Taxonomy & naming
Ehrhardt's cory was described by the Austrian ichthyologist Franz Steindachner in 1910, from material collected in affluents of the Jaraguá mountains near Joinville, in Santa Catarina State, Brazil — still the type locality. The specific epithet honours the German collector Wilhelm Ehrhardt, who gathered specimens in southern Brazil in the early twentieth century. The species has long been known as Corydoras ehrhardti, the name under which it is traded and written about, and the one we keep at the head of this page; Corydoras meridionalis is treated as a synonym.
That name is now in transition. The genus Corydoras Lacépède, 1803 had grown to well over 170 species and molecular work showed it was not a single natural group. In 2024 Dias, Tencatt and colleagues published a large phylogenomic revision of the subfamily Corydoradinae (Zoological Journal of the Linnean Society, DOI 10.1093/zoolinnean/zlae053) that split the old genus into several, resurrecting Brochis, Hoplisoma, Gastrodermus and Osteogaster among others. Under that scheme Ehrhardt's cory becomes Hoplisoma ehrhardti, and Eschmeyer's Catalog of Fishes now lists Hoplisoma ehrhardti (Steindachner 1910) as the valid combination, placing it in the large resurrected genus Hoplisoma. Because the epithet honours a person it stays 'ehrhardti' regardless of the new genus's gender. The change is recent and the hobby still overwhelmingly says 'Corydoras ehrhardti.'
Morphology
This is a small, fairly slender cory, reaching about 1.5 in in standard length (roughly 2 in including the tail); females grow larger and noticeably broader than males, a difference best judged looking down on the fish from above. Like all corydoradines it is armoured rather than scaled: two overlapping rows of bony plates (scutes) run down each flank and meet in a ridge along the back, the head is a hard blunt helmet, and three pairs of short barbels fringe the downturned mouth.
The ground colour is a pale silvery to greyish base, warmed in places to an olive or pinkish tone — hence the 'olive cory' name — overlaid on the upper flank by a soft iridescent blue-to-green sheen that brightens over the shoulder under good light. A fine dark lateral stripe runs along the midline from behind the gill cover toward the tail, the belly is pale, and the fins are largely clear. As in all corys the dorsal, pectoral and adipose fins each carry a stiff, mildly venomous spine. As the fish matures, males tend to develop more pointed fins while females stay rounder.
Habitat
Corydoras ehrhardti is endemic to south-east Brazil, occurring in the Atlantic coastal rivers of Santa Catarina and Paraná — from the Ribeira de Iguape drainage south to the Itajaí-Mirim — and also in the upper Iguaçu and Paranapanema systems inland, across the states of São Paulo, Paraná and Santa Catarina. The type locality, in the Jaraguá hills near Joinville, is typical: a short, clear coastal stream draining the coastal range.
It is a demersal fish of small streams and shallow rivers with soft substrates, found in slow-flowing reaches, on the margins of larger rivers, and in marshes, ponds and lakes — rarely in stagnant water, and able to persist in habitats subject to moderate human disturbance. As with its coastal neighbours this is subtropical, not tropical, country: the water runs cool, often around 66–72 °F, soft to moderately hard and slightly acidic to neutral. Like other corys it is a facultative air-breather, gulping air at the surface and absorbing oxygen through a vascularised hindgut, which helps it cope with warm, oxygen-poor shallows.
Feeding
Ehrhardt's cory is a benthic omnivore. In the wild it works the bottom continuously, sifting sand and leaf litter with its barbels to taste out small invertebrates, insect larvae, worms, crustaceans and the organic film of detritus, expelling cleaned sand through the gills. It is an opportunist rather than a specialist, and that broad diet underpins its hardiness. In the aquarium it readily takes all prepared foods that reach the bottom — sinking tablets, granules and larger flakes — along with live and frozen foods such as bloodworm, Daphnia, Tubifex and white or grindal worm, the last being especially useful for conditioning a group for spawning. It should not be left to subsist only on tankmates' leftovers; a deliberate sinking diet keeps it in condition.
Mating
Corydoras are egg-laying shoalers with no pair bond and the distinctive, well-studied courtship shared across the group. When a female is ready she cleans a site — a leaf, a stone, the aquarium glass — and the group becomes animated; a male positions himself at right angles to her head in the classic 'T-position,' and she appears to take his milt into her mouth. Sperm is thought to pass rapidly through her gut to fertilise the few eggs she clasps in a basket formed by her pelvic fins, which she then carries to the cleaned surface and presses into place. The sequence repeats, a few eggs at a time, until a clutch of sticky eggs is distributed across the chosen surfaces.
Breeding
Ehrhardt's cory has been bred in the hobby — Planet Catfish records several breeding reports — and its cooler requirements are central to success. As with related coastal corys, spawning is typically triggered by a large, cool water change that mimics the arrival of subtropical winter rains, dropping the temperature several degrees; a group skewed toward more males than females tends to be most productive. The adhesive eggs are laid in small batches on plants, wood, stones or glass and receive no parental care — the adults will eat them, so breeders move either the eggs or the parents. Eggs hatch in roughly three to five days depending on temperature, and once the fry have absorbed their yolk sacs they take microworm and newly-hatched brine shrimp as first foods. The fry are hardy if kept cool and clean.
In the aquarium
This is a rewarding cory for a keeper who can give it what it actually needs, the chief requirement being temperature: it is a subtropical fish and does best around 66–72 °F, cooler than a typical community tank, and it should not be kept in warm tropical water long-term. Within that band it is hardy and undemanding.
Like every cory it is a shoaling species and is genuinely stressed kept alone or in pairs — keep six or more and it rewards you with constant, confident foraging. A group is comfortable in a tank of around 23.5 in or more, where footprint and bottom area matter more than height. Substrate is close to mandatory: provide soft sand or, as an imperfect alternative, smooth rounded fine gravel, because corys feed by sifting and sharp or chipped gravel erodes and infects their barbels. Border the open foraging area with driftwood and planting, with overhanging wood, rock or floating plants for shade, and keep the water well-oxygenated and clean with regular changes; soft to moderately hard, slightly acidic to neutral water suits it. It is a peaceful, shoaling community fish ideal with other calm, cool-tolerant species. Two standing cautions apply to all corys: the fin spines can catch in a net (a cup is safer), and a distressed cory can release skin toxins that, in the confined volume of a bag or bucket, can kill the whole group — so transport them with care.
Conservation
Corydoras ehrhardti is assessed by the IUCN Red List as Least Concern (assessment dated 2018). It is reasonably widespread across the coastal and upper-river drainages of south-east Brazil, ecologically flexible, and able to persist in streams subject to moderate human disturbance, so it does not meet the thresholds for a higher category. That tolerance should not be mistaken for security: the Atlantic coastal rivers of Santa Catarina and Paraná lie in a densely settled, intensively used region, and deforestation, urban and agricultural run-off, water abstraction and pollution all degrade the shallow, soft-bottomed habitats it favours. As a species it is in no immediate danger, but its habitat is under steady local pressure, and the wider reshuffling of regional corys into resurrected genera means some forms may eventually warrant assessment on their own terms.