figures

FIGURE 1. Maximum-Likelihood phylogram based on the concatenated Cytb and ND4 gene fragments. Bootstrap supports are indicated on the top left of the nodes and “*” depict values above 97%. The existence of two lineages of S. vierai in the tree is merely due to the absence of overlap of the loci across samples.

FIGURE 3. Occurrence records of Sibon spp. from northern South America and central America that have been associated with the name S. nebulatus. White-bordered circles indicate records supported by molecular data; black-bordered circles indicate specimens that have been examined morphologically; points without contour indicate records from iNaturalist, Faune Guyane or other credible sources. “T” symbols indicate type localities. The northernmost records in Mexico are out of the focal area.

FIGURE 4. Pictures of preserved specimens of the Sibon nebulatus species complex and schematic drawings of the head in lateral, dorsal, ventral and frontal views. From left to right: two specimens of S. nigralbus sp. nov., a specimen of S. nebulatus from French Guiana, the holotype of S. nebulatus, and a specimen of S. aff. nebulatus from Central America. Some scales in the drawings are filled in red to highlight important differences in shape and a thin red line indicates the postcephalic dark patch.

FIGURE 5. Pictures of preserved (including types) and in life specimens of S. nebulatus, S. aff. nebulatus and S. hartwegi.

FIGURE 6. Holotype and one of the paratypes of Sibon nigralbus sp. nov. in preservative and picture of specimens in life.