Taxonomy & naming
Anostomoides passionis was described by Santos & Zuanon in 2006 in Zootaxa, based on specimens collected from the rio Xingu in Pará, Brazil. It belongs to the family Anostomidae — the headstanders — within the order Characiformes, and to the small genus Anostomoides, whose few species are confined to the Amazon and Orinoco basins and share the family's oblique, head-down grazing posture.
The Catalog of Fishes (Eschmeyer, California Academy of Sciences) lists Anostomoides passionis as a currently valid name with Santos & Zuanon 2006 as the describing authority; the type locality is given as the rio Xingu at Viracebo do Arroz Cru (3°25'15.7"S, 51°55'08.3"W). Some more recent taxonomic treatments have proposed folding this species into a broader Insperanos nattereri concept, and hobbyist and aggregator sources should be read with that unsettled status in mind; this article follows the Catalog of Fishes and keeps the genus as Anostomoides pending a fuller resolution.
Morphology
Anostomoides passionis has the elongated, torpedo-shaped body typical of headstanders in the genus, with an underslung, angled mouth adapted for grazing at a steep, head-down posture rather than feeding level in the water column. Published data from the type series give a maximum size of 12 in standard length — a substantial fish, well above the size of most aquarium-trade headstanders such as the smaller Chilodus species.
Detailed colouration and fin-count data beyond the original description are sparse in the available sources; as with many single-river Anostomoides, the species is best distinguished from close relatives by the combination of body proportions, dentition and locality rather than by a single obvious field mark.
Habitat
Anostomoides passionis is documented from the rio Xingu, a clearwater Amazon tributary in Pará, Brazil, known for stretches of exposed bedrock and boulders under strong to moderate current. The species has been recorded specifically from these rocky, current-swept reaches, a habitat that shapes both its body plan and its unusual diet.
Distribution notes associated with the species mention the wider Orinoco and Amazon basins, but the confirmed, well-documented population is the rio Xingu one; specific water chemistry values (pH, hardness, temperature) for the type locality were not captured in the available sources, so none are asserted here beyond noting it as a warm, well-oxygenated, current-swept Amazonian river habitat.
Feeding
The most distinctive fact documented about Anostomoides passionis is its diet: stomach-content analysis of the type series found the gut contents dominated by freshwater sponge fragments, consistent with grazing directly on sponge colonies that encrust the submerged rock surfaces of its rapids habitat. This is an unusual specialisation even among headstanders, most of which graze more generally on algae, biofilm and aufwuchs.
Aquarium diet has not been documented for this species. Given its close relatives' general grazing habits, a keeper should expect to offer a vegetable-forward diet — algae wafers, blanched vegetables and spirulina-based foods — alongside some protein, while recognising that no published account confirms how well this particular species accepts substitute foods for its natural sponge-heavy diet.
Mating
No courtship or pairing behaviour has been documented for Anostomoides passionis in the available sources. Headstanders in general are egg-scattering characins without pair-bonding rituals reported in detail, and there is no basis in the cache to describe species-specific courtship here; this is noted as data sparse rather than inferred.
Breeding
Anostomoides passionis has not been reported as bred in the aquarium. Headstanders as a group are typically egg-scatterers with no parental care, and a few relatives in genera such as Leporinus and Anostomus have occasionally been spawned by hobbyists or farmed commercially, but no breeding account — home or commercial — exists for this species in the sources reviewed. Given its large adult size, current-loving rapids habitat and sponge-based wild diet, it should be assumed effectively unbred in captivity until documented otherwise.
In the aquarium
Anostomoides passionis is rarely, if ever, seen in the aquarium trade, and there is no established husbandry record to draw on. Its documented adult size — over 12 in standard length — rules out anything but a large, long, well-filtered aquarium with strong water movement approximating its rapids habitat, and a keeper taking one on should be prepared for a big, active, current-loving fish rather than a small community grazer.
Because its wild diet leans heavily on freshwater sponge, replicating adequate nutrition in captivity is an open question; a varied diet of vegetable matter, algae-based foods and some protein is the reasonable starting point, following general headstander husbandry, but this is inference from the genus rather than a documented feeding regimen for the species itself. As with other Anostomoides and Leporinus-type headstanders, some individuals can become territorial and nippy with age, so tankmates and space should be chosen with that possibility in mind.
Conservation
Anostomoides passionis is listed as Data Deficient by the IUCN Red List. Its narrow known range in the rio Xingu is notable because that river has undergone major hydrological disruption from the Belo Monte dam complex, which has altered flow and flooded or dewatered stretches of the very rocky-rapids habitat this species depends on; while no species-specific impact assessment was found in the sources reviewed, the general pressure on Xingu rapids-dwelling fish from damming is well documented for the river as a whole. Given how little range and population data exist, further survey work would be needed before its status could be upgraded from Data Deficient with confidence.