Hynobius stejnegeri Dunn 1923

The body of Hynobius stejnegeri is slender. The snout vent length of H. stejnegeri ranges averages 59.8 mm in males, and 61.7 mm in females. The head of H. stejnegeri is moderately depressed, and is longer than it is broad. There are 13 costal grooves present (Matsui et al. 2008).The limbs of H. stejnegeri are short and stout. The forelimbs and hind limbs are separated by two costal folds when adpressed. The fifth toe is very short, compared to other species in the Hynobius genus (Matsui et al. 2008). The tail is shorter than the trunk. It is vertically oval at its base and middle, and its is not keeled, gradually flattening to a round tip. There is no small protuberance at the anterior tip of the vent (Matsui et al. 2008).Hynobius stejnegeri has a distinct dorsal markings, which differentiates it from H. boulengeri and H. katoi, both of whom have nearly monotonous, dark-brown dorsum. Compared to three taiwanese species, H. arisanensis, H. formosanus, and H. sonani, H. stejnegeri has a markedly shorter vomerine tooth series (Matsui et al. 2008).The other four Japanese species, H. kimurae, H. naevius, H. okiensis, and H. ikioi are more similar in appearance to H. stejnegeri. However, H. okiensis has a posteriorly compressed tail that is longer than that of H. stejnegeri. In males, H. okiensis is also equipped with a small protuberance at the anterior tip of the vent, whereas H. stejnegeri lacks such a protuberance. Finally, H. okiensis has long, fragile limbs, whereas H. stejnegeri has short and stout limbs. Hynobius kimurae also differs from H. stejnegeri through its coloration. Hynobius kimurae has clear yellow spots on the dorsum and no markings on its flanks and ventral side. However, H. stejnegeri has brownish-white mottling on its dorsum and white lateral and ventral markings. Hynobius naevius is most similar to H. stejnegeri. However, H. naevius is larger in body size, and has a longer and less cylindrical tail than H. stejnegeri. They can also be differentiated by the vomerine tooth series, as H. naevius has a less deeply curved series. The coloration also differs slightly. The dorsum of H. naevius is bluish-purple, and lacks a mottling pattern, as opposed to the mottled reddish purple dorsum of H. stejnegeri. Additionally, H. naevius has pale white markings on its lateral side, whereas the white markings on H. stejnegeri are much more vivid (Matsui et al. 2008). Hynobius stejnegeri has a decidedly smaller body than H. ikioi. Their coloration is also clearly different, as H. ikioi has orange-yellow blotches on a black dorsum, while H. stejnegeri has brownish-white markings or white dots, on a reddish-purple dorsum. Hynobius ikioi also has a better-developed fifth toe, compared to the much smaller fifth toe of H. stejnegeri. Hynobius ikioi also has a much longer tail than H. stejnegeri, although their medial tail width and height do not differ much. The tail of H. ikioi is also compressed posteriorly, unlike that of H. stejnegeri (Matsui et al. 2017). In life, the dorsum of H. stejnegeri is reddish-purple in color, and has brownish-white mottling or white dots. White markings are present on the lateral and ventral sides. In preservative, the color is faded all over. The dorsum is light brown, and has discontinuous brownish-white markings, which tend to fuse on the tail. The ground color on the ventral side is lighter brown with continuous white markings (Matsui et al 2008).There appears to be some slight sexual dimorphism in H. stejnegeri. Females tend to have a slightly larger snout vent length, a longer trunk, and more numerous upper and lower jaw teeth. Males have a relatively larger head and longer limbs and tails (Matsui et al. 2008).There is also slight variation among different populations. Chibu-Kinki and Kyushu individuals have a longer vomerine tooth series and relatively shorter tails than Shikoku individuals. Some Shikoku and Kyushu samples have well-developed toes compared to samples from other districts. Shikoku individuals have fewer jaw teeth than other districts compared to Kyushu samples, and fewer vomerine teeth than Chubu-Kinki samples (Matsui et al. 2008).The species authority is: Dunn, E. R. 1923. "New species of Hynobius from Japan." Proceedings of the California Academy of Sciences, 4th Series 12: 27–29.Phylogenetic analysis was done on the 12SrRNA and 16SrRNA genes of mtDNA using Bayesian inference. The analysis revealed differences among different species of Hynobius, with four species being especially close: Hynobius stejnegeri, H. kuishiensis, H. guttatus, and H. tsurugiensis. Each of the four species was monophyletic, and H. stejnegeri and H. kuishiensis formed a clade that is sister to the other two species: H. guttatus and H. tsurugiensis (Tominaga et al. 2019).Genetic analysis of the cyt b mitochondrial gene was done using Bayesian Inference on the other side of the Hynobius genus. This genetic analysis revealed that H. stejnegeri and H. naevius were sister to a clade formed by H. katoi and H. hirosei. These four species form a clade that is sister to H. shinichisatoi, H. osumiensis, H. amakusaensis, and H. ikioi (Nishikawa and Matsui et al. 2014). Hynobius stejnegeri was previously used to describe a species of Hynobius that was commonly called the “amber-colored salamander.” However, after a reevaluation of the genus in 2017, it was found that the amber-colored salamander did not actually match the description of H. stejnegeri that Dunn offered in 1923 (Matsui et al. 2017). As a result, a new name of H. ikioi was given to the amber-colored salamander. However in 1947, the name H. yatsui was given to the small-blotched salamander (Matsui et al. 2008), which was later revealed to be the species of this account. This posed a problem, since there were now two names that described the small-blotched salamander: H. stejnegeri and H. yatsui. The name H. yatsui was relegated as a subjective junior synonym of H. stejnegeri, so now the official name of the small-blotched salamander is indeed, H. stejnegeri (Matsui et al. 2017).

Hynobius stejnegeri, at the time of its description, was known from the western part of Japan: the Chubu-Kinki Districts, with the exception of the Kyoto and Hyogo Prefectures, the Shikoku District, and the Kyushu Districts, with the exception of the Saga and Nagasaki Prefectures (Sparreboom 2014).

The breeding season of H. stejnegeri is in early May in Chubu, and from late May to June in Shikoku (Matsui et al. 2008).Despite extensive surveys of the Kyushu Districts, few egg sacs and larvae have been found in nature. In the Shikoku and Chubu Districts, the eggs of H. stejnegeri are found in very small, underground streams, where larvae hatch and metamorphose without feeding. The clutch sizes of seven female individuals ranged from 7 to 19, with a median of 14. The diameters of the ova from the females ranged from 4.6 to 5.0 mm, with an average of 4.66 mm (Matsui et al. 2008). The ova in the females is unpigmented (Matsui et al. 2008).Hynobius stejnegeri occurs synoptically with H. naevius in northeastern to central Kyushu, with H. kimurae in Chubu and northern Kinki, with H. boulengeri in southern Kinki, Shikoku, and central Kyushu, and with H. ikioi from central to southwestern Kyushu. However, in all of these species, larval growth and oviposition occur in relatively wide, open montane streams, unlike in H. stejnegeri (Matsui et al. 2008).

Although H. stejnegeri is listed as "Least Concern" on the IUCN Red List because of its wide range and large population, the species is experiencing habitat loss in the southernmost part of its range (Matsui and Angelo 2009).

The amber salamander, amber-colored salamander, tortoiseshell salamander, or Stejneger's oriental salamander (Hynobius stejnegeri) is a species of salamander in the family Hynobiidae, endemic to Japan. Its natural habitats are temperate forests and rivers. It is threatened by habitat loss.