Onychodactylus koreanus is a medium-sized, slender hynobiid salamander with a snout-vent length range of 36.0 – 77.9 mm for males and 41.8 – 78.6 in females. The head is longer than wide and reasonably depressed, attached to a slender long neck. In the dorsal view the snout short and rounded with a gradual slope from eyes to nostrils. The dorsolaterally oriented nostrils are small, not protuberant, and positioned widely and distant from snout tip. The protuberant eyes are very large and close together with well-developed eyelids. The salamander has indistinct parotid glands. There is a distinct longitudinal postorbital groove from the posterior corner of eye towards the corner of the mouth, but does not reach the mouth. The cylindrical body is elongated and slender with a narrow pectoral region. The skin on the dorsum and venter are smooth and slimy, with many tiny granular glands particularly on the dorsum. There is a middorsal groove present that extends from the base of the head to the base of the tail. Along the sides of the body 12 - 14 costal grooves are clearly visible. The cloaca is elongate with Y-shaped longitudinal vent. The long tail begins cylindrically, then becomes slightly laterally compressed in the middle third of its length and distinctly laterally compressed in the last third. The limbs are slender and well-developed. The hindlimbs are more distinctly robust and a bit longer than the forelimbs. The four fingers and five toes do not meet when forelimb and hind limb are adpressed against the flank. There are no distinct palmar or plantar tubercles are present on palms or feet. Tips of digits are rounded with distinct black cornified claws that slightly curved and pointed at the ends. The fifth toe has a large fleshy skinfold that begins at the distal phalanx and extends along the posterior edge of the hindlimb, which are more pronounced in breeding males (Poyarkov et al. 2012).
Onychodactylus koreanus can be differentiated from the similar salamanders in the region, including a sympatric Hynobius species, by its slender body, long tail with almost cylindrical base that flattens lateral towards the end, smooth skin and distinct costal grooves, and small and elongated head with rounded snout. Onychodactylus koreanus also possesses morphological characteristics unique to Onychodactylus including absence of lungs, dermal flaps on the hindlimbs of breeding males, presence of black horny claws on digit tips of both sexes during the breeding season; two short transverse arch-shaped rows of vomerine teeth, premaxillary fontanelle, large fully separating nasals, caudal ribs present on more than 5 – 6 postsacral tail vertebrae. Onychodactylus koreanus can be specifically differentiated from Karsenia koreana, sympatric plethodontid salamander, by absence of nasolabial groove, the lateral compression of the tail towards the end, and larger body size. From other Onychodactylus, O. koreanus differs in the average number of trunk vertebrae (19), costal grooves (13), and typical coloration of juveniles and adults — dark ground color with numerous bright, small round spots and/or ocelli and no distinct dorsal band (Poyarkov et al. 2012).
Larvae have square-shaped heads with a rounded snout that are wider than the body. The eyes are slightly prominent. Like adults they have cone-shaped sharp, horny claws. Larvae possess a fold of skin along the outer edge of fore- and hind limbs that disappears before metamorphosis. The short gills are covered at the base with an opercular skin fold. They have low tail fins that start 1/3 of the length below the cloaca on the ventral side of the tail. The dorsal tail fin higher than the ventral fin (Poyarkov et al. 2012).
In life, the dorsum is background color is blackish-brown to dark-chocolate brown, becoming dark lavender-brown on the flanks and tail and dark reddish brown on the dorsal surface of head and tail. Dorsal limb ground coloring is slightly lighter than the dorsal background coloration. Numerous golden yellow to golden orange spots and ocelli are present on the dorsal side of the body, head, and tail. Spots vary in size from very small on the head to noticeably large and slightly longitudinally elongated ocelli with black centers on the scapular area and longitudinally elongated and sometimes merging spots on the dorsal side of tail and hindlimbs. The sides of tail and body have a lavender to pinkish tint and are obviously lighter than the dorsum with a gray-beige or reddish-gray color. The costal segments are slightly lighter than the costal groves. The lateral sides of the head have a background color of pinkish-beige to light orange-beige. The venter background color is dark pinkish-gray, or lavender-gray and slightly darker on the chest, but still lighter than the dorsum. The skin on the ventrum is semitransparent. The cloacal area the same color as the ventral side of the tail, gray-beige with a slight pinkish hue. The ventral sides of the limbs are more lightly colored than the venter and range from pinkish-gray or orange-beige. The eyes are noticeably lighter than the dorsal head background color. The area around the eye, including the upper eyelid is gray-bluish to grayish-violet. Iris in life is golden-brown with tiny orange and yellow spots. Upper and lower marginal sides of the iris are lighter and brighter than the medial part surrounding the pupil. The pupil is horizontal and oval- shaped. The cornified claws are black. In preservative, the bright colors tend to fade and disappear; the base color of the dorsal surfaces usually fades to dark gray or brownish; the pale spots fade to a creamy white, and the dark brown is slightly less warm in tone, turning grayish-brown. Background color of premetamorphic larvae usually dark-brown or gray-brown dorsally and cream-colored or beige ventrally (Poyarkov et al. 2012).
