Description
Hynobius oni is a large salamander described from 12 males and one female with a male snout-vent length range of 73.6 - 87.5 mm and a female snout-vent length of 84.6 mm. The large head is oval-shaped, longer than it is wide, and somewhat depressed. The round snout extends beyond the lower jaw with the nostrils near the snout tip. There is no labial fold. The eyes are somewhat inset from the edge of the head in the dorsal view, and are large and protruded. They have well-developed eyelids that are shorter than the snout. The distinct gular fold curves anteriorly and the parotid gland is evident, sitting between the jaw angle and the gular fold. The postorbital grooves are also distinct. One runs anteriorly from the posterior jaw angle to the lower jaw, while the other is longer and runs posteriorly from the posterior jaw angle to the parotid gland. The vomerine teeth are wider than they are long and V-shaped, with an anterior margin distal to the choanae, and the broad tongue is free from the floor of the mouth on both sides. There are 13 costal grooves between the axilla and groin, and adpressed limbs are separated by two costal folds. The forelimbs and hind limbs are long and thick. The relative finger lengths are I < IV < III < II and the relative toe lengths are I < V < II < IV < III with a well-developed fifth toe. The cloaca has a longitudinal slit and no genital tubercle on the anterior. The tail is short and thick, becoming increasingly compressed posteriorly from its cylindrical base to the tail tip, which is rounded in lateral view (Kanamori et al. 2022).

DIAGNOSIS:

Within the Hynobius genus, H. oni resembles H. boulengeri, H. katoi, H. shinichisatoi, H. osumiensis, and H. hirosei due to the uniform dorsal color. Hynobius oni has a smaller body size than H. boulengeri (mean male snout-vent length in H. oni is 79.4 mm while H. boulengeri is 93.9 mm) and H. hirosei (85.5 mm), while it is larger than H. katoi (58.4 mm) and H. osumiensis (68.4 mm). Compared to H. shinichisatoi, H. oni has a shorter tail length (mean tail length of H. oni is 80.4% of snout-vent length while H. shinichisatoi is 90.4%) and shallower vomerine teeth (vomerine teeth width in H. oni is 1.6 mm and 1.2 mm in H. shinichisatoi). Compared to H. hirosei, in addition to a smaller body size, H. oni has a longer axilla-groin distance, shorter tail length, shorter internarial distance, longer upper eyelid length, and larger medial tail width. Based on mtDNA, H. oni is genetically closer to H. sematonotos than H. hirosei. However, H. oni does not have any large body markings and has a longer snout-vent length, while H. sematonotos has silvery spots and a shorter snout-vent length (Kanamori et al. 2022).

COLORATION:

In life, the adult dorsum is uniformly dark reddish brown and immaculate with no markings. The ventral side is lighter than the dorsum and the underside of the tail is somewhat ochre. The iris is a solid dark brown. In preservatives, there are no obvious changes to the coloration besides the dorsal side fading and becoming gray-brown (Kanamori et al. 2022).

VARIATION:

All specimens have generally similar body size and proportions. The dark reddish-brown adult dorsal color may be less red or darker. Young individuals have white dots scattered on the dorsum. The one female paratype, with a snout-vent length of 84.6 mm, is as large as the male holotype, with a snout-vent length of 82.7 mm. However, the female’s upper eyelid width is smaller than that of the twelve male specimens (3.1% of the snout-vent length for the female and 3.2 - 3.8% in males). The female paratype’s tail is also lower and thinner (Kanamori et al. 2022).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Japan

View distribution map in BerkeleyMapper.

Hynobius oni is only known to occur from the Onigajo and Sasayama Mountains in the Ehime and Kochi prefectures on the island of Shikoku in western Japan. They are found occupying areas around mountain streams with partially-exposed bedrock. Specifically, they inhabit deciduous broad-leaved forests, though they may also be artificial or mixed forest, at elevations from 650 to 1,700 m (Kanamori et al. 2022).

Life History, Abundance, Activity, and Special Behaviors
Hynobius oni is lotic-breeding and breeds in montane streams. Out of the breeding season, adults and juveniles were found under stones, rotten wood, and debris near the stream (Kanamori et al. 2022).

The breeding season is assumed to be in late April as egg sacs have been found under rocks in streams in late April. The ova are large and pigmentless. The long, crescent-shaped egg sacs have a distinct whiptail structure on the free end. The animal and vegetal poles are both cream in color, with eggs forming either a single or double row in each sac. The envelope of the egg sac is thicker than that of other congeners, but thinner than that of H. boulengeri, and of similar thickness to H. naevius, H. oyamai, and H. sematonotos. Hynobius oni has a small clutch size with a range of 16 - 36 eggs (Kanamori et al. 2022).

Larva
For one larva at the onset of metamorphosis in late July, the snout-vent length was 20.6 mm. The head is rounded in dorsal and lateral views with a short, broadly rounded snout. The eyes are inset from the edge of the head in dorsal view and slightly protruded. The labial fold is distinct at the posterior of the upper jaw. The external gills are developed. The caudal fin is higher than the head with the dorsal fin beginning at distal half to three-fourths of the trunk and the ventral fin starting from the vent. The dorsal fin is higher than ventral fin. The tail tip is slightly pointed. The limbs are slender, with fingers and toes that lack claws (Kanamori et al. 2022).

The larvae are most similar to those of H. hirosei, but have a smaller body size, longer axilla-groin distance, shorter tail length, shorter internarial distance, longer upper eyelid length, and larger medial tail width (Kanamori et al. 2022).

The dorsum is light brown with small, dark-brown dots and blotches in life. The venter is whitish and transparent and golden dots are scattered on the tail fin. In preservative, the dorsal coloration usually fades to a light brown and the golden dots fade to white (Kanamori et al. 2022).

Little is known about the larval life history of H. oni, however the density of overwintered larvae was found to be lower than that of H. hirosei (0.25 per square meter for H. oni and 10 per sq m for H. hirosei). Metamorphosis occurs in late July, and H. oni larvae might have a smaller metamorphosing size than H. hirosei. Hynobius oni were not observed in sympatry with other hynobiid salamanders (Kanamori et al. 2022).

Trends and Threats
Hynobius oni occupies a small habitat range, which has been degraded. Private over-collecting can also threaten natural populations. Thus the conservation status of H. oni should be "Endangered". (Kanamori et al. 2022).

Relation to Humans
Hynobius oni might be collected for private purposes, similar to Onychodactylus tsukubaensis in the Ibaraki prefecture (Kanamori et al. 2022).

Possible reasons for amphibian decline

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

Comments
Maximum Likelihood analysis of cyt B from the H. hirosei complex showed that there are three genetic groups: Tsurugi, Central, and Nanyo. Based on this analysis H. oni was split from the H. hirosei Central group and is sister to the H. hirosei Tsurugi group. The analysis also found the Tsurugi and Central groups were distributed parapatrically, having a hybrid zone between them, while the Nanyo group was distributed allopatrically from the other groups and was morphologically distinguishable. The Tsurugi and Central groups could not be divided in this study, while the Nanyo group is described as H. oni (Kanamori et al. 2022).

ETYMOLOGY: (Origin or explanation of scientific name)

References

Kanamori S, Nishikawa K, Matsui M, Tanabe S. (2022). "A new species of lotic breeding salamander (Amphibia, Caudata, Hynobiidae) from Shikoku, Japan." PeerJ, 10, e13891. [link]

Species Account Citation: AmphibiaWeb 2022 Hynobius oni: Nanyo salamander <https://amphibiaweb.org/species/9564> University of California, Berkeley, CA, USA. Accessed Apr 23, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 23 Apr 2024.

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