Retroculus spp. are unsuitable for the standard community aquarium, and should be maintained in a set-up designed to resemble a flowing river. The substrate should be composed of soft sand mixed with small pebbles if the aim is to breed the fish (see ‘Reproduction’). Rocks, boulders, roots, branches, and aquatic plants can also be added, although the la…

This species can be distinguished from all congeners except P. bachi and some specimens of P. furcata by possessing only faint spots and saddles on the body.

It can be told apart from P. bachi by possession of narrow (vs. wide) pelvic-fin spines, having the eyes positioned high (vs. low) on the head, and distinctly round (vs. appearing more as a mottling) spots on the body.

Described from ‘Igarapé Candiru-Mirim, Rio Capim, Pará, Brazil’, a relatively short river draining into the rio Pará near the city of Belem, eastern Brazil.

Records of similar-looking fish exist from Santarém, Pará state, in the central Amazon region, and the coastal city of Fortaleza, Ceará state, much further southeast. These may require confirmation.

R. vulpinus is the most widely-distributed member of the family Cynodontidae.

Type locality is given simply as ‘Brazilian rivers’, but this species is currently understood to be distributed in the Amazon basin from the Río Ucayali system in Peru, eastward as far as the rio Xingu in Brazil, plus the rio Tocantins and Rio Capim basins.

It’s also known from the Río Orinoco ba…

This species is currently the only described member of its genus although colour pattern and morphology vary considerably across its range.

You may see reference to this species being the only fish species to possess infra-red vision, but other species also possess this ability and we suspect that infra-red plays an important role in the reproductive cycle of many fishes.

The genus Acarichthys remains monotypic and was resdescribed by Kullander (1986) who characterised it by the following: lack of epibranchial lobe; comparatively few (12-14)rays; pelvic fins with relatively broad tips (branches of the first ray are equal in length or the inner is slightly longer); possession of a single supraneural plus 14-15 vertebrae (these characters taken as a combination); no caudal extensions to swim bladder; no caudal ribs.

Its current taxonomic position is based largel…