Species account

Onychodactylus japonicus is a slender, elongate salamander. 13 to 14 costal grooves. Tail slightly longer than snout-vent length. Small elongated head with prominent eyes and moderately developed paratoids. The skin is smooth.

Males and females show a number of morphometric differences. Males are on average longer than females; they have a relatively longer tail and longer forelegs (Akita, 1985, in Kuzmin, 1995). Unique among salamanders is a distinct sexual character, consisting of a hypertrophied lateral fold on the posterior surface of the male hindlegs, extending from the fifth toe to the tibia. This feature, which effectively extends the surface of the hindlegs, is most prominent during the reproductive period. Tailtip of the female is more pointed than that of the male. The male cloaca is V-shaped, the female cloaca is more rounded. In the breeding season, the male cloaca is somewhat swollen (Sato, 1943; Kuzmin, 1995). Both sexes have black asperities on the inner surface of the hindlegs, the males also on the forelegs.

Colour is brown or grey-brown, with dark dots. A dorsal stripe extends over the body and tail, sometimes divided into large spots. In the western part of the range, the stripe may also be vermillion to orange, contrasting with the sides (Sato, 1943; Goris & Maeda, 2004).

Total length of adult animals 15 to 19 cm in males, 13,5 to 18 cm in females (Akita, 1985).

Diagnosis
Onychodactylus japonicus is closely related to and very similar to O. fischeri, but differs from that species in dorsal colour pattern. Dorsal band in O. fischeri is less bright (Dunn, 1923). External sexual differences are manifested to a higher degree in O. japonicus than in O. fischeri (Kuzmin, 1995) and the number of costal grooves in O. japonicus (13-14) appears to be lower on average than in O. fischeri (14-15) (Dunn, 1923).

Two paired egg sacs, elliptical in shape and joined together at the distal end are attached to a stone by a mucous stalk. Many different sacs may be found attached to a single stone. Mean clutch size is 11-24 eggs, egg diameter 4.5-5.4 mm. Size of the egg sac may differ per site and depends on female size and perhaps time of deposition (Kuzmin, 1995 and references therein). The maximum number of ovarian eggs (36) was observed in the largest female (17 cm) (Akita, 1985, in Kuzmin 1995).

Hatchlings apparently stay at the oviposition sites at head waters for several months, then migrate downstream and begin to feed. Iwasawa & Kera (1980, in Kuzmin, 1995) describe the stages in the normal development of egg and larva. Claws appear at a total length of 25 mm after 142 days of development. Larvae begin metamorphosis and go on land in the fourth summer of their lives at a total length of about 8 cm (Hayase & Yamane, 1982).

The species is endemic to Japan and is widely distributed on the islands of Honshu and Shikoku.

Habitat
Onychodactylus japonicus is a typical high altitude species. It occurs at average altitudes from 300 to 2000 m, but can also be found at lower elevations, down to 20 m. It inhabits moist, cool and shady places covered by mixed forests, with streams, moss-covered rocks and stony ground. Dense woodlands near small rivers and lakes with clear and cool water with temperatures ranging from 9 to 12˚C, usually of northern exposure, are preferred habitats. The animals remain close to springs and waterfalls and are found under rocks, stones, fallen trees and logs (Kuzmin, 1995).

The salamander is a locally abundant species. It feeds on a variable diet of aquatic, but mainly terrestrial invertebrates (Kuzmin, 1995). During spring or September-December migrations, high concentrations of salamanders may be found, for instance on Hodatsu mountain, Ishikawa prefecture (Akita & Miyazaki, 1991, in Kuzmin, 1995). Breeding period, depending on altitude and latitude, may start mid-May and last to end of June (Thorn, 1969), or April to June and July to early August in the northern part of its range (Ohno, 1981, in Kuzmin, 1995). There may even be two breeding periods per year, in spring and in autumn, at a single site. Males are assumed to be biennial breeders and females probably triennial (Akita & Miyazaki, 1991, in Kuzmin, 1995). A breeding site on Hodatsu mountain was described in detail by Akita. Breeding occurred in the underground part of a stream. Pairs of egg sacs were attached to stones in the inside of a small underground cavity where cold springs emerged from under stones to form the headwater of a mountain brook. (Akita, 1982, 1983, in Kuzmin, 1995). For reproduction the salamander is strongly dependent on cool, permanent water and availability of hiding places. At such places, a high density of clutches may be observed.

Egg deposition and fertilization have not been observed, but are probably similar to what was observed in O. fischeri (Serbinova & Solkin, 1995).

The species is widely distributed throughout the country, including several protected areas, and not uncommon. It is not included in the Red Data Book of Japan. Locally salamanders are collected during their migrations, sold for medicinal purposes or served as a delicacy in restaurants.

Successful experiments have been conducted to breed O. japonicus with the help of hormone injections (references in Kuzmin, 1995).

Many references to Japanese literature are given in Kuzmin (1995), who extensively quotes papers on ecology and reproduction of this species by Y. Akita.