Hynobius sumidai Sugawara, Naito, Iwata, and Nagano, 2022

Hynobius nebulosus (part, as Hiroshima morphotype) Okawa and Utsunomiya, 1989, 142.

Hynobius nebulosus (part, as Aki morphotype) Okawa et al., 2005, 11.

Hynobius akiensis (part) Matsui et al., 2019, 32.

Holotype: HMNH-AM-102, from Tamaricho, Takehara-shi, Hiroshima Prefecture.

Paratype: KPM-NFA 945, from the same locality of the holotype.

Specimens referred in the present paper. Hiroshima Prefecture: Takehara-shi, KUHE 64274, from Nika-cho, collected by Y. Tomimori on 2 May 2022. KUHE 20220610 _1, 20220610_3, from Nika-cho, collected by K. Sakamoto on 22 April 2021. KUHE 61404-61406, from Tamari-cho, collected by Y. Tomimori on 4 Jury 2019. Higashihiroshima-shi, KUHE nb8-7, from the former Kurose-cho (Date of collection and collector unknown). KUHE 8682, 8683, 8685, from the former Kurose-cho, collected by M. Matsui on 10 March 1986. U 103–104, from former Kurose-cho, collected by T. Utsunomiya on 24 March 1986. KUHE OU 0297– 0299, from former Saijocho, collected by H. Okawa on 28 March 2007; KUHE OU 0300, from former Happonmatsu-cho, collected by H. Okawa on 28 March 2007; KUHE OU0318, 0319, from the former Akitsu-cho, collected by H. Okawa on 13 May 2007. KUHE OU 0340–0342, from former Happonmatsu-cho, collected by H. Okawa on 28 March 2008. Kure-shi U0842–U0845, from Gohara-cho, collected by T. Utsunomiya on 22 March 1990.

Diagnosis. A small species of the genus Hynobius that breeds in hilly wetlands (adult male SVL 36.2–56.9 mm). Phylogenetically, H. sumidai forms a clade with H. geiyoensis in mitochondrial and nuclear DNA, but has a smaller SVL. than the latter. The dorsum of an adult dark brown and yellow stripes on upper and lower edges of tail often indistinct. Short limbs and tail; forelimb and hindlimb tips barely meeting (overlap of -2 to 0 costal folds in males). The fifth toe undeveloped. Egg sac short, loosely coiled into a string, with slightly fewer clutch size than in other lentic breeding species of the genus. Most similar to H. akiensis, but with shorter internarial distance (RIND 5.7% vs. 6.3%), interorbital distance (RIOD 6.0% vs. 6.4%), forelimb (RFLL 23.6% vs. 25.3%), and hindlimb (RHLL 29.5% vs. 30.8%).

Color. In life, dorsum pale brown with minute black spots. Small white spots on ventral side, and yellowish brown stripe on dorsal edge of tail often indistinct, less distinct on ventral side. In preservative, ventral color tends to be faded to grayish-brown and white nuptial color on throat not clear.

Variation. Morphological measurement data for males are summarized in Table 2. In 19 males examined, SVL 47.6 ± 5.2 (36.2–56.9) mm, tips of adpressed limbs separated or touching, the fifth toe generally weakly developed, with few individuals having only four toes (about 5.0% of the specimens examined). Body light brown with black spots, and some individuals with silvery white spots on dark brown background.

Range. The species is known only in the low hills of southern area in the middle region of Hiroshima. Higashihiroshima-shi (including the former Saijo-cho, Kurose-cho, Fukutomi-cho, Toyosaka-cho, Akitsu-cho, and Happonmatsu-cho), Takehara-shi, and Kure-shi (Fig. 1). Altitude of the localities extends from 160.0– 455.0 m with the mean±SD of 296.3± 121.1 m (n=11).

Natural history. Breeding usually occurs in hilly streams, wetlands, and paddy ditches from March to April. Egg sacs are string-like in shape and coiling. number of eggs is 26 ± 9.5 (3–67) (Average ± SD [range]) (Okawa et al. 2019).

Morphological Comparisons. Hynobius sumidai has a yellowish-brown body color with small black dots on the dorsal side, a body color pattern that is often seen in lentic Hynobius (e.g., H. geiyoensis, H. akiensis, H. setouchi, H. utsunomiyaorum, H. abuensis, H. iwami, H. bakan, H. setoi, and H. kunibiki). However, H. iwami, H. bakan, H. setoi, and H. kunibiki can be distinguished from H. sumidai by distinct yellow stripes on dorsal and ventral sides of tail, while H. sumidai has an indistinct yellow stripe on dorsal side, if present. In addition, this species differs from H. iwami, which always lacks the fifth toe. Hynobius sumidai (mean male SVL 48.0 mm) is distinctly smaller than H. geiyoensis (60.4 mm) and H. abuensis (57.3 mm). Hynobius sumidai is distinguishable from the genetically related H. geiyoensis by the following ratio values: longer head, shorter interorbital, larger eyes, shorter trunk and tail, narrower medial tail, higher basal tail, shorter hindlimb and fifth toes, and wider vomerine teeth series. Hynobius sumidai significantly differs from H. akiensis in the following ratio values: shorter internarial, interorbital, forelimb, and hindlimb. Hynobius sumidai significantly differs from H. setouchi in the following ratio values: shorter snout, internarial, interorbital, tail, forelimb, hindlimb, third toe, and fifth toe. Hynobius sumidai is distinguishable from H. utsunomiyaorum by its significantly smaller SVL and following ratio values: larger head, longer lower jaw, larger eyes, narrower medial tail, higher basal tail, longer third finger and fifth toe, and larger vomerine teeth series.

Protection: H. sumidai is in the Japanese Red List as Endangered (EN) under the name of H. akiensis (Ministry of the Environment Government of Japan 2020) and is listed as Vulnerable (VU) by the Red Data List Hiroshima (Okawa [2021] as H. akiensis). This species is presently protected by Higashihiroshima-shi, Hiroshima Prefecture.