Taxonomy & naming
Physopyxis cristata was described in 2005 by the Brazilian ichthyologists Leandro Sousa and Lúcia Rapp Py-Daniel. It belongs to Doradidae, the thorny or talking catfishes, a large South American family known for producing audible sounds by stridulating their pectoral spines. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Physopyxis cristata as the valid name in the genus Physopyxis, a small doradid genus erected for a handful of diminutive, heavily armoured species.
Doradid taxonomy has been reshuffled repeatedly as the family's many small, superficially similar genera have been revised; species now placed in Physopyxis have at times been grouped with other small doradids in the literature. The genus and species combination used here follows the original 2005 description and the Catalog of Fishes.
Morphology
Physopyxis cristata is minute for a doradid, reaching only about 1 in standard length (1 in per FishBase) — extremely small even among the small-bodied thorny catfishes. Like its relatives it lacks true scales; the body is instead protected by bony plates and scutes, with an incomplete lateral line and a series of small, low spines running along the dorsal midline, a hallmark of the genus. The armour is the same basic doradid design seen in far larger relatives, just scaled down to a fish that would fit on a fingertip.
Because the skin is plated rather than scaled, this species — like other doradids — is more sensitive than a fully scaled fish to copper and to some medications, a point that matters for anyone treating disease in a tank that holds it.
Habitat
The species is endemic to the Rio Negro basin in Brazil, one of the great blackwater tributaries of the Amazon. It lives specifically among submerged leaf litter, the tangled banks of decaying leaves that accumulate along blackwater margins and provide both cover and a hunting ground for small invertebrates.
Blackwater habitats in the Rio Negro are characteristically soft and acidic, stained tea-brown by dissolved tannins from the surrounding leaf litter and forest. Data sparse on exact field pH and temperature readings for this species specifically, but it shares the general blackwater profile typical of Rio Negro leaf-litter specialists.
Feeding
Physopyxis cristata is a nocturnal benthic omnivore, foraging over and within the leaf litter mostly after dark for small invertebrates and detritus rather than actively hunting by day. This matches the general feeding pattern of small doradids, which pick through submerged debris for prey too small or slow-moving to require pursuit.
In captivity it takes the usual ornamental fish foods — sinking pellets and frozen foods sized appropriately for its tiny mouth — but as with other nocturnal leaf-litter catfish, offering food after lights-out gives it a better chance of feeding without being out-competed by more active tankmates.
Mating
Data sparse: no published account of courtship or spawning behaviour for Physopyxis cristata was found in the sources consulted. Given its family and its leaf-litter habitat, any account should not assume open-water pairing without direct evidence; the species has not been documented breeding in the aquarium.
Breeding
There is no confirmed record of Physopyxis cristata being bred in captivity, and the reproductive biology of this species has not been documented in the sources reviewed here. Breeding mode should not be assumed to be open-water egg-scattering by default — many small doradids and their relatives use more specialised strategies — but nothing specific can be stated for this species without invented detail. Anyone keeping it should treat it as effectively unbred in the hobby.
In the aquarium
This is about as far from the size-trap end of the catfish spectrum as it gets: adult Physopyxis cristata tops out at roughly 1 in standard length, so it never needs anything like the swimming room larger doradids demand. That said, it is a specialist, not a generalist community fish. It needs a genuine leaf-litter setup — a deep bed of submerged leaves (aquarists working with similar leaf-litter doradids commonly recommend 12 in or more of litter depth) that mimics its blackwater habitat and gives it both cover and foraging material.
Two care points carry over from its family even at this small size. First, like other doradids it is scaleless and plated rather than truly scaled, so it is medication-sensitive — dose copper and other treatments conservatively. Second, despite its size it carries small, sharp, numerous spines that tangle very easily in fine aquarium nets; move it in a container rather than netting it. It is most active at night, so cover and subdued lighting suit it better than a brightly lit, heavily trafficked tank. It reaches the trade only occasionally, usually as an incidental catch mixed in with other Rio Negro leaf-litter fish rather than as a deliberately targeted export.
Conservation
The IUCN Red List assesses Physopyxis cristata as Least Concern, with the assessment dated November 7, 2018. As a narrow-range Rio Negro basin endemic tied to a specific leaf-litter microhabitat, its long-term status depends on the health of that blackwater system — deforestation, sedimentation and habitat disturbance along the Rio Negro are the general pressures relevant to species of this kind, though no species-specific threat data was found in the sources consulted.
Because it enters the trade only incidentally rather than through targeted collection, aquarium demand is not considered a significant pressure on the species.