Description
Hynobius fossigenus is a large salamander. Adult H. fossigenus males have a snout-vent length range of 66.0 – 80.9 mm while the female range is larger, approximately 76.2 – 82.5 mm. The large, elliptical head is longer than it is wide, and it is slightly dorso-ventrally compressed. The snout is wide and rounded. The small, circular nostrils are placed laterally on the snout. The eyes are widely spaced. The upper eyelids are distinct, and the eyes laterally protrude. The parotoid glands are visible and the neck is distinct. The tubiform body is slightly narrowed at the chest. The skin is smooth. There is a faint indent of a dorsal groove that runs from the head to the tip of the tail. There are 12 - 13 costal grooves (typically 13) on either side of the body and 11 visible costal grooves on the ventral surface. The cloaca is slightly bulbous with a lengthwise vent. The tail is slightly shorter than the body length, and it narrows toward the tail tip. The forelimbs are shorter and less brawny than the hind limbs. The forelimbs have four fingers, with digits II and III longer than digits I and IV. There is a visibly flattened sore-like palmar tubercle on both the left and right sides of the palm. The hind limbs have five toes, with a descending length order of III > IV > II > V > I. At the base of toes I and V, two flat, rounded metatarsal tubercles can be observed. All digits are unwebbed, and all fingertips and toe tips are rounded. The digits are stout (Okimiya et al. 2018).

Hynobius fossigenus is most similar morphologically to other Japanese lotic Hynobius species, but can be diagnosed based on its size and breeding behaviors. Hynobius fossigenus is large, and its size allows it to be distinguished from the Taiwanese congeners H. fucus, H. formosanus, H. glacialis, H. arisanensis, and H. sonani. Hynobius fossigenus breeds in mountain streams like other lotic-breeding Hynobius species, but they lay a small clutch of 12 to 16 yellow-colored eggs. Egg sacs have a crescent or loosely spiraled shape with a smooth envelope that has a vivid blue-violet iridescent hue in water and terminate in a whip-like tail; this egg sac morphology is only observed for members of the H. kimurae-H. boulengeri species complex. Many other Hynobius species have completely dark dorsal colorations, occasionally with minimal light markings. Hynobius fossigenus, however, has a blackish-violet dorsum with tiny yellow spots, making it easily distinguishable from most other lotic species. Hynobius kimurae is the sister species of H. fossigenus, but the males and females of the latter species are consistently larger than those of H. kimurae. Vomerine teeth in H. fossigenus form a narrow U-shape, and are composed of 43 - 66 teeth in both sexes; H. kimurae, however, has long V-shaped teeth (Okimiya et al. 2018).

In life, the dorsum is dark violet-brown to black. The ventral side is paler than the dorsal side. There are distinct golden spots of varying sizes speckled across the dorsum from the head to the tail. Spots are smaller in size and fewer in number on the limbs and tail. The ventral surface lacks spots. The irises are dark brown. The vent is gray in hue and tinged with blue (Okimiya et al. 2018).

In preservation, Hynobius fossigenus did not exhibit many color changes. The dorsal markings became a beige color, and the pink tint of the underside faded to light gray (Okimiya et al. 2018).

Males are smaller than females, with heads that are shorter and wider. Males have tails that are relatively longer and thicker, and a shorter trunk length. Males have longer forelimbs than females. Some males may have 12 costal grooves instead of 13. Males’ cloacal areas are noticeably more swollen than females’ during the breeding season. Variation also exists in the number of golden spots on the dorsum (Okimiya et al. 2018).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Japan

View distribution map in BerkeleyMapper.

Hynobius fossigenus are found in the Gunma, Saitama, Tokyo, Kanagawa prefectures of the Kanto District of Japan. They are also reported to be seen in the Yamanashi, Shizuoka, Nagano, and northeastern Aichi Prefectures of the Chubu District. There appear to be at least five, likely isolated, geographic populations occurring around mountains; these populations appear to be isolated from each other. Although most abundantly found in elevations of 400 - 900 m above sea level, they are recorded at elevations from 300 - 1100 m above sea level. Hynobius fossigenus are found at the headwaters of small mountain streams, usually under leaf litter, rocks, stones, or decomposed logs (Okimiya et al. 2018).

