Thursday, June 25, 2020

[Herpetology • 2020] Second Specimen of the Rare Bornean Snake Xenophidion acanthognathus (Serpentes: Xenophidiidae) and Confirmation as A Distinct Species from X. schaeferi

Xenophidion acanthognathus Günther & Manthey, 1995

in Fukuyama, Hossman & Nishikawa, 2020.
 Raffles Bulletin of Zoology. 68

We report a second specimen of Xenophidion acanthognathus, collected from Lambir Hills National Park, Malaysian Borneo. We investigate the taxonomic status and relationship between this species and its only other congener X. schaeferi, as possible synonymy was suggested. Morphological and genetic analysis confirmed that X. acanthognathus and X. schaeferi are heterospecific. Here, we provide a detailed morphological description and novel natural history observations of this rare species. 

Key words. cyt b, Southeast Asia, Squamata, Sundaland, taxonomy, Xenophidion


Snakes of the family Xenophidiidae consist of one genus, Xenophidion Günther & Manthey, 1995, which includes two species: X. acanthognathus Günther & Manthey, 1995, and X. schaeferi Günther & Manthey, 1995, both of which are rare and enigmatic snakes. 

Interspecific comparisons: 
Xenophidion acanthognathus can be differentiated from its sister species, X. schaeferi by the following characteristics: larger number of ventrals (181–185 vs. 176–178); larger number of subcaudals (51–55 vs. 43–45); and shorter tail (SVL/tail length = 4.20–4.52 vs. 4.69–4.84).

Fig. 2. The live juvenile of Xenophidion acanthognathus (SRC 00961).

Fig. 1. The juvenile Xenophidion acanthognathus (SRC 00961) trying to escape under the bark in situ. 

Fig. 3. Ventral colouration of Xenophidion acanthognathus (SRC 00961) in life. Scale bar = 10 mm. 
Fig. 4. A, right-lateral; B, dorsal; C, ventral; and D, left-lateral aspects of the head of Xenophidion acanthognathus (SRC 00961). Scale bar = 10 mm. 

Natural history. The new specimen (SRC 00961) was found moving slowly on the trunk of a tree ca. 20 cm above the ground in a lowland dipterocarp forest, at 2117 h (Figs. 1, 5). The air temperature at the site was 24.7°C and it had not rained for several days except for a very light rain during the day, thus the ground was dry. When the first author spotted the snake under the beam of his flashlight, it tried to escape under the bark (Fig. 1). The holotype of the species was collected under moss covering a rock, 10 m from a stream, at 0815 h (Günther & Manthey, 1995). One photographed individual was found lying vertically on a damp, mossy log, at 2015 h (Rowntree et al., 2017). The only known food item of the species is a skink (Sphenomorphus sp.) that was found in the gut of the holotype (Wallach & Günther, 1998).

 The results of our study demonstrate that Xenophidion acanthognathus and X. schaeferi are sufficiently distinct morphologically and genetically to warrant specific status. With the acquisition of a second specimen of X. acanthognathus, this study is the first to examine the intraspecific variation within the species. However, additional specimens of both species are still needed to clarify their interspecific and intraspecific morphological variations. Genetic material of Xenophidion from Sumatra is also urgently needed to clarify its taxonomic status and phylogenetic placement. The distribution of the genus that is spread across Borneo, Peninsular Malaysia, and presumably Sumatra reflects the connection between these land masses until 400 kya as indicated by recent biogeographical and geological studies (Husson et al., 2019; Sarr et al., 2019). In addition, Xenophidion was shown to be closely related phylogenetically to the family Bolyeridae that is found only in the Mauritius (Lawson et al., 2004; Figueroa et al., 2016). Only a few genera of reptiles show similar relictual distributions in Sundaland like Xenophidion, such as the cat gecko, Aeluroscalabotes, which is the most basal linage and the only genus distributed in Sundaland in the family Eublepharidae, and the false gharial, Tomistoma, which is the sister species of Gavialis in South Asia (Willis et al., 2007; Jonniaux & Kumazawa, 2008). Comprehensive molecular phylogenetic analyses of these taxa may provide new insights into the biogeographical history of Sundaic fauna.

 Inger & Voris (2001) regarded X. acanthognathus as fossorial or secretive, but Quah et al. (2018) discussed the possible ecological niche of the genus and suggested that they may be of semiaquatic or semi-scansorial habit, and not burrowers. Our observation of the new specimen crawling on the trunk of a tree may support the hypothesis of Quah et al. (2018) that they are semi-scansorial by nature. Although many herpetological surveys were conducted in Malay Peninsula and Borneo, this genus has only been documented six times. It is possible that the extremely low encounter rate in the field with members of this genus is related to their low density as suggested by Günther & Manthey (1995), or may be due to specialised ecological habits. Further observations are essential to understand the natural history of the genus.

Ibuki Fukuyama, Mohamad Yazid Hossman and Kanto Nishikawa. 2020. Second Specimen of the Rare Bornean Snake Xenophidion acanthognathus (Xenophidiidae, Serpentes, Reptilia) and Confirmation as A Distinct Species from X. schaeferiRaffles Bulletin of Zoology. 68; 214–219.