Taxonomy & naming
Hypancistrus phantasma was described by Heok Hee Tan and Jonathan W. Armbruster in ZooKeys volume 552 (2016), doi:10.3897/zookeys.552.5956, from material collected at Taracuá on the Rio Uaupes, a major tributary of the Rio Negro in Amazonas State, Brazil (approximately 0.1°N, 68.47°W). The Catalog of Fishes (Eschmeyer, CAS) recognises the name as valid within Hypancistrus, subfamily Hypostominae, family Loricariidae.
The species is one of two spotted Hypancistrus described from the Rio Negro drainage in that publication. Its diagnosis centres on a reversal of the colour pattern typical of the most familiar Hypancistrus: where H. zebra and several relatives bear a dark base with pale spots or stripes, H. phantasma carries a tan base overlaid with small brown spots. That distinction, combined with the type locality, separates it from all described congeners.
No L-number was ever assigned. The species was never collected in an undescribed form that passed through the German aquarium trade — the usual route by which Hypancistrus taxa acquired their codes — because, as far as the scientific record shows, it has not been encountered since the original 1924 collection.
The epithet phantasma is Latin for 'phantom', chosen by the authors with deliberate precision: it refers simultaneously to the species' extreme elusiveness and to its pale, ghostly coloration. Few specific epithets in the loricariid literature have been so literally apt.
Morphology
The holotype measures 5 in SL (5 in), the largest specimen from the type series; this figure is treated as the approximate maximum size since no other specimens are known. Body form is that of a typical Hypancistrus: moderately depressed and armoured with keeled bony scutes dorsally, with a naked ventral surface.
Coloration in preservation is described as tan on the dorsal and lateral surfaces, overlaid with small, evenly distributed brown spots. The interorbital region is paler than the flanks, and this pale zone extends posteriorly along the mid-dorsal surface. The naked ventral skin is white. Spots increase in size toward the caudal fin. Fin membranes are hyaline (clear) with darker margins along the distal edges of the dorsal and caudal fins.
This pattern — tan ground with dark spots — is the reverse of the configuration seen in H. zebra (dark with pale spots) and is the primary diagnostic character separating H. phantasma from that well-known species and its closest relatives. In life, the tan coloration may be more vivid, but no live photographs of H. phantasma exist.
Sexual dimorphism is not documented for this species. In Hypancistrus generally, males develop more prominent interopercular odontodes (cheek spines) and tend toward a slimmer body profile; gravid females are noticeably broader-bodied. These patterns are inferred, not confirmed, for H. phantasma.
Habitat
The type locality, Taracuá, sits on the upper Rio Uaupes at the border of Brazil and Colombia. The Uaupes — known as the Vaupés in Colombia — is a substantial black-water tributary of the middle Rio Negro, itself one of the great blackwater rivers of the Amazon basin.
The Rio Negro system is defined by its chemistry: highly acidic (pH typically 3.5–5.5), extremely soft, stained dark with dissolved organic acids from decomposing vegetation, and very low in dissolved minerals. Water temperatures in the tropical Amazon basin range broadly from about 79–86 °F. These conditions — warm, soft, acidic blackwater — characterise the probable habitat of H. phantasma, though no site-specific water chemistry data were recorded in 1924 and none exists for subsequent collections, because there have been none.
Whether the species persists in the Uaupes drainage today is unknown. It may have a restricted range naturally suited to conditions in a specific stretch of the river, or it may occur more widely but remain uncollected in an area that receives relatively little ichthyological survey effort.
Feeding
No dietary information exists for Hypancistrus phantasma. The Hypancistrus genus as a whole is characterised by reduced, fine teeth adapted for scraping soft algae and biofilm from hard surfaces rather than the coarser rasping or wood-eating seen in related genera. Members of the genus tend toward omnivory in captivity, accepting high-protein invertebrate foods (bloodworm, brine shrimp, small pellets) alongside algae and vegetables.
Data sparse — feeding ecology of this species is undocumented and cannot be inferred beyond general genus-level biology.
Mating
No reproductive observations exist for Hypancistrus phantasma. The species has never been maintained in captivity. Based on the biology of the genus — particularly the well-studied H. zebra and H. inspector — Hypancistrus males are territorial around cave sites and use interopercular odontodes in male–male competition. Females are coaxed into a prepared cavity for egg deposition.
Data sparse — mating behaviour for this species is entirely undocumented.
Breeding
No captive breeding records exist for Hypancistrus phantasma. The species is not in the aquarium trade and has not been maintained in any recorded aquarium setting. Genus-level inference — Hypancistrus are cave-spawning, paternally guarding loricariids — is the limit of what can be said.
Data sparse — breeding biology for this species is entirely undocumented.
In the aquarium
Hypancistrus phantasma is not available in the ornamental trade and is not kept in home aquaria. The species has no L-number and no history of collection for the hobby. If living populations exist and were ever to become accessible, the blackwater Rio Negro origin would indicate soft, acidic, warm water as the logical target chemistry — similar to conditions used successfully for H. zebra and other Rio Negro Hypancistrus.
Given the species' extreme apparent rarity and the conservation concern warranted by a century without records, any commercial collection would be inappropriate without substantial baseline ecological data.
Conservation
Hypancistrus phantasma has not been evaluated by the IUCN Red List. Its conservation status is formally Not Evaluated (NE). This gap is itself informative: the species was described only in 2016 from 1924 material, giving limited time for a formal assessment, and the near-total absence of subsequent collection records makes population data essentially nonexistent.
The facts that are available are sobering. A single collecting event circa 1924, in a relatively well-explored region of the upper Amazon, followed by approximately 100 years of ichthyological activity in the broader drainage without a single additional record, suggests either extreme rarity, a very restricted microhabitat, or both. Whether the species persists at Taracuá or elsewhere in the Uaupes drainage is not known.
The Rio Negro basin faces ongoing pressures from mercury contamination associated with artisanal gold mining, habitat alteration from agricultural expansion in the surrounding cerrado, and episodic commercial ornamental-fish collection pressure on Hypancistrus populations more broadly. Without any population data for H. phantasma, its response to these pressures cannot be assessed. A formal IUCN evaluation treating extent-of-occurrence as critically small would be warranted given available information.