To deter predators, Japanese fire-bellied newts contain high levels of tetrodotoxin, a neurotoxin accumulated mainly from their diet.

The Japanese fire-bellied newt first diverged from its closest relative in the Middle Miocene, before splitting into four distinct varieties, each with a mostly separate range, although all four are formally recognized as composing a single species.

The species was first scientifically described by German zoologist Heinrich Boie in 1826 as Molge pyrrhogaster,[note 1] based on specimens brought from Japan to Europe.

[7][2] A study of mitochondrial DNA in 2001 indicated that its supposed fellow members of Cynops, C. cyanurus and C. wolterstorffi, may belong to a different genus.

[12] In particular, the validity of the Sasayama and intermediate races has never been proven, with one study finding no behavioral differences between the two supposed forms.

Cynops pyrrhogaster diverged from its close relative C. ensicauda about 13.75 million years ago (Mya; during the Middle Miocene).

The common ancestor of the two species would have lived in an area of the Eurasian mainland which is today the East China Sea and the central Ryukyu Islands.

The study that identified them concluded that the four clades represent separate taxonomic units, although their exact relationship is unclear.

[14] On the newt's upper body, the skin is dark brown, approaching black, and covered in wartlike bumps.

[17] Cynops pyrrhogaster is endemic to Japan, being found on several islands in the archipelago, including Honshu, Shikoku, and Kyushu.

Ecosystems they are found in include forests, grasslands, shrublands, wetlands, lakes, marshes, and cultivated environments.

The male then deposits two to four spermatophores, one at a time, moving several centimeters after each, which the female attempts to pick up with its cloaca, sometimes unsuccessfully.

[24] Females lay eggs separately on underwater objects, such as leaves and submerged grass roots, fertilized one by one from the spermatophores they carry.

[20] Cynops pyrrhogaster is regarded as an ideal vertebrate model for investigating the mechanism(s) involved in the transition from mitosis to meiosis during spermatogenesis.

[15] Adults at one particular sub-alpine moor in the Azuma Mountains of Fukushima Prefecture were found to like both live prey and carrion.

They consume many insect varieties, such as members of Odonata, which include dragonflies and damselflies, whose larvae have been found whole in newt stomachs, but only pieces of adults; Brachycera, a suborder of Diptera (flies); Hymenoptera, which include sawflies, wasps, bees, and ants; and Coleoptera (beetles).

In Mainland Japan the newts must also avoid mammalian predators, which cannot distinguish colors as well as avian hunters.

This behavior difference is likely because newts from the mainland are adapted to escape from mammalian hunters, which are less likely to be repelled by such a display.

[29] Wild Japanese fire-bellied newts contain high levels of the neurotoxin tetrodotoxin (TTX).

[30] This toxin inhibits the activity of sodium channels in most vertebrates, discouraging predation by both birds and mammals.

When raised in captivity with no source of TTX, 36- to 70-week-old juveniles did not contain detectable levels, but wild specimens from the same original habitat had high toxicity.

In younger captive-reared newts some TTX was still detected, which was inferred to have been transferred by adult females to their eggs.

[1] Japanese fire-bellied newts with mysterious skin lesions at Lake Biwa in Japan's Shiga Prefecture were found to be suffering from infections caused by a single-celled eukaryote in the order Dermocystida.

There is concern that this affliction could spread to other nearby species, including Zhangixalus arboreus and Hynobius vandenburghi.

Then, in 2016, a trio of researchers discovered that newts on the Chita Peninsula were very likely the same variant due to their similar morphological traits.

Both groups share a preference for cooler temperature and have smooth and soft bodies, pale dorsal regions, and yellowish undersides.

[34] Japanese fire-bellied newts serve as a highly useful model organism in laboratory settings, but they become more difficult to care for after metamorphosis.

[25] Japanese fire-bellied newts produce motilin, a peptide that stimulates gastrointestinal contractions, identified in many vertebrates.

[35] This species, as well as other Urodele amphibians, is capable of regrowing missing body parts, including limbs with functional joints and the lower jaw.

The difference in time is purely due to the size of the eye, and regenerative ability does not change; the discovery of this fact contradicted a popular claim that juvenile animals are quicker to regenerate than adults.