Taxonomy & naming
Centromochlus existimatus was described by the Dutch ichthyologist G. F. Mees in 1974, in his monograph on the Auchenipteridae and Pimelodidae of Suriname, based on material from the Amazon basin; the holotype is held at the Academy of Natural Sciences of Philadelphia (ANSP 73154) and the type locality is given as the harbor of Manaus, on the Amazon River, Brazil. It belongs to the family Auchenipteridae, the driftwood catfishes, within the genus Centromochlus, a group of small South American catfishes closely tied to submerged wood and root habitats.
Genetic work on populations attributed to Centromochlus existimatus has found at least five cryptic mitochondrial and nuclear lineages, several of them living side by side in the same rivers, which suggests the name currently covers a species complex still awaiting full taxonomic revision. Aquarists and researchers alike should treat the name as provisional for now — the Catalog of Fishes treats Centromochlus existimatus as the currently valid name, and that is the name used here.
Morphology
Centromochlus existimatus is a small catfish, reaching only about 3.5 in standard length, well within the modest size range typical of the genus. Like other driftwood cats it has a somewhat flattened head, a slender tapering body, and three pairs of barbels used to feel out food and structure in low light. As in virtually all catfishes the skin is naked and scaleless rather than covered in bony plates, which makes it noticeably more sensitive than scaled fish to copper-based medications and to some other treatments — doses should be conservative.
The fins carry stiff, serrated spines at the leading edge of the dorsal and pectoral fins that can lock erect; these easily snag in an aquarium net and can jab a careless hand, so this species — like most catfish — is best scooped and moved in a container rather than netted. In males the anal fin is further modified into an elongated, rod-like intromittent organ used for internal fertilisation, a structural giveaway of the family's unusual mating system (see "Mating").
Habitat
The species is native to the Amazon River basin, with records spanning Brazil (Amazonas, Pará and Acre states), Colombia and Peru, including the Ucayali, Solimões and Madeira drainages. It is a demersal, freshwater fish of moving water — related Centromochlus have been documented as rheophilic, favouring highly oxygenated, flowing forest streams rather than still backwaters.
It shelters by day among submerged driftwood, root tangles and leaf litter along the margins of these streams, emerging at night to forage. Closely related congeners are reported from soft, weakly acidic to neutral water (roughly pH 6.0–7.5) at warm tropical temperatures of about 75–82 °F, and this is a reasonable working range for Centromochlus existimatus as well, though dedicated water-chemistry data for this exact species is sparse.
Feeding
Centromochlus existimatus is a nocturnal benthic micro-predator and omnivore. In the wild it forages over the stream bottom and among wood debris chiefly after dark, taking aquatic insect larvae, small crustaceans and other invertebrates it can find and overpower.
In the aquarium it accepts sinking pellets, frozen foods such as bloodworm and brine shrimp, and other prepared foods, but because it is genuinely nocturnal it does best when fed after the lights go out — in a mixed tank it will otherwise be outcompeted by diurnal fish before it has even emerged from cover.
Mating
Driftwood catfishes are one of the very few catfish groups with internal fertilisation, and Centromochlus existimatus follows the family pattern. The male's anal fin is modified into an intromittent organ that is used to transfer sperm directly to the female during a brief mating contact, rather than the pair shedding eggs and milt together over open substrate as most catfishes do.
Following mating, the female is able to store sperm and does not necessarily spawn immediately; fertile eggs can be laid some time — potentially weeks — after mating has taken place. This is a genuinely unusual strategy among catfishes generally and is worth stating plainly rather than assuming the more typical egg-scattering pattern seen in most Siluriformes.
Breeding
Set facts.breeding.mode to internal fertilisation via the modified male anal fin, with the female storing sperm and spawning later; this is what is documented for the family and for related Centromochlus species. Parental care after spawning appears to vary among close relatives — some studies note a degree of female guarding of eggs in one congener (C. perugiae) against no post-spawning care at all in another (C. reticulatus) — so care in this species specifically is not confirmed and should not be assumed one way or the other.
Centromochlus existimatus is not documented as an aquarium-bred species; there is no established home-breeding protocol in the hobby literature, and given the internal-fertilisation strategy and the delayed, hard-to-predict timing of spawning, it should be considered essentially unbred in captivity at present. Aquarists keeping this species should not expect fry from a community setup.
In the aquarium
This is a genuinely small catfish — about 3.5 in SL at most — so it does not fall into the size trap that catches keepers of larger driftwood cats and other Siluriformes; even so it should be given a tank with real current, oxygenation and, above all, plenty of submerged driftwood and root tangles to hide in by day, echoing the flowing forest-stream habitat it comes from. It is strictly nocturnal and will rarely be seen unless fed and observed after dark or under red/blue night lighting.
Being scaleless, it is medication-sensitive: treat copper and other harsh chemicals conservatively, often at reduced dose, exactly as with loaches, puffers and other naked-skinned catfish. Its dorsal and pectoral spines can lock and snag a net, so move it in a cup or container rather than netting it directly. Keep it with peaceful tankmates suited to flowing, well-oxygenated water; as a nocturnal micro-predator it may take very small fish or fry but poses no threat to normal community-sized companions.
Conservation
The IUCN Red List assesses Centromochlus existimatus as Least Concern, last evaluated in 2020. It is known from a wide swath of the Amazon basin across Brazil, Colombia and Peru, and is not currently considered at risk at the species level.
That said, the discovery that the name likely covers several cryptic lineages means the conservation picture for any one of those individual lineages is not really known — a broad "Least Concern" for the complex as currently defined could mask a narrower range, and therefore higher vulnerability, for some of the constituent populations once the taxonomy is resolved. Amazon streams generally face pressure from deforestation, damming and habitat degradation, which is relevant background even where a species itself is not flagged as threatened.