Taxonomy & naming
Corydoras hypnos was described in 2022 by Luiz Tencatt, Willian Ohara, Leandro Sousa and Marcelo Britto, from the Rio Jamanxim and a small tributary in the Rio Tapajós basin, Novo Progresso Municipality, Pará State, Brazil. The describers named it for Hypnos, the Greek god or personification of sleep, and described it in the same work as Corydoras thanatos, named for Thanatos, the personification of death — in mythology the twin brother of Hypnos. The paired names underscore two closely similar species that differ in fine detail, the sleeping twin and the death twin. In the hobby the fish carries the collection code CW173.
The genus Corydoras Lacépède, 1803 had become an unwieldy, non-monophyletic assemblage of over 170 species. The 2024 revision of the Corydoradinae by Dias, Tencatt and colleagues split it into several lineages, and under that scheme this species becomes Hoplisoma hypnos — Hoplisoma being the largest of the resurrected genera. The combination is recent and the hobby still says 'Corydoras hypnos,' so we keep that heading while noting the current placement.
Morphology
This is a small cory, reaching only about 1.5 in in standard length; females grow somewhat larger and deeper-bodied than males. It has the standard Corydoras armour — two rows of overlapping bony scutes along each flank, a hard helmeted head, and three pairs of barbels around a small ventral mouth.
The ground colour of the body is light, overlaid by a greenish-yellow iridescent sheen, and a dark stripe runs from the front lower edge of the eye toward the upper lip — part of the fine head patterning that helps separate it from its twin Corydoras thanatos and other similar corys. The dorsal, pectoral and adipose fins each bear the stiff locking spine typical of the genus.
Habitat
Corydoras hypnos is known from the Rio Jamanxim, a clearwater tributary in the Rio Tapajós basin of southern Pará, Brazil, ranging from the main channel into its small tributary streams. It is associated with sandy substrates, leaf litter and fine gravel in stretches with submerged branches — the structured, sediment-and-wood bottoms typical of Tapajós clearwater streams.
These are warm tropical waters; the species has been recorded in conditions around 72–81 °F and pH 6.0 to 7.2, soft and mildly acidic to neutral. Like all corys it is a facultative air-breather, periodically surfacing to gulp air and absorb oxygen through a vascularised hindgut, an adaptation to the warm, sometimes oxygen-poor forest streams it occupies.
Feeding
Corydoras hypnos is a benthic omnivore that feeds by sifting the bottom: it works its snout and barbels through sand, fine gravel and leaf litter to taste out small invertebrates, insect larvae, micro-crustaceans and organic detritus, passing cleaned substrate out through the gills. It is an opportunist rather than a specialist. In aquaria it takes sinking prepared foods, frozen bloodworm and brine shrimp and other small sinking fare, but should be fed directly rather than left to scavenge leftovers.
Mating
Spawning of Corydoras hypnos has not been documented in detail, but corys share a stereotyped courtship that this species is expected to follow. They are egg-laying shoalers without a pair bond: a ripe female cleans a surface and the group becomes active, a male takes the classic 'T-position' at right angles to her head, and the female appears to take milt into her mouth. The sperm passes through the gut to fertilise the small clutch of eggs she clasps in a pelvic-fin basket, which she presses onto the cleaned site, repeating the sequence a few eggs at a time.
Breeding
There is no published aquarium breeding record for Corydoras hypnos, a recently described and seldom-traded species, so clutch size, egg size and fry development are not documented for it. By analogy with related corys, spawning would be expected to follow a cool, large water change mimicking the onset of the rains, producing adhesive eggs scattered on plants, wood and glass with no parental care, hatching in a few days, with fry taking microworm and baby brine shrimp once free-swimming. These are inferences from relatives, not records for this species.
In the aquarium
Corydoras hypnos is a small, attractive but rarely available cory suited to a quiet clearwater or blackwater community. Like all corys it is a shoaling fish and should be kept in groups of six or more, where it is confident and active; kept singly it is stressed and reclusive. Because it is small, it can be housed in a modest tank, provided the bottom area is generous — footprint matters more than height.
Provide soft sand or very fine, smooth, rounded gravel, since corys feed by sifting and sharp or coarse substrate erodes and infects their barbels, leaving the blunted faces that mark poor husbandry. Soft, mildly acidic, warm and well-oxygenated water with leaf litter and some wood best reflects its Tapajós origins. It is peaceful and belongs only with small, gentle tankmates such as small tetras, pencilfish and dwarf cichlids. The usual cory cautions apply: the fin spines tangle in nets, so a cup is safer, and a badly stressed cory can release a skin toxin that can be lethal in the small volume of a transport bag.
Conservation
Corydoras hypnos has not been evaluated by the IUCN, so it carries no formal Red List status. As a newly described species with a known range apparently restricted to the Rio Jamanxim and its tributaries, its conservation standing is uncertain — narrowly distributed clearwater fishes of this kind can be vulnerable to localised disturbance. The Tapajós basin faces pressure from gold mining, deforestation and proposed and existing hydropower, all of which can alter the sediment regime, water quality and connectivity of the streams it inhabits. Until a formal assessment is made, a precautionary view of its status is warranted, and it is not a significant aquarium-trade fish.