Life History, Abundance, Activity, and Special Behaviors
Adults are found under leaves, stones, debris, or decomposed logs close to breeding streams (Okimiya et al. 2018). Hynobius fossigenus reproduce in cold (below 20°C), adequately oxygenated headwaters of mountain streams that are small in size and are typically located in evergreen forests or mixed forests. Breeding streams are usually less than 1.5 m wide and 20 - 30 cm deep. Adults begin gathering by the headwaters in November and are found hiding under partly submerged rocks or stones. Breeding begins in December and ends in April. The peak months for egg laying are early February and the middle of March when water temperatures range from 5.5 - 7.0°C (Okimiya et al. 2018).

Hynobius fossigenus reproduce underwater, beneath stones in the stream. After copulation, females lay crescent or loosely spiraled shape egg sacs in pairs that terminate in a whip-like tail and attach them under big stones. They may also attach them onto smaller stones that are close to a waterfall. The egg sacs are very thick and appear iridescent when in water. Egg sacs contain12 to 16 yellow-colored eggs. While females leave the streams after oviposition, males linger in the streams and can often be found underneath the same stone with egg clutches (Okimiya et al. 2018).

Embryonic development lasts about 60 days, and larvae hatch between April to mid-May. Hynobius fossigenus adolescents remain in their larval form throughout the winter months. Many larvae in the Tokyo population metamorphose towards the end of May and early in June of the following year after overwintering the last stages of their development. Older larvae have been observed to cannibalize younger larvae (Okimiya et al. 2018). Larvae feed on larval caddis flies and mayflies, amphipods, and sometimes prey on conspecifics (Okimiya et al. 2018).

After undergoing complete metamorphosis, H. fossigenus prey on insects and other smaller invertebrates like spiders and earthworms. Males take at least 5 years to mature in the Tokyo population, while females require at least 7 years (Okimiya et al. 2018).

Larva
When the larvae are at Akita larval stage 57, the head appears dorso-ventrally compressed. The dorsal tail fin begins at the middle of the total length. The end of the tail tapers, and the maximum tail height is at the middle of the tail’s length. The external gills are visible, and no eyelids can be seen (Okimiya et al. 2018).

In life, larval H. fossigenus have a golden-brown dorsum and are covered with multiple dark brown spots. The ventral surface is pale surface and has no spots (Okimiya et al. 2018).

Trends and Threats
Currently, H. fossigenus is locally abundant, but there may be isolated populations that are negatively impacted by habitat destruction and other human activity and influence. In general, Hynobius fossigenus is not currently under any protections and is categorized as "Least Concern" by IUCN, but in the Gunma prefecture, H. fossigenus is protected as its sister species, H. kimurae (Okimiya et al. 2018).

Possible reasons for amphibian decline

General habitat alteration and loss
Habitat modification from deforestation, or logging related activities

Comments

Hynobius fossigenus is a sister species to H. kimurae, and both are members of the H. kimurae–H. boulengeri species complex. Phylogenetic analyses – using both Maximum Likelihood estimates and Bayesian Inference – of 16S rRNA, tRNA^(Glu), and cyt b, mitochondrial gene fragments and RAG1 nuclear gene fragments was used to support the monophyly of this species complex. The observed level of genetic divergence between H. fossigenus and other lotic Hynobius points to a species level of differentiation. It’s estimated that the split between H. fossigenus and its sister species, H. kimurae , was approximately 5.2 million years ago. The next closest species is H. boulengeri (Okimiya et al. 2018).

The species epithet, "fossigenus", is derived from the Latin word “fossa” which means “pit” or “hollow”, and “-genus” meaning “born in;” this alludes to the distribution of the species, as they are located on either side of the Itoigawa-Shizuoka Tectonic Line, Fossa Magna’s western boundary. It is thought that the uplifting of the Central Highlands in the Fossa Magna area in the early Pliocene resulted in the geographic split between H. fossigenus and H. kimurae (Okimiya et al. 2018).

In Japanese, the common name is Higashi-hida Sanshouuo (Okimiya et al. 2018).

References

Okimiya, H., Sugawara, H., Nagano, M., Poyarkov, N. A. (2018). "An integrative taxonomic analysis reveals a new species of lotic Hynobius salamander from Japan." PeerJ [link]

Species Account Citation: AmphibiaWeb 2022 Hynobius fossigenus: Japanese Rift Salamander <https://amphibiaweb.org/species/8868> University of California, Berkeley, CA, USA. Accessed Jan 14, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 14 Jan 2025.

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