Description
Chthonerpeton indistinctum are smooth-skinned caecilians that can reach up to 530 mm long, with an average width of 34 mm. The body width is greatest at the mid-length, followed by the width at the neck region, and is the thinnest at the rounded terminal tip. The head is dorso-ventrally compressed and widens posteriorly. The oval-shaped nostrils are oriented dorsolaterally and are closer to the snout than the eyes. The tentacle is between the eye and the snout. This species has four series of teeth, with no correlation between teeth number and length of the individual. The series of teeth are: premaxillary-maxillary (mode: 37 teeth), prevomeropalatine (mode: 24 teeth), dentary (mode: 26, 28, 30 teeth), and splenial (mode: 6 teeth). There are 70 - 81 primary annuli, which are incomplete at the dorsal and ventral midlines, and no secondary annuli. The vent is located in a depressed area on the posterior ventral surface (Gudynas et al. 1988).

This species may be confused for congenerics but can be easily differentiated via locality and primary annuli number. As of 2025, this is the only Chthonerpeton species to occur outside of Brazil other than C. onorei, which is only found in northeastern Ecuador. It is characterized by its stocky body, wide head, and superficial choanal valves. Many characters can be compared to differentiate C. indistinctum from similar species such as C. braestrupi, C. exile, C. noctinectes, and C. perissodus (Gudynas et al. 1988, da Silva et al. 2003). Specifically, C. indistinctum is distinguishable by its 82 - 86 vertebrae and 70 - 81 primary annuli, which are fewer than C. noctinectes (100 - 108 vertebrae, 94 -103 primary annuli), C. perissodus (103 - 108 vertebrae, 95 - 101 primary annuli), and C. exile (99 vertebrae, 88 primary annuli). Additionally, C. indistinctum has fewer premaxillary-maxillary teeth (37) than C. perissodus (41 - 46). Lastly, C. indistinctum’s dark coloration can be used to easily distinguish the species from C. braestrupi, which is bicolor with a dark dorsum and cream venter (Nussbaum and Wilkinson 1987).

In life, dorsal coloration of C. indistinctum is bright black, while the ventral coloration is black-gray. The entire body contains small white spots, which are thought to be mucous gland apertures. The tentacle and eye are additionally surrounded by a white coloration, which may be solid or diffused depending on the individual. The area surrounding the vent is cream-white (Gudynas and Williams 1992).

There is variation on an individual level with some individuals having solid versus diffused white coloration around the tentacle and eye. Additionally, sexual dimorphism has been observed with males having larger vents and body width at the cloaca (Gudynas et al. 1988).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Argentina, Brazil, Paraguay, Uruguay

	View distribution map in BerkeleyMapper.
	View Bd and Bsal data (15 records).

Chthonerpeton indistinctum is thought to be distributed in neotropical South America and has been found in Argentina, Brazil, Paraguay, and Uruguay. The species is associated with large rivers and river systems and is often found near river shores and muddy areas (da Silva et al. 2003).

Life History, Abundance, Activity, and Special Behaviors
This species is known for being semi-aquatic, nocturnal, and viviparous (Gudynas et al. 1988).

The gestation period for the species is thought to be four months, with a single female birthing 6 - 10 newborns. Copulation is in the austral spring, from August through September (Gudynas et al. 1988).

A study that revised the stomach contents in 17 specimens revealed insight into the diet, which includes arachnids, odonata larvae, and other amphibians. They are also known to have nematodes, likely parasitic, in their digestive tract (Gudynas et al. 1988).

Larva
This species is viviparous (da Silva et al. 2003).

Comments
As of 2025, the Chthonerpeton genus is comprised of nine species and is organized into two species-groups based on morphology, focusing on the number of vertebrae and primary annuli. These species-groups, which may be monophyletic sister groups, are the C. indistinctum group, which includes the titular C. indistinctum, C. braestrupi, C. exile, C. noctinectes, and C. perissodus, and the C. viviparum group, which is comprised by C. viviparum and C. onorei. The C. indistinctum group is associated with fewer vertebrae and primary annuli. Another Chthonerpeton species is C. arii, which has not been organized into a species-group due to its intermediate amount of vertebrae (da Silva et al. 2003). The newest species to be described in the genus is C. tremembe, which was described in 2015 and is ungrouped as well (Maciel et al. 2015).

References
da Silva, H. R., de Britto-Pereira, M. C., and Caramaschi, U. (2003). A new species of Chthonerpeton (Amphibia: Gymnophiona: Typhlonectidae) from Bahia, Brazil. Zootaxa, 381(1), 1–11. [link]

Gudynas, E., and Williams, J.D. (1992). Chthonerpeton indistinctum (Reinhardt and Lütken). Catalogue of American Amphibians and Reptiles. Society for the Study of Amphibians and Reptiles, 531.1-531.2. [link]

Gudynas, E., Williams, J. D., and de las Mercedes Azpelicueta, M. (1988). Morphology, ecology and biogeography of the South American caecilian Chthonerpeton indistinctum (Amphibia: Gymnophiona: Typhlonectidae). Zoologische Mededelingen, 62(2), 5–28. [link]

Maciel, A. O., Leite, J. M., Leite, R. R., Leite, J. R., and Cascon, P. (2015). A new species of Chthonerpeton Peters 1880 (Amphibia: Gymnophiona: Typhlonectidae) from the State of Piauí, Northeastern Brazil. Journal of Herpetology, 49(2), 308–313. [link]

Nussbaum, R. A., and Wilkinson, M. (1987). Two new species of Chthonerpeton (Amphibia: Gymnophiona: Typhlonectidae) from Brazil. Occasional Papers of the Museum of Zoology, University of Michigan, 716, 1–15. [link]

Species Account Citation: AmphibiaWeb 2025 Chthonerpeton indistinctum: Argentine caecilian <https://amphibiaweb.org/species/1953> University of California, Berkeley, CA, USA. Accessed May 13, 2025.

Citation: AmphibiaWeb. 2025. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 13 May 2025.

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