The Ezo salamander (Hynobius retardatus), also known as the Hokkaido salamander, Noboribetsu salamander, or Ezo Sanshouo in Japanese is a species of salamander in the family Hynobiidae. Prior to 1923 the species was also classified by the binomial scientific name Satobius retardatus. H. retardatus is endemic to Japan's northernmost prefecture, the island of Hokkaido. The species is a pond-type salamander, and its natural habitats are temperate forests, temperate shrubland, swamps, freshwater marshes, intermittent freshwater marshes, freshwater springs, irrigated land, canals, and ditches. In 1923, the species was discovered by American herpetologist Emmett Reid Dunn, the same year the Hida and Amber salamander species were discovered. H. retardatus was first observed on the eastern edge of the Noboribetsu region within Lake Kuttara (Kuttara-ko), although this specific population is now extinct. The name Ezo comes from Yezo, the old name of Hokkaido. Description Adult Ezo salamanders can reach 115–200 mm(4.52-7.87 inch) in length. Hindlimbs of the terrestrial adults have five digits. Adults are dark brown in coloration and aquatic juveniles are a similar dark brown sprinkled with golden flecks across the body. The Ezo salamander is distantly related to others of its genus. A unique characteristic of this species is that individuals posses only 20 chromosome pairs, a significantly smaller amount than the 28-29 pairs found in other salamanders of the Hynobius genus. Habitat and Distribution The Ezo Salamander lives in alpine forests and grasslands only on the island of Hokkaido, and is not found on any of the surrounding islands. The Ezo salamander likely migrated to the island of Hokkaido from continental Eurasia via land bridges. These bridges connected the two regions until the end of the last glacial period, approximately 10,000 years ago. The divergence of H. retardatus vastly predates this migration, occurring between 14 and 18 million years ago. This time frame encompasses parts of the Miocene, Pliocene, and Pleistocene epochs, where the Japanese archipelago first rose from the Pacific Ocean. Population Structure, Speciation, and Phylogeny Geographically separated populations of H. retardatus showcase significant differences in morphology, with some individuals of the Obihiro-shi population in eastern Hokkaido displaying larger adult body size as well as larger egg sac size and diameter. These differences in size have compelled some researchers to tentatively separate the species into “large” and “common” morphs. Despite the visual morphological difference between large and common types, they are not genetically distinct enough to be separated by