Description
Hynobius unisacculus males have a snout-vent length range of 38.31 – 60.26 mm and females have a range of 37.48 – 59.93 mm. The head is a partially depressed ovoid, and the base of the head is not distinctive from the neck. The overall head size is somewhat small, with a head length to snout-vent length ratio of about 0.25. The head is distinctly narrow, with a total head length to head width ratio of 1.64. Hynobius unisacculus has a snout that is short and thick, with a snout length to the total head length ratio of 0.38. The nostrils are side-facing, non-protruding, and are placed centrally between the snout-tip and the eye corners. The eyes are small, with a ratio of eye diameter to total head length of 0.21. The eye length, as well as the diameter, is rather small when compared to the snout length with a ratio of 0.71. The eyelids are present and prominent, and the eyes are distinctly extended from the body when viewed from the side, and slightly so from the dorsal view. There are 13 - 21 vomerine teeth. Hynobius unisacculus has a notable gular fold, no labial grooves, no paratoid glands, but has a bloated paratoid area. The subparotid groove begins above its jaw, and extends past the head, curving ventrally at the base of the head, but is not very prominent (Min et al. 2016).

The main trunk is elongated, and possesses 11 - 12 prominently developed costal grooves that are visible from the ventral side as well. There is a weak mid-dorsal groove on the dorsum that extends from the base of the head to the base of the tail. Males have swollen cloacas with a longitudinal vent during the breeding season. The tail is notably shorter than the total body, with tail to the snout-vent length ratio of 0.77. The tail is laterally compressed for the majority of its length, with the first fifth being uncompressed (similar to body height) and then distinctly flat for the last two-fifths until reaching the dorsal fin. The tip of the tail is acute, with the widest point being located at the base of the tail. The limbs are short and thin, and the forelimbs are slightly shorter and less powerful than the hindlimbs. There are no cornified structures on the limbs. When adpressed, the digits of each limb do not touch and the limbs are separated by 1.5 - 3 costal folds. Relative finger lengths are IV < I < III < II and relative toe lengths are V < I < II < IV < III, and there is no webbing between digits. The tips of the toes are rounded. The tubercles on the palms and pes are not developed. The skin on the dorsum and ventrum is smooth, slimy, and scattered with diffuse microscopic glands over the whole body. The ventral abdominal vein appears through the translucent skin (Min et al. 2016).

Species of the genus Hynobius are grouped into three discrete groups based on morphology, phylogenetics, chromosome structure, and life history traits. Hynobius unisacculus is morphologically and genetically most similar to lentic-breeding species from the Korean Peninsula, which include, H. leechii, H. quelpaertensis, and H. yangi. The focal species can be differentiated from H. leechii from the mainland by H. unisacculus having a shorter tail, a shorter head, a longer body, and by having more than one costal fold gap between adpressed limbs. Populations of the H. leechii complex found on islands near H. unisacculus’ range should have their taxonomic statuses investigated. The focal species can be differentiated from H. quelpaertensis by H. unisacculus having a shorter tail, shorter limbs that do not touch or overlap when adpressed, a longer body, and fewer vomerine teeth. The focal species is similar to H. yangi in size and body proportions, but can be differentiated by H. unisacculus having a longer body, shorter limbs that do not touch or overlap when adpressed, and slightly deeper and more curved vomerine tooth series. From an undescribed sister linage, coined Hynobius clade 1 that is found in the coastal and lowland areas of southern Gyeongsangnam-do Province, the focal species differs by having more costal folds, a shorter tail, shorter hindlimbs, a smaller head, a shorter eye to snout distance, and limbs that do not overlap or touch when adpressed. Other lentic-breeding Hynobius can be differentiated based on geographic range, coloration, morphological characters, and genetics. Lastly, H. unisacculus is unique in producing egg sacs that are not attached to substrates by gelatinous stalks (Min et al. 2016).

In life, the species’ dorsal coloration has a dark brown background and indistinct olive to tan to dark brown markings. Yellowish-brown, bronze, or copper speckles have been observed on the trunk, head, and limbs of some specimens. Some specimens also have light silvery-grey, or whitish-blue speckles on the lateral sides of the tail, trunk, and head, but these markings are more common in sub-adults and juveniles. In individuals with light-colored speckles, the markings become bronze and tend to morph together on the tail, but do not form any large blotches or bands. The sides of the trunk and belly are lighter than the dorsum, varying from light beige to purple-gray to grey. The coloration lightens more to slightly purplish or pinkish on the ventrum. The skin on the ventrum is translucent and the bluish ventral abdominal vein can be seen through the skin. The throat, chest, ventral sides of the limbs and first anterior third of the tail are pinkish. The ventral surface of the posterior two-thirds of the tail is brownish. Bluish-gray speckles have also been observed on posterior sides of the tail. The irises have black markings on a dark bronze background. In preservative, the coloration on the dorsum fades to light brown or brownish-grey and coloration on the ventrum fades to yellowish-grey. However markings remain the same if not more distinct (Min et al. 2016).

