Amazon puffers from the Tocantins River drainage (formerly classified as Colomesus asellus) are now described as a separate species, Colomesus tocantinensis. However, it appears that both species are very similar in external appearance, and differentiating the two requires close examination of some minor anatomical details as well as slight differences in coloration.

Amaral CR, Brito PM, Silva DA, &Carvalho EF, 2013. A New Cryptic Species of South American Freshwater Pufferfish of the Genus Colomesus (Tetraodontidae), Based on Both Morphology and DNA Data. PLoS One, 8(9).

Open-access paper here.

Abstract: The Tetraodontidae are an Acanthomorpha fish family with circumglobal distribution composed of 189 species grouped in 19 genera, occurring in seas, estuaries, and rivers between the tropical and temperate regions. Of these, the genus Colomesus is confined to South America, with what have been up to now considered only two species. C. asellus is spread over the entire Amazon, Tocantins-Araguaia drainages, and coastal environments from the Amazon mouth to Venezuela, and is the only freshwater puffer on that continent. C. psittacus is found in coastal marine and brackish water environments from Cuba to the northern coast of South America as far south as to Sergipe in Brazil. In the present contribution we used morphological data along with molecular systematics techniques to investigate the phylogeny and phylogeography of the freshwater pufferfishes of the genus Colomesus. The molecular part is based on a cytochrome C oxidase subunit I dataset constructed from both previously published and newly determined sequences, obtained from specimens collected from three distinct localities in South America. Our results from both molecular and morphological approaches enable us to identify and describe a new Colomesus species from the Tocantins River. We also discuss aspects of the historical biogeography and phylogeography of the South American freshwater pufferfishes, suggesting that it could be more recent than previously expected.

Colomesus tocantinensis is diagnosed by the following features:

Colomesus species diagnosed by six to seven basal pterygiophores and nine rays in the anal fin (contra ten to eleven in both C. asellus and C. psittacus); ten basal pterygiophores and rays in the dorsal fin (contra eleven for both C. asellus and C. psittacus); the absence of dermal flaps across the chin (contra its presence uniquely in C. asellus); a caudal peduncle with eight vertebrae; and an opercle with a posterior ventral border subdivided in a ventral and a posterior region, the herein called "inverted V" shape (contra the triangular opercle exhibited by both C. asellus and C. psittacus).