Description
Hynobius amakusaensis has a medium-sized body with male snout-vent length ranging from 64.8 - 81.8 mm and female snout-vent length ranging from 71.5 - 83.7 mm, and their means being 75.4 and 76.5 respectively. The species has a moderately depressed oval head that is distinctly longer than it is wide. The rounded snout projects slightly past the lower jaw. Their nostrils are close to their snout tip, which does not have a labial fold. The eye is large with a well-developed upper eyelid. They have 12 - 14 costal grooves between the axilla and groin. They have short, yet robust limbs; adpressed limbs separated by three costal folds. The relative length of their fingers is I < IV < II < III and their toes are I < V < II < IV < III, with a well-developed fifth toe. The cloaca is a longitudinal slit that lacks a genital tubercle on the anterior cloaca. Finally, their tail is short and thick, slightly cylindrical at the base with a well-developed dorsal fin posteriorly, and a rounded tip from the lateral view (Nishikawa and Matsui 2014).

In their late larvae stage, H. amakusaensis has an average snout-vent length of 23.4 ± 1.4 m. Their head is a rounded trapezoidal shape with a short snout, and they have robust limbs with claws present on the fingers and toes. They have a moderately pointed tail tip. Their external gills are well developed (Nishikawa et al. 2003, Nishikawa and Matsui 2014).

DIAGNOSIS:

Hynobius amakusaensis is a medium-sized species in the lotic breeding genus, Hynobius, and is most similar to H. naevius. The main morphological differences between H. amakusaensis and all other lotic breeding Hynobius can be seen mainly through the numerous white dots on the body. This trait is only shared by H. yatsui and H. naevius, with the difference in H. amakusaensis being the dots are fine in size and dull in color without forming a pattern. Hynobius amakusaensis does not have a silvery mottling pattern, unlike H. yatsui and H. naevius. In males, H. amakusaensis overlaps in size with H. naevius (means of 75.4 and 75.1 mm, respectively) but is significantly larger than H. yatsui (58.0 mm mean). Although H. amakusaensis and H. naevius overlap in size, H. amakusaensis has a greater separation of the fore- and hind limbs, shorter limbs and toes, a shorter, wider tail, and longer vomerine tooth series when compared to H. naevius (Nishikawa and Matsui 2014).

COLORATION:

In life, adult H. amakusaensis are uniformly light purplish brown on their dorsal side, with slightly lighter coloration on their ventral side. They have small, dense, white dots on the whole body, which are most evident on the lateral and ventral sides, limbs, and tail. Their iris is a dark brown without any significant markings. In preservation their dorsal coloration fades to a duller grey brown, but otherwise there is no noticeable change (Nishikawa and Matsui 2014).

In life, the larvae have a dark brown dorsum with a beige dorsal stripe, and large black spots on the lateral sides of the body, tail, and tail fins. They also have smaller golden dots on the tail fin. As with the adults, in preservative, their dorsal coloration fades and their golden dots fade to white (Nishikawa and Matsui 2014).

VARIATION:

Variation in H. amakusaensis was identified from 22 adult males and 12 adult females. Females are slightly larger than males, but the difference was statistically insignificant. Males have significantly longer heads, snouts, and upper eyelids than females. Males also have a wider head, internarial distance, and a longer, wider, and higher tail. However, females have a longer trunk. Females also have a significantly greater separation of adpressed limbs (2.5 - 4.0 folds) than males (1.5 - 3.5 folds). The final key variation between the sexes is that the development of a tail fin is only found in breeding males. The number of costal grooves did not differ between males and females, with grooves varying from 12 to 14 but most often being 13. The sexes also do not show any variation in the shape of their vomerine tooth series. They have 17 or 18 presacral vertebrates, with adults often having 17 while the juveniles have 18. The juveniles also seem to have a denser distribution of white spots when compared to the adults (Nishikawa and Matsui 2014).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Japan

View distribution map in BerkeleyMapper.

Hynobius amakusaensis are found on Kamishima Island of the Amakusa Islands, Kumamoto Prefecture, Japan. They have been found at altitudes between 410 and 430 meters above sea level (Nishikawa and Matsui 2014).

Life History, Abundance, Activity, and Special Behaviors
During the day, H. amakusaensis can be found in the leaf litter and gravel along small montane streams (Nishikawa et al. 2003).

