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.

This time frame encompasses parts of the Miocene, Pliocene, and Pleistocene epochs, where the Japanese archipelago first rose from the Pacific Ocean.

[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.

[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.

Some Japanese brown frog tadpoles (R. pirica) have adapted thicker bodies to reduce predation by H. retardatus larvae (as they are gape-limited).

[10] H. retardatus larvae artificially fed exclusively conspecific prey were found to more likely develop into the cannibalistic morph when subjected to high-density conditions.

For larvae fed sewage worms (Tubifex)--a heterospecific prey—under the high-density conditions, head width increased, but body size remained unchanged.

[3] The population of H. retardatus found within Lake Kuttara exhibited paedomorphosis, a mechanism where adults maintain larval features even after metamorphosing and reaching sexual maturity.

These H. retardatus individuals were sexually mature but morphologically similar to the larvae of the species, with features such as external gills and caudal fins.

The three individuals were 14% smaller than metamorphosed males when comparing snout-vent lengths and 2.26 times larger than the aquatic larvae of the species.

Two of the three males also exhibited three small external gill ramus pairs and vomerine teeth arranged in an inverted “V” as seen on H. retardatus larvae.