Species account

Dorsal parts reddish brown with irregular yellowish spots. Underside is lighter without markings.

Total length 12-13 cm (Goris & Maeda, 2004).

Diagnosis
Differs from H. naevius in following characteristics: Vomerine teeth series in U shape. Limbs moderately developed. When limbs are appressed fingers and toes overlap. Colour is red-brown with yellow spots (Thorn, 1969).

Eggs and larvae
There is no detailed description of the egg sacs. Larvae are abundant in the small tributaries of mountain brooks. Larvae may have claws, but these are poorly developed in larvae living in slow-moving rivulets (Goris & Maeda, 2004; but see Sato, 1940, in: Matsui et al., 2007, who notes absence of claws). Some larvae metamorphose and leave the water end August, early September, while others overwinter as larvae (Goris & Maeda, 2004).  

Distribution
The species only occurs on the island of Dogo in the Oki islands, Shimane Prefecture. (IUCN, 2010).

Habitat
The species occurs from sea level to the top of the mountains, inhabiting evergreen forest and coniferous plantations. It breeds in streams (IUCN, 2010).

Behaviour
In late autumn many adults gather in the vicinity of the headwaters of mountain brooks, where they overwinter. The breeding season of Hynobius okiensis is from end February to early April. It is thought that they breed in underground stretches of the brooks (Goris & Maeda, 2004).

Threats and conservation
This species is considered very vulnerable in view of its limited range. Due to habitat destruction, several populations have been extinguished. Conservation efforts are needed (Environment Agency of Japan, 2000). It is classified as critically endangered (Stuart et al., 2008; IUCN, 2010).

Observations in captivity
The species has been kept in captivity but there are no published reports.

Comments
Hynobius okiensis is a lotic-breeding species with characteristics of the lentic-breeding H. nebulosus group such as absence of claws on the tip of the digits in larvae, posteriorly compressed tail, long and fragile limbs, presence of a small protuberance at the anterior tip of the vent in the adult male, and diploid chromosome number of 2n=56 like all species of lentic-breeding Japanese Hynobius, instead of 58 chromosomes as in the lotic breeders (Sato, 1940; Matsui et al., 2007). Electrophoretic analysis shows that H. okiensis clusters with H. nebulosus and H. dunni and is more closely related to these species than to H. naevius or H. kimurae. This suggests that lotic-breeding H. okiensis shares a common ancestor with lentic-breeding H. nebulosus and H. dunni, and has secondarily acquired the habit of breeding in streams (Matsui et al., 2007).

 
Range of Hynobius okiensis
© 2010 IUCN, Conservation International & NatureServe. (click on map to enlarge)