Taxonomy & naming
Corydoras acrensis was described by Han Nijssen in 1972 in the Netherlands Journal of Zoology, based on a single holotype (the species is known from a unique type) collected at the Furo do Lago São Francisco, a left-bank tributary of the Rio Juruá in Acre State, Brazil. The specific epithet 'acrensis' simply means 'from Acre'. For five decades it sat in the vast genus Corydoras Lacépède, 1803, the dominant genus of South American armoured catfishes in the family Callichthyidae.
That genus had grown to well over 170 species and was shown by molecular work not to be a single natural lineage. In 2024 Dias, Tencatt and colleagues published a large phylogenomic revision (Zoological Journal of the Linnean Society) that split Corydoras into several genera — among them Brochis, Hoplisoma, Gastrodermus and Osteogaster. Under that scheme this species is placed in the resurrected genus Hoplisoma and its valid combination becomes Hoplisoma acrense (the neuter ending agreeing with the genus). The Catalog of Fishes now lists the current status as 'Valid as Hoplisoma acrense (Nijssen 1972)'. The aquarium trade still largely uses 'Corydoras acrensis', so we keep that as the heading while recording the new combination.
Morphology
This is a small Corydoras, reaching only about 1 in in standard length — among the more diminutive members of the group. Like all corys it is armoured rather than scaled, with two overlapping rows of bony plates (scutes) running down each flank and meeting along the back, a hard helmet-like head, and three pairs of sensory barbels around the downturned mouth.
Detailed colour and pattern information is scarce, because the species is known from very little material and has been written about hardly at all since its description. As in related short- to intermediate-snouted Hoplisoma, the dorsal, pectoral and adipose fins each carry a stiff leading spine. The pectoral spines in particular are capable of piercing skin and can deliver a painful 'sting'. We avoid inventing diagnostic detail the literature does not support; what is certain is its small size and the standard Corydoradinae body plan.
Habitat
Corydoras acrensis is known only from the Juruá River basin in Acre State, southwestern Brazilian Amazonia — a region of clear- and whitewater forest streams in the headwaters of one of the Amazon's great white-water tributaries. The type locality, the Furo do Lago São Francisco, is a small left-bank tributary channel of the upper Rio Juruá.
No detailed field data on its preferred microhabitat have been published, but by analogy with its congeners it is a fish of shallow, quieter water over soft substrates with leaf litter and submerged wood. FishBase lists a broad tolerance envelope typical of lowland Amazonian corys — a temperature band of roughly 70–75 °F and a wide pH range — though these are general estimates rather than measurements taken at the type locality. Like all Corydoradinae it is a facultative air-breather, gulping surface air and absorbing oxygen through a vascularised hindgut, an adaptation for warm, oxygen-poor shallows.
Feeding
No diet study exists for this species specifically, but Corydoradinae catfishes are uniformly benthic omnivores and detritivores. They work the bottom continuously, burying snout and barbels in sand and litter to taste out small invertebrates, insect larvae, worms, micro-crustaceans, fallen seeds and the organic film of detritus, expelling cleaned substrate through the gills. C. acrensis can be assumed to feed in the same way — an opportunistic forager of the substrate rather than a specialist.
Mating
There are no published observations of courtship or spawning in Corydoras acrensis. The genus as a whole follows a well-studied pattern: corys are egg-laying shoalers with no pair bond, in which a receptive female cleans a site, a male takes up the classic 'T-position' at right angles to her head, and a few sticky eggs at a time are clasped in a basket formed by her pelvic fins and pressed onto the cleaned surface. Whether this species deviates from that template is simply unknown.
Breeding
Corydoras acrensis has, as far as the literature records, not been bred in captivity, and no account of its reproduction in the wild has been published. By analogy with related small corys it would be expected to be an adhesive-egg layer producing modest clutches with no parental care, the eggs scattered across firm surfaces and the fry feeding on micro-foods after absorbing the yolk sac. We flag this as inference rather than documented fact: the species' rarity in the trade means its breeding remains essentially undescribed.
In the aquarium
This is not a fish most aquarists will ever encounter; it is rare-to-absent in the trade and there is little husbandry experience to draw on. Should specimens appear, the requirements would be those of any small Corydoras: keep it as a shoal of six or more, never singly, on soft sand or very smooth fine gravel — sharp substrate erodes and infects the barbels, a common sign of poor husbandry. Provide gentle, well-oxygenated, well-filtered water in the low-to-mid 20s Celsius, with cover and a clean bottom they can sift through. As a peaceable, small bottom-dweller it would suit a calm community of similarly small, gentle fish. The usual cory cautions apply: the fin spines can tangle in nets (a cup is safer), and a stressed cory can release skin toxins that, in the confined volume of a bag, can harm the whole group.
Conservation
Corydoras acrensis (as Hoplisoma acrense) was assessed by the IUCN in 2020 as Least Concern. Despite being known to science from very little material, the assessment judged it to be at low risk. It is not a target of any meaningful fishery and faces no species-specific threat that has been identified. That said, a fish known from such a narrow slice of the upper Juruá warrants attention: the broad pressures on western Amazonia — deforestation, road-building, agricultural run-off and pollution of headwater streams — could affect a narrowly distributed species disproportionately, and its true range and population are poorly known.