Onychodactylus koreanus have unique dorsal pattern of small light spots and ocelli. There may be seasonal changes in color intensity, with the terrestrial phase being duller than during the breeding coloration. While, there is no known variation in coloration between the sexes, O. koreanus has morphometric sexual variation with, male O. koreanus having significantly longer tails, longer and narrower heads , and larger eyes than females (Poyarkov et al. 2012).
The total chromosome number (2n) is 78 and was described from a specimen reported as O. fischeri from Korea. The chromosomal formula is as follows: 6M + 6SM + 8ST + 10T + 48a. The estimated size of the O. koreanus genome is 102.5–103.5 pg per diploid nucleus (Poyarkov et al. 2012).
The species authority is: Poyarkov, N. A., Che, J., Min, M.-S., Kuro-o M., Yan, F., Li, C., Iizuka, K., Vieites, D. R. (2012). "Review of the systematics, morphology and distribution of Asian Clawed Salamanders, genus Onychodactylus (Amphibia, Caudata: Hynobiidae), with the description of four new species." Zootaxa, 3465, 1-106.
Phylogenetic analyses suggest that Onychodactylus koreanus is more distantly related to is sister to O. fischeri from Russia, from which it was split. Instead O. koreanus is more closely related to Onychodactylus populations from Liaoning Province of China and to species inhabiting Japanese islands. The koreanus species complex, includes O. koreanus and its sister taxon, O. zhaoermii, from Liaoning. There are two distinct lineages of O. koreanus, highlighting the species’ cryptic diversity. Those lineages include lineage D is found in the mountain regions throughout the country and lineage E from an isolated mountain area of Yangsan of Gyeongsangnam-do in the southwest region of the peninsula. These lineages were consistently recovered as reasonably divergent and monophyletic (p-distances of 5.6 to 5.8% in COI and 1.9% in 16S rRNA; Poyarkov et al. 2012).
Onychodactylus koreanus can be found with Hynobius species (H. leechii complex), brown frogs of the genus Rana (Rana coreana, and Rana dybowskii), and has been found under stones together with the plethodontid salamander, Karsenia koreana (Poyarkov et al. 2012).
The specific name koreanus refers to the Korean peninsula inhabited by this species (Poyarkov et al. 2012).
Onychodactylus koreanus has a wide distribution across the Korean peninsula. It is locally common in some areas but scarce in other parts of the range. The species has only been documented from South Korea, where it can be found in mountainous areas at elevations of 10 – 1915 m of the Korean peninsula, excluding Jeju Island and offshore islands of Korea. Populations in the southern part of the Korean peninsula may be isolated from the more northern primary range. The species can likely also be found in nearby mountains of North Korea, but no known specimens or collections have been made in the area (Poyarkov et al. 2012).
Like other Onychodactylus species, O. koreanus can be found in cool, moist, and shady places in mountain forests. This species is usually found in pine trees or deciduous forest streams with clean, fast-running water, but it has also been found in caves. They prefer streambeds with numerous logs, moss-covered stones, rocks, and pebbles. It appears that humidity and water temperatures are important habitat characteristics. The species is found in streams with water temperatures that range from 6 – 14°C. Onychodactylus koreanus hibernate from approximately late October to April, when temperatures are below to 3 – 5°C (Poyarkov et al. 2012).
Breeding typically takes place in underground streams. Breeding males and females aggregate together in male biased groups (estimates of 20 to 1 ratios). Onychodactylus koreanus females lay two small, white, elliptical, paired egg sacs with semitransparent walls. Females attach egg clutches to submerged rocks and stones by a mucous-like stalk while males congregate and attempt to grasp the egg sac before oviposition is completed forming a mating ball for access to the egg sacs. Many sacs may be found attached to a single stone (Poyarkov et al. 2012).
Larvae appear in brooks at lengths of 30 - 40 mm and remain there for two or three years, metamorphosing in late summer or early autumn and leaving the water (Poyarkov et al. 2012).
Adults feed on a variety of invertrbrates, which include dragonflies (Odonata), millipedes (Diplopoda), caddisflies (Trichoptera), bees (Apoidea), spiders (Arachnida), and earthworms (Lumbricidae; Poyarkov et al. 2012).
Because of Onychodactylus koreanus’ distributional range, with presumably large populations and its presence in some protected areas it has been recommended for Least Concern status by the authors who described it (Poyarkov et al. 2012). Onychodactylus koreanus is currently not listed in IUCN.