Taxonomy & naming
The species was described by the American naturalist Edward Drinker Cope in 1872 as Corydoras ambiacus, based on a single specimen from the Río Ambyiacu (Ampiyacu) near Pebas in northeastern Peru, which is the type locality. The epithet is taken from that river name; the Catalog of Fishes treats 'ambiacus' as an indeclinable noun. Corydoras grafi and Corydoras longirostris are treated as synonyms.
For a century and a half the fish sat in Corydoras Lacépède, 1803, the enormous genus of South American armoured catfishes in the family Callichthyidae. Because that genus had ballooned past 170 species and molecular work showed it was not a single natural group, Dias, Tencatt and colleagues split it in a 2024 revision (DOI 10.1093/zoolinnean/zlae053) into several genera including Brochis, Hoplisoma, Gastrodermus and Osteogaster. Under that scheme the spotted cory becomes Brochis ambiacus, the combination now carried by Eschmeyer's Catalog of Fishes; Planet Catfish likewise files it in Brochis. We keep 'Corydoras' as the display heading because that is still the universal trade and hobby name, while stating the current science plainly. The bigger everyday problem with this fish is identification, not nomenclature: ambiacus belongs to a complex of near-identical finely spotted upper-Amazon corys, and the names ambiacus, agassizii and leopardus are routinely swapped in shops. A great many fish sold as 'agassizii' are in fact ambiacus, and a great many 'ambiacus' are something else again.
Morphology
The spotted cory is a small, fairly deep-bodied catfish reaching roughly 2 in in standard length, the females growing larger and rounder than the males. It has the standard cory build: two rows of overlapping bony plates down each flank in place of scales, a hard helmeted head, three pairs of barbels around a downturned sifting mouth, and stiff leading spines in the dorsal, pectoral and adipose fins.
Its pattern is a pale tan-to-silvery ground covered in fine, fairly even dark peppering — small spots distributed densely over the body and head and extending into the fins. Compared with the closely similar C. agassizii, which tends to show a bold, well-defined black blotch dominating the dorsal fin and somewhat coarser, more row-like body spotting, ambiacus is generally more uniformly and finely speckled, with a less emphatic dorsal marking. These are tendencies rather than hard rules: the species are genuinely difficult to separate, the features overlap and vary between populations, and even experienced keepers disagree over individual fish. Honest sources treat single-feature identifications of this complex with caution.
Habitat
Corydoras ambiacus is an upper-Amazon species. The Catalog of Fishes gives its range as the upper Amazon River basin across Peru, Ecuador, Colombia and Brazil, and the type locality lies on the Río Ambyiacu in northeastern Peru. This is lowland rainforest river country, and like its relatives the fish occupies soft-bottomed shallow margins — the edges of streams, side-channels, floodplain pools and slow river reaches over sand and mud, among leaf litter and submerged wood.
The waters it lives in are warm, soft and quiet. The parameters associated with the species are a temperature of about 70–78 °F, a pH from acidic to slightly alkaline (roughly 6.0–8.0) and soft to moderately hard water. As with other corys it is a facultative air-breather, rising to gulp atmospheric air that it absorbs through a modified, vascularised hindgut, which lets it cope with the warm, oxygen-poor backwaters typical of its habitat.
Feeding
This is a bottom-feeding omnivore and detritivore. It forages continuously over the substrate, driving its snout and barbels into sand and litter to taste out small invertebrates, insect larvae, worms, micro-crustaceans, seeds and the organic film of detritus, and expelling the cleaned substrate through its gills. It is an opportunistic generalist, not a specialist feeder. In the aquarium it takes sinking pellets and tablets along with frozen and live foods such as bloodworm, Daphnia, grindal worm and brine shrimp, which are also useful for conditioning a group for spawning; it should be fed deliberately at the bottom and not left to live on scraps.
Mating
The spotted cory is an egg-laying shoaler with no pair bond and the characteristic corydoras courtship. When a female comes into condition she cleans a spawning surface and the group becomes active and excited; a male adopts the classic 'T-position', lying at right angles across the female's head. The female holds a small number of eggs clasped in a basket formed by her pelvic fins while the male fertilises them, then carries them to the cleaned site and presses the very sticky eggs into place. The sequence is repeated many times over, a few eggs at a pass, until a clutch of dozens to roughly a hundred eggs has been distributed across plants, wood, stones or the aquarium glass.
Breeding
Breeding follows the standard corydoras template and is generally prompted by a large, cooler water change that mimics rainy-season conditions, ideally after conditioning the group on rich live and frozen foods. The adhesive eggs are scattered in small batches across firm surfaces and receive no parental care — the adults will eat them, so breeders move the eggs or the parents to a separate container. Eggs hatch within a few days depending on temperature, and the fry, after absorbing the yolk sac, are reared on microworm, baby brine shrimp and powdered foods with clean water and frequent feeding. Deliberate aquarium spawning of this exact species is reported only occasionally, in part because of the chronic uncertainty over whether a given group of 'spotted corys' is truly ambiacus or one of its look-alikes.
In the aquarium
The spotted cory is a peaceful, hardy community fish with the usual cory needs. It is a shoaling species and must be kept in a group of six or more; a lone or paired individual is stressed and withdrawn, while a proper group is confident and busy. Give it a soft sand substrate, or at most very smooth rounded fine gravel — corys feed by sifting, and sharp or coarse gravel abrades and infects their barbels, producing the eroded, stubby faces that mark poor husbandry. Keep the bottom clean, provide shade from wood and plants with open feeding space, and maintain warm (about 70–79 °F), well-oxygenated water with gentle current and regular changes; the species tolerates a wide pH and hardness range.
It belongs with calm community fish — tetras, rasboras, dwarf cichlids and similar — and never with large or aggressive tankmates. As with all corys, two practical cautions apply: the stiff fin spines snag easily in a net, so a rigid cup is safer for catching and moving them, and a severely stressed cory can release a skin toxin that may kill the whole group in the confined volume of a bag or bucket, so transport them carefully and avoid crowding. And if precise identity matters to you, buy from a supplier who knows the fish's origin, since much of what is sold under this name — and much sold as 'agassizii' — comes from the same confused spotted complex.
Conservation
Corydoras ambiacus has not been formally assessed by the IUCN and currently carries no Red List category (Not Evaluated). It appears to be a reasonably widespread upper-Amazon fish, collected in numbers for the aquarium trade, and as a small, fecund, fast-maturing species it is unlikely to be threatened by collection itself; the broad pressures on Amazonian freshwaters — deforestation, agriculture, mining and pollution — are the more relevant long-term concerns. The honest caveat is the same one that runs through everything about this fish: because ambiacus is so easily confused with agassizii, leopardus and other finely spotted corys, both trade volumes and distribution records may lump several species together, which makes the true status and range of any one of them hard to pin down.