Description
Onychodactylus sillanus is a medium-sized hynobiid salamander with a slender body. It has a snout-vent length of about 54.8 mm. However, of the four specimens used to describe this species, one was a juvenile and the other four were subadults. Their 18 - 22 vomerine teeth are arranged in short arch-shaped series connected with each other. The outer branches of their vomerine teeth are slightly longer than the inner branches. They possess 11 - 12 costal grooves. Their tails are longer than the lengths of its head and body combined, and seem cylindrical at the base. The tails are also slightly compressed laterally at the distal end. Breeding males have dermal flaps on the posterior edges of their hind limbs. The larval and adult forms of Onychodactylus sillanus have black claw-like keratinous structures on both of their fore- and hind-limbs (Borzee et al. 2022).

Onychodactylus sillanus has features that distinguish it from other members of the genus. For instance, they differ from O. koreanus as the focal species has about 11 costal grooves compared to 12 - 13 in O. koreanus. Onychodactylus sillanus also has 18 - 22 vomerine teeth in contrast to 10 - 18 of the latter. While the inner branch of vomerine teeth are curved forward at the medial end in O. sillanus, they is not curved in O. koreanus. They also differ from O. zhaoermii by having an average of 20 vomerine teeth compared to less than 16 per series. Furthermore, they differ by marking patterns. In O. sillanus, there are light dorsal markings in forms of blotches and spots as well as light ocelli. On the other hand, O. zhaoermii does not have light ocelli and their light dorsum markings show a reticulated pattern. Lastly, they are overall more closely related to O. koreanus in terms of number of teeth, but closer to O. zhaoermii in tooth shape (Borzee et al. 2022).

Males in breeding condition have dermal flaps attached to the posterior edges of their hind limbs (Borzee et al. 2022).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Korea, Republic of

View distribution map in BerkeleyMapper.

Onychodactylus sillanus was first recorded in Unmun Mountain as O. fischeri. Its geographic range was estimated to be between 258.3km² to 792.7km² in high elevation regions in South Gyeongsang Province based on ecological niche modeling. However, the lower end of the range is expected to decrease as these maps exhibited a sharp decline in suitable habitat for the species; models that gave a prediction for the future showed an even greater decrease (Borzee et al. 2022).

Larva
Skinfolds on the posterior edges of fore- and hind limbs of O. sillanus larvae were observed. Furthermore, the larvae of O. sillanus exhibit contrasting limb lengths from those of O. koreanus(Borzee et al. 2022). Juveniles have dark ventral trunks with large yellowish blotches on their dorsum and tails (Borzee et al. 2022).

Trends and Threats
According to different scenarios created based on the trends of climate change, scientists have estimated that there would be a 87.6% to 97.3% decline in suitable habitat within the following three generations. Thus, O. sillanus has been considered “Critically Endangered”. Specifically, the population inhabiting southern Gyeongsang is highly threatened by habitat loss (Borzee et al. 2022).

Comments
Based on Maximum Likelihood and Bayesian analyses on fragments of 16S rRNA, cyt b, and COI mitochondrial DNA on the O. koreanus group, O. sillanus was concluded to be phylogenetically closest to , O. koreanus, and an undescribed species of Onychodactylus found in Gangwon Province. The latter two and O. koreanus form a monophyletic group (Borzee et al. 2022).

The species epithet, “sillanus”, was derived from Silla, a Korean kingdom that ruled south-eastern Korea, which overlapped with the geographic range of the species (Borzee et al. 2022).

References
Borzée A, Shin Y, Poyarkov NA, Jeon JY, Baek HJ, Lee CH, An J, Hong YJ and Min M. 2022. Dwindling in the mountains: description of a critically endangered and microendemic Onychodactylus species (Amphibia, Hynobiidae) from the Korean Peninsula. Zoological Research 43(5): 750-755. [link]

Species Account Citation: AmphibiaWeb 2023 Onychodactylus sillanus: Yangsan Clawed Salamander <https://amphibiaweb.org/species/9646> University of California, Berkeley, CA, USA. Accessed Apr 18, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 18 Apr 2024.

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