Hynobius amakusaensis is a part of the lotic breeding genus Hynobius, meaning that they breed in fast moving fresh water such as rivers and streams. During breeding season, adults are found underwater in local streams, while metamorphs are found under stones or debris near the streams. Larvae can be found at night between early summer and autumn. Most larvae undergo metamorphosis in the autumn after the breeding season, however it can take up to two years. (Nishikawa and Matsui 2014).

Males develop a tail fin only during the mating season (Nishikawa and Matsui 2014).

Fertilization is external (Nishikawa and Matsui 2014).

During the breeding season, one captive female Amakusa salamander laid 19 eggs, with a mean diameter of 5.4 ± 0.3 mm. Both vegetal and animal poles were cream in color, with a short-coiled egg sac and no indication of a whiptail structure (Nishikawa and Matsui 2014).

Although, as of 2021, there is no specific diet information for H. amakusaensis, another lotic breeding salamander of the same genus, H. shinichisatoi, had its stomach contents examined. It contained a variety of invertebrates, including earthworms and arthropods. Predators of the Hynobius genus in Korea often include crayfish, leeches, invertebrates, and loaches. They typically feed on the eggs of amphibians, while many other fish have difficulties removing the jelly coat of salamander eggs (Bae et al. 2019).

Trends and Threats
At the time of the species description, H. amakusaensis was listed on the Japanese Red Data Book under its previous name H. boulengeri and, through that name, was under protection by the Kumamoto Prefecture. However, further protections are needed (Nishikawa and Matsui 2014).

As of 2021, the species has an IUCN Red List status of “Critically Endangered” because of its small range of less than 100 km², continued habitat loss and degradation, over-harvesting for personal collection, and from negative interactions with native wild boar populations (IUCN 2021).

Relation to Humans
Breeding adults of this species are over-harvested for personal collections by hobbyists, but as of 2021 were not found in the pet trade (IUCN 2021).

Possible reasons for amphibian decline

General habitat alteration and loss
Subtle changes to necessary specialized habitat
Loss of genetic diversity from small population phenomena

Comments
Bayesian analysis of cytb mtDNA sequences indicate that H. amakusaensis belongs to a clade that also includes H. osumiensis, H. shinichisatoi and H. stejnegeri with H. amakusaensis being sister to H. stejnegeri. However allozyme analyses indicate that H. amakusaensis is more closely related to Osumi Penisula Hynobius than to H. stejnegeri (Nishikawa and Matsui 2014)

Hynobius amakusaensis was previously classified as a subspecies of H. boulengeri (Nishikawa and Matsui 2014).

The species epithet, “amakusaensis” was derived from the Amakusa Islands, which includes the island of Kamishima where the species is found (Nishikawa and Matsui 2014).

The common name is Amakusa-sanshouo (Nishikawa and Matsui, 2014).

With the addition of H. amakusaensis and two other species; Kyushu had nine total lotic breeding Hynobius species, which is a much larger number than any other similar sized areas in Japan. These species have very small distributional ranges, which put them at a higher threat risk to habitat loss and degradation (Nishikawa and Matsui 2014).

References

Bae, Y., Kong, S., Yi, Y., Jang, Y., Borzée, A. (2019). "Additional threat to Hynobius salamander eggs: predation by loaches (Misgurnus sp.) in agricultural wetlands." Animal Biology, 69(4), 451-461 [link]

IUCN SSC Amphibian Specialist Group. 2021. "Hynobius amakusaensis." The IUCN Red List of Threatened Species 2021: e.T79075007A177019904. Downloaded on 30 September 2021.

Nishikawa, K., Matsui, M. (2014). ''Three new species of the salamander genus Hynobius (Amphibia, Urodela, Hynobiidae) from Kyushu, Japan.'' Zootaxa, 3852(2), 203-226.

Nishikawa, K., Matsui, M., Tanabe, S., Sakamoto, M. (2003). "Occurrence of a lotic breeding Hynobius salamander (Amphibia, Urodela) on Kamishima of the Amakusa Islands, Japan." Current Herpetology, 22(1), 1-8. [link]

Species Account Citation: AmphibiaWeb 2021 Hynobius amakusaensis: Amakusa-sanshouo <https://amphibiaweb.org/species/8218> University of California, Berkeley, CA, USA. Accessed Apr 20, 2024.

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 20 Apr 2024.

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