Taxonomy & naming
Scleromystax reisi was described in 2016 by Marcelo R. Britto, Clayton K. Fukakusa and Luiz R. Malabarba, in the journal Neotropical Ichthyology, from the laguna dos Patos drainage of Rio Grande do Sul. The paper was titled for its central finding — that the species extends the meridional (southern) distribution of genera endemic to the Atlantic Forest — and the fish honours the Brazilian ichthyologist Roberto E. Reis. It was described directly in Scleromystax Günther, 1864, the small genus of south-eastern and southern Brazilian corydoradines marked by the erectile cheek odontodes of breeding males and by strong sexual dimorphism.
Unlike the older corys, this species never sat in the catch-all Corydoras, so the 2024 revision of the Corydoradinae by Dias, Tencatt and colleagues — which split Corydoras into Brochis, Hoplisoma, Osteogaster, Gastrodermus and others — simply confirmed Scleromystax as a valid lineage and left S. reisi in place. The Catalog of Fishes and FishBase list it as Scleromystax reisi, the combination we use. It is diagnosed from its congeners in part by the second infraorbital bone being expanded ventrally.
Morphology
This is a small Scleromystax, reaching about 2 in in standard length (the male holotype measured 2 in). It has the deep-bodied, flat-bellied, bony-plated build of the corys, with a blunt head and three pairs of barbels around the mouth. Diagnostically, the second infraorbital bone is expanded downward, adjacent to a small, almost imperceptible naked area between the sphenotic, compound pterotic, opercle and preopercle — fine skeletal characters that separate it from its relatives.
In colour it shows the freckled, blotched pattern typical of the genus, with roughly oblique, elongate dark-brown markings along the body. As in other Scleromystax, mature males develop patches of stiff cheek odontodes, and the sexes differ in pattern and proportions, the females being the larger, rounder sex.
Habitat
Scleromystax reisi is endemic to small tributary streams of the rio Jacuí and rio Camaquã systems within the laguna dos Patos drainage of Rio Grande do Sul, Brazil — the southernmost known occurrence of the genus. The type locality is the arroio Demétrio at Morungava, Gravataí.
It is a fish of very small waters: streams only about 2–7 ft wide and 12–23.5 in deep, with slow-flowing, clear water over sandy bottoms. This is a cool, subtropical setting in the far south of Brazil, and the species should be regarded as a cool-water fish rather than a warm tropical one. Its habitat lies within the southern fringe of the Atlantic Forest, a biome whose small streams are heavily exposed to agriculture and land-use change.
Feeding
As a corydoradine, S. reisi is a benthic foraging omnivore. In its narrow sandy creeks it works the bottom, using its barbels to detect and sift out small invertebrates, insect larvae, micro-crustaceans and organic detritus, expelling cleaned sand through the gills. In aquaria the genus reliably accepts sinking dried foods and takes small live and frozen items — chironomid larvae (bloodworm), Tubifex and similar — and a varied diet keeps it healthy and in condition.
Mating
No detailed account of spawning in this very recently described species has been published, but reproduction can be expected to follow the universal corydoradine pattern: an egg-scattering shoaler with no pair bond, in which a ripe female cleans a site and a male takes up the 'T-position' at right angles to her head. A few eggs at a time are fertilised and carried in the basket of the female's pelvic fins to be pressed onto the chosen surface, the sequence repeating until the clutch is laid. The cheek bristles of males are presumed, as in related Scleromystax, to play a role in courtship and male–male interaction.
Breeding
Scleromystax reisi is barely present in the aquarium hobby and there is no established captive-breeding account for it. By analogy with its better-known congeners it would be expected to spawn after cool, fresh water changes that mimic seasonal rains, laying small batches of adhesive eggs on firm surfaces with no parental care, the eggs best separated from the adults to hatch. Given its origin in small, cool, clean streams, success would almost certainly depend on cool, pristine, well-oxygenated water and small live foods for the fry. For now this remains informed inference rather than documented practice.
In the aquarium
This is a rare fish, essentially unknown in general trade and of interest mainly to dedicated corydoradine specialists. Husbandry should follow the template for southern, cool-water Scleromystax: a group of six or more over soft sand, with leaf litter, wood and gentle, well-oxygenated flow, in cool subtropical temperatures rather than tropical warmth. Soft, near-neutral water suits it. It is a small, peaceful, retiring species that would be overwhelmed by boisterous tankmates and is best kept either in a species set-up or with other small, calm, cool-water fish. As always with corys, the smooth substrate protects the barbels and a cup is safer than a net for handling.
Conservation
Scleromystax reisi has not been evaluated by the IUCN and carries no Red List status. It is a narrow-range endemic, known only from small streams in the laguna dos Patos drainage of Rio Grande do Sul, the kind of restricted, small-stream distribution that is inherently vulnerable. Its habitat sits in a region of intensive agriculture at the southern edge of the much-reduced Atlantic Forest, where small watercourses are readily degraded by clearance, abstraction and pollution. The species was described only in 2016, and a formal assessment of its range and population status remains an open and worthwhile task.