Ezo salamander

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.[5]

The Ezo salamander is distantly related to others of its genus.[6] A unique characteristic of this species is that individuals have only 20 chromosome pairs, a significantly smaller amount than the 28-29 pairs found in other salamanders of the Hynobius genus.[7]

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. [8]

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.[9]

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 taxonomic rank.[7] Morphological differences between H. retardatus individuals may be attributable to inducible defenses of their prey and the prevalence of cannibalism between larvae of the species.[10]

H. retardatus is an abundant endemic species on the island of Hokkaido, with a wide range across the island.[7] They are the northernmost species of the Hynobius genus in Japan.[3][7] Within Japan, populations have been found in the Iburi subprefecture,[11] Kayabe district,[3] the cities of Obihiro, Chitose,[7] Hakodate, and Kitami, as well as the towns of Erimo, Teshio, and the Nopporo Shinrin Kōen Prefectural Natural Park.[9]

Ezo salamanders can only live within lentic freshwater habitats, so their mobility is restricted, and separate populations show very low levels of gene flow. However, relatively high levels of gene flow have been recorded between the Nopporo and Kitami populations despite the mountain range separating them.[9]

Adults of the species feed on invertebrates like snails and worms. H. retardatus larvae are known to consume the tadpoles of multiple frog species such as the Japanese brown frog (Rana pirica)[10][11] and the Hokkaido frog,[4] as well as sewage worms (Tubifex),[12] gammaridean amphipods, caddisfly larvae,[11] and possibly larvae of Siberian salamanders.[13] Cannibalism by the larvae is not uncommon when prey items are scarce.[4] H. retardatus, like other salamander species, is a gape-limited predator that swallows its prey whole.[10]

Ezo salamander in shallow water

The breeding season of the Ezo salamander runs from April to May, with eggs hatching between early- and mid-May.[10][12][9] The species has a biphasic life cycle, with the first stage of life spent as aquatic larvae and the second, after metamorphosis, spent on land as a terrestrial adult.[11]

Ezo salamander in aquarium

H. retardatus possess a rare trait among amphibians where the transition from larval hemoglobin to the adult type occurs independently of thyroid activity and can occur for up to two years, well past the actual metamorphosis period. During early stages of development, the liver of H. retardatus is the main erythropoietic organ. The spleen then takes over further into development but before the hemoglobin transition. After the transition the spleen resumes its function as the main source of erythropoiesis, and it continues to do so throughout the life of an H. retardatus individual. The transition from juvenile to adult hemoglobin also occurs entirely within a single population of red blood cells, in contrast to the complete replacement of red blood cells from larval to adult type found in many other amphibian species. A similar method of hemoglobin switching has been seen in mice.[14]

Ezo salamanders are commonly sold through the internet as part of the pet trade. They are also endangered by the introduction of invasive species, as seen in the extinction of the Lake Kuttara population likely due to the introduction of salmon into the lake.[11] Raccoons, a non-native species introduced to Japan by humans, are known to prey upon H. retardatus.[15] Urbanization and modification of wetlands and similar habitats have caused population decline for Ezo salamanders living in habitats in close proximity to humans.