Taxonomy & naming
Physopyxis ananas was described by Sousa & Rapp Py-Daniel in 2005, in the same paper that redescribed Physopyxis lyra and introduced a second new species in the genus. It belongs to the family Doradidae, the thorny or talking catfishes, a large South American group known for bony body armour and stridulating pectoral spines. The Catalog of Fishes treats Physopyxis ananas as the currently valid name; the genus Physopyxis itself is a relatively small, taxonomically distinct doradid lineage and the species has not been placed under any of the older, broader catfish genera historically used as catch-alls (such as Doras).
The genus name Physopyxis is built from Greek roots referring to a bladder or bubble-like box, an allusion to the swollen, box-like body form typical of the genus. The species epithet ananas is Latin for "pineapple", almost certainly chosen for the fish's compact, plated, faintly segmented body outline, which recalls the fruit's textured rind.
Morphology
Physopyxis ananas is one of the smallest doradid catfishes known, reaching only about 1 in standard length at maturity — genuinely tiny even by the standards of small thorny catfishes. Like other doradids it is armoured with rows of bony plates and scutes rather than true scales, and it carries the family's signature stiff, serrated pectoral spines, which in doradids can be rasped against the pectoral girdle to produce an audible sound (the family's "talking catfish" name).
Because it lacks true scales, its skin should be treated as medication-sensitive in the same way as other scaleless and armoured catfishes — copper-based treatments and many standard medications need conservative dosing. Given its size, the locking spines are more a handling and netting hazard for the fish itself (they easily snag in net mesh) than a real risk to the keeper, but the same principle applies as with larger doradids: move it in a container rather than a net when possible.
Habitat
The species is known from the lowlands of the entire Amazon basin, including the Rio Negro, and from the Essequibo River basin in Guyana, with records spanning Brazil, Guyana, Peru and Venezuela. The type locality is the confluence of the Mazaruni and Cuyuni Rivers at Kartabo Point, in the Essequibo basin of Guyana's Cuyuni-Mazaruni region.
In the wild it lives among submerged organic litter — leaf packs, twigs and other decaying plant debris — in shallow water, and has been found in both turbid, sediment-laden rivers and blackwater habitats. This debris-dwelling habit, rather than a fixed water chemistry, appears to define its niche across a wide geographic range.
Feeding
As a small benthic doradid, Physopyxis ananas is best treated as a nocturnal micro-predator and scavenger, foraging through leaf litter and detritus after dark for small invertebrates and organic matter, in keeping with the feeding style typical of small thorny catfishes. Specific dietary studies for this species were not found, so this should be taken as a reasonable inference from its habitat and family rather than a directly documented account.
In the aquarium, offer small sinking foods — fine sinking pellets, crushed flake, and small frozen foods such as chopped bloodworm or daphnia — after the lights go out, since this tiny species is easily out-competed by more active tankmates at feeding time.
Mating
No courtship or spawning behaviour has been documented for Physopyxis ananas in the aquarium literature. Given its very small size and litter-dwelling habits, any wild spawning is likely to take place hidden within leaf packs and debris rather than in the open, but this is inference rather than an observed account.
Breeding
Breeding has not been reported for this species in aquarium literature, and no established captive-breeding method exists. It should be considered essentially unbred in the hobby at present. Prospective keepers should not expect to reproduce it and should be honest that any wild-collected stock is not being replaced by captive breeding.
In the aquarium
Physopyxis ananas stays genuinely tiny — around 1 in standard length — so unlike many catfish it does not spring the usual "size trap" of a juvenile bought small that later outgrows its tank; it is suited to modest aquaria, with sources suggesting tanks from about 12 in upward are workable for a small group. What it does need is the right furnishings: a fine sand substrate, wood, and floating plants with dense root systems, along with a layer of leaf litter or other debris it can bury into, replicating the submerged organic litter it inhabits in the wild. A small, robust internal filter with gentle flow suits its natural low-current habitat better than a powerful outflow.
As with other doradids, its skin lacks true scales, so treat medications and copper-based treatments conservatively. Its spines can snag in net mesh, so scooping it with a container is gentler than netting. Because it tolerates both turbid and blackwater conditions in nature, it is not a fussy specialist on water chemistry, but its small size means only small, peaceful tankmates are appropriate — anything large enough to see it as food will eat it.
Conservation
Physopyxis ananas has not been evaluated by the IUCN Red List and carries no formal conservation status (Not Evaluated). Broader doradid family assessments show a mix of outcomes — many assessed species come back Least Concern, with a smaller number Near Threatened or Data Deficient — but no species-specific data exists for P. ananas.
It is known from a wide span of the Amazon and Essequibo lowlands rather than a single isolated site, which is reassuring for its prospects, but with no dedicated assessment its population trend and specific threats are simply unknown. It is not established in the aquarium trade in any numbers, so trade pressure is presumed minimal.