All individuals appear relatively similar in morphology and proportions. However, there is variation in the number of costal grooves, 11-12. Additionally there is variation in the number and type of speckles on individuals that may be age related. In males, the longitudinal vent slit that breaks into a y-shape, with grooves running from the end of the primary slit. Females have a simpler longitudinal cloacal slit. Lastly, males have higher tail-fins that females (Min et al. 2016).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Korea, Republic of

View distribution map in BerkeleyMapper.

The species has been found in four areas within South Korea – Giheung, Suncheon, Bosoeung, and Yeosu areas on the mainland. Besides these rural mainland areas, populations been found on human-populated islands off the southern coast of Korea in the Jeollanam-do area, including Naro Islands (Oenaro-do Island). When breeding they can be found in lentic water bodies including ditches and rice-paddies. When not breeding, adults stay in forest areas, generally resting in general forest litter (Min et al. 2016).

Life History, Abundance, Activity, and Special Behaviors
The breeding season for H. unisacculus is from late February to late March. During that time, adults move to still water, often man-made ditches or rice-paddies and can be found during the day, hiding in dead leaves that they use for breeding. The cloacal areas of males are notably swollen during the breeding period (Min et al. 2016).

Females lay two small (less than 150 mm), mucous egg sacs, one per oviduct, that are in the shapes of either Cs or Os. The sacs have a transparent envelope, have many irregular folds, and contain 17- 88 eggs. The eggs, themselves, are pigmented and arranged in one or two rows. Hynobius unisacculus lays the egg-sacs in pairs that are initially attached to substrate via mucous stalks. The stalks are particularly weak and often break during the egg laying process. This results in single egg sacs being found freely deposited in the body of water, not attached to any particular substrate. The unattached egg characteristic of breeding is non-typical in the Hynobius genus (Min et al. 2016).

Individuals leave after reproduction in April to return to the surrounding forest where they hide in forest litter or other shelters. Metamorphosis occurs during the latter half of the summer (Min et al. 2016).

Trends and Threats
The species is subject to habitat destruction via economic growth in Southern Korea. As agricultural areas begin to dissipate, the breeding grounds of the species shrink. Other areas are subject to forest fragmentation. Chytridiomycosis is a threat to all Korean amphibians (Min et al. 2016).

Given the above threats and the apparent narrow range of H. unisacculus, the species is recommended for an IUCN Red List category of “Vulnerable (Vu2a)” (Min et al. 2016).

Relation to Humans
The species often use irrigation ditches for agriculture as breeding pools (Min et al. 2016).

Possible reasons for amphibian decline

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

Comments
The species authority is: Min M.S., Baek, H.J., Song, J.Y., Chang, M.H., Poyarkov Jr, N.A. (2016). A New Species of Salamander of the genus Hynobius (Amphibia, Caudata, Hynobiidae) from South Korea. Zootaxa. 4169(3):475-503

Based on Bayesian Inference and Maximum Likelihood analyses of the mtDNA genes, cyt b (partial, 907 base pairs), COI (1,097 base pairs), and 12S rRNA (788 base pairs) using H. unisacculus and other Korean hynobiids, the focal species was revealed to be sister to the clade formed by a yet unnamed species (at H. unisacculus’ description called Hynobius clade 1). The combined clade of H. unisacculus and Hynobius clade 1 are sister to H. quelpaertensis (Min et al. 2016).

The species epithet, “unisacculus”, means “single” (uni) “small sac” (sacculus) in Latin and refers to the single small egg sac the species makes during breeding (Min et al. 2016).

References

Min M.S., Baek, H.J., Song, J.Y., Chang, M.H., Poyarkov Jr, N.A. (2016). ''A New Species of Salamander of the genus Hynobius (Amphibia, Caudata, Hynobiidae) from South Korea. .'' Zootaxa, 4169(3), 475-503.

Species Account Citation: AmphibiaWeb 2017 Hynobius unisacculus: Korean small salamander; Kkoma-Dorongnyong (Korean) <https://amphibiaweb.org/species/8521> University of California, Berkeley, CA, USA. Accessed Mar 20, 2023.

Citation: AmphibiaWeb. 2023. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 20 Mar 2023.

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