AmphibiaWeb - Hynobius retardatus


(Translations may not be accurate.)

Hynobius retardatus Dunn, 1923
Hokkaido Salamander, Ezo Salamander, Ezo Sansho-uo (Japanese)
Subgenus: Satobius
family: Hynobiidae
subfamily: Hynobiinae
genus: Hynobius
Species Description: Dunn, E. R. 1923. New species of Hynobius from Japan. Proceedings of the California Academy of Sciences, 4th Series 12: 27–29.
Hynobius retardatus
© 2016 Henk Wallays (1 of 92)
Conservation Status (definitions)
IUCN Red List Status Account Least Concern (LC)
National Status None
Regional Status None
conservation needs Access Conservation Needs Assessment Report .


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Hynobius retardatus is a stocky, medium-to- large salamander; it has a snout-vent length of 60-85 mm and a total length of 115-200 mm. It has 11 costal grooves and its hindfeet have 5 toes. Its color is generally a patternless dark brown, but juveniles are liberally sprinkled with gold flecks. This gold speckling may be retained in the adults in many locations (Goris and Maeda 2004).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Japan

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View distribution map in BerkeleyMapper.
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Hynobius retardatus occurs from lowland to alpine zones in forests and grassland (IUCN 2006). It is found widely throughout Hokkaido (the second largest island in Japan), with the exception of the outlying islands. It is not found elsewhere (Goris and Maeda 2004).

Life History, Abundance, Activity, and Special Behaviors

Hynobius retardatus belongs to the family Hynobiidae, one of only two salamander families exhibiting external fertilization of eggs. This species is one of several where the males take part in a "scramble competition" for females (Hasumi 1994)[3756]. H. retardatus breeds from April to May in the lowlands, but may breed as late as July at higher altitudes. Breeding takes place mainly in still waters of permanent ponds or temporary puddles, but a small percentage of H. retardatus also breed in running water. When spring thaw begins, males leave their hibernacula and make their way to the breeding site. There they position themselves on twigs or other vegetation just below the surface of the water to wait for females. When a female arrives, she swims up to the twig, attaches one end of the egg sac to it, then releases her hold. A nearby male approaches and pushes her cloacal region with his hind legs while pulling out the egg sac with his forefeet. He is quickly joined in this by other males. Once freed of the egg sac, the female floats to the bottom and hides. The males fight over the egg sac, embracing it and ejecting sperm, so that the eggs are fertilized by many males (Goris and Maeda 2004). During the breeding phase, males also undergo a noticeable increase in head width (Hasumi and Iwasawa 1990; Hasumi 1994).

The egg sac generally forms a shapeless, transparent mass, although in some localities it is opaque. One clutch contains 60-150 eggs, which may hatch in 30-40 days. The larvae metamorphose 40-50 days later. In very cold regions, however, 2-3 years may elapse between egg laying and metamorphosis. The larvae often feed on the tadpoles of Rana pirica, and cannibalism is not uncommon when other prey items are scarce. Little is known of the habits of the juveniles and adults outside the breeding season (Goris and Maeda 2004).

Trends and Threats
In general, most of this species habitat is in good condition although populations near urban and suburban sites are threatened by modifications of wetlands, increasing road construction and other conversion of habitat to farming and housing. The IUCN also reports that increasing predation by (e.g., raccoons, Procyon lotor) or toxicity introduced species (e.g., Japanese toads, Bufo japonicus formosus), may be concerning for the future (IUCN 2020).

The name "Yezo Salamander" has also been used in the literature. "Ezo" (or "Yezo," the old pronunciation of Ezo) is a historical name for the island of Hokkaido (Goris and Maeda 2004).

This species was featured in News of the Week May 1, 2022:

Paedomorphosis, the retention of juvenile characteristics into adulthood, is well known in salamanders. In salamanders, paedomorphosis occurs via neoteny, the truncation of somatic development even as the gonads become mature. Extreme paedomorphic (obligatorily neotenic) forms typically look like larva and never transform, remaining aquatic throughout their lives. Perhaps the most famous of neotenic salamanders is the critically endangered Axolotl (Ambystoma mexicanum), known only from Lakes Xochimilco and Chalco in Central Mexico. This species has become an important model organism for developmental biology and is widely available in the pet trade, and thus has been distributed around the world. In Japan, the axolotl goes by the humorous name "Wooper Looper", ascribed to the species from a 1985 TV commercial that caught the fancy of the public. There is even a Wooper Looper Pokémon. Now Japan has its own Wooper Looper– a facultatively neotenic hynobiid salamander (the Ezo salamander, Hynobius retardatus). In this species, most individuals transform into terrestrial adults, but they are developmentally flexible and sometimes remain in the water, failing to transform while nevertheless achieving reproductive maturity. Neotenic adults were first collected in 1924 and observed again in 1932 in Lake Kuttara in Hokkaido, but neotenic adults were not seen again until Dr. Hisanori Okamiya rediscovered the phenomenon in the form of three specimens collected in a pond in south Hokkaido in December 2020 and April 2021 (Okamiya et al. 2021). Hynobius retardatus and Batrachuperus londongensis are the only hynobiid species (of 87 total species) that exhibit facultative neoteny. (Written by Jim McGuire)


Goris, R.C. and Maeda, N. (2004). Guide to the Amphibians and Reptiles of Japan. Krieger Publishing Company, Malabar, Florida.

Hasumi, M. (1994). ''Reproductive behavior of the salamander Hynobius nigrescens: monopoly of egg sacs during scramble competition.'' Journal of Herpetology, 28(2), 264-267.

Hasumi, M. and Iwasawa, H. (1990). ''Seasonal changes in body shape and mass in the salamander, Hynobius nigrescens J.'' Journal of Herpetology, 24(2), 113-118.

IUCN SSC Amphibian Specialist Group. 2021. Hynobius retardatus. The IUCN Red List of Threatened Species 2021: e.T59101A177215489. Accessed on 01 May 2022.

IUCN, Conservation International, and NatureServe. 2006. Global Amphibian Assessment. < >. Accessed on 28 November 2006.

Originally submitted by: Nichole Winters (first posted 2007-01-30)
Edited by: Kellie Whittaker, Michelle S. Koo (2022-05-01)

Species Account Citation: AmphibiaWeb 2022 Hynobius retardatus: Hokkaido Salamander <> University of California, Berkeley, CA, USA. Accessed Jul 21, 2024.

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Citation: AmphibiaWeb. 2024. <> University of California, Berkeley, CA, USA. Accessed 21 Jul 2024.

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