Taxonomy & naming
Hydrolycus wallacei was described in 1999 by Mônica Toledo-Piza, Naércio Menezes and Aléssio Santos, one of four species currently recognised in the genus Hydrolycus within the family Cynodontidae (the payaras and their relatives), part of the order Characiformes. The genus name combines the Greek hydro ("water") and lykos ("wolf"), a fitting description for a group of fast, fanged, open-water predators; the species epithet honours Alfred Russel Wallace, the British naturalist and co-discoverer of the theory of evolution by natural selection, who collected extensively along the Rio Negro and its tributaries in the 1850s.
The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Hydrolycus wallacei as a valid, distinct species separated from its congeners — chiefly H. scomberoides and H. armatus — on details of dentition, fin-ray counts and body proportions worked out in the original 1999 revision of the genus. It is a comparatively recently described and poorly studied species, and much of what little is published about the genus's biology comes from the more widely distributed and better-known H. scomberoides.
Morphology
Like its relatives, Hydrolycus wallacei has the classic payara build: a long, slender, laterally compressed, silvery body shaped for sustained fast swimming in open water, with a deeply forked tail and a small, forward-set dorsal fin. The most striking feature is the lower jaw, which carries a pair of greatly enlarged, sabre-like fangs that project upward and pass through dedicated sockets in the snout when the mouth is closed — the source of the "vampire fish" nickname and a hunting adaptation for impaling fast-moving prey fish. FishBase records a maximum length of 14 in standard length for H. wallacei, notably smaller than the roughly 1-metre size reported for H. scomberoides, though as a recently described and rarely encountered species, its full size range is not well documented.
Colouration is countershaded silver typical of an open-water pursuit predator — bright, reflective flanks and a darker dorsal surface — with no strongly documented pattern differences reported between the sexes.
Habitat
Hydrolycus wallacei is known from the Rio Negro and upper Orinoco River basins in Brazil, Venezuela and Colombia, occupying open channel and pelagic habitat in large tropical rivers rather than vegetated margins or still backwaters. FishBase describes the species' habitat as pelagic, open-water freshwater — consistent with an active pursuit predator that needs long, unobstructed stretches of water to chase down prey rather than ambushing from cover.
Specific water chemistry data for H. wallacei is sparse in the sources consulted, but the Rio Negro system it partly inhabits is characteristically warm, blackwater and acidic; keepers and researchers dealing with the genus generally describe large-river tropical conditions. Given the limited documentation, prospective keepers and researchers should treat precise temperature and pH figures for this specific species as unconfirmed rather than assume extreme blackwater parameters without direct evidence.
Feeding
Hydrolycus wallacei is an obligate piscivore. Related, better-studied Hydrolycus species are described as exclusively fish-eating pursuit predators, using their fanged jaws to strike and impale prey fish in open water, and there is no indication that H. wallacei differs in this respect. In the wild it almost certainly targets small to medium schooling fish that share its open-river habitat.
In captivity — where the species is exceptionally rarely kept — accounts of the genus note that payaras are difficult to condition onto anything but live or freshly killed fish, and that they can be reluctant or slow to accept prepared foods. Any tankmates or feeder fish must be far too large for the payara to engulf, since anything it can fit in its mouth is at risk of being treated as prey.
Mating
No courtship or spawning behaviour has been documented for Hydrolycus wallacei in the sources available. As with the rest of the genus, the species is a large-river fish presumed to migrate and spawn in open water rather than pairing off or defending a nest site, but no field or captive observations describing pairing, courtship display or sexual dimorphism could be confirmed for this species.
Breeding
Hydrolycus wallacei has no established aquarium or aquaculture breeding record. As a payara — a large-river, open-water predator of the Rio Negro and Orinoco systems — it almost certainly relies on a big-river spawning strategy similar to other pelagic Neotropical characins, but no confirmed details of its reproductive biology (spawning migration, egg type, fecundity) were found in the cache for this species.
The species is essentially absent from the aquarium trade — a commercial importer's notes describe obtaining only a single specimen in more than a decade of dealing in South American fish — so there is no hobbyist or commercial breeding history to draw on. Any claim of captive breeding for H. wallacei should be treated with strong suspicion; the honest state of knowledge is that it is not bred in captivity.
In the aquarium
Hydrolycus wallacei is not a realistic aquarium fish for almost any keeper. It is a fast, fragile, open-water pursuit predator that needs a long, unobstructed swimming space to feel secure and to feed properly; payaras are notorious for panicking in confined tanks and injuring or killing themselves by striking the glass or hardscape, and their delicate skin and jaw structure make them poor shippers and poor survivors outside large, specialised systems. A commercial exotic-fish importer that has dealt in the species for years reports handling only a single specimen since 2012 — a clear indicator of how rare it is in the trade, not a fish a hobbyist should expect to source, let alone house long-term.
Any attempt to keep this species would require an exceptionally long, unfurnished tank (public-aquarium scale for the genus generally), pristine, well-oxygenated water, and a diet of live or freshly killed fish sized appropriately for its 14 in adult length; even then, payaras have a poor reputation for surviving transition to captivity. Given the near-total absence of husbandry data specific to H. wallacei, this is a species best appreciated in its native rivers or in public aquaria rather than attempted at home.
Conservation
The IUCN Red List assesses Hydrolycus wallacei as Least Concern, and FishBase and the U.S. Fish & Wildlife Service both describe it as restricted to the Rio Negro and upper Orinoco basins of Brazil, Venezuela and Colombia — a wide enough range within intact Amazonian blackwater habitat that it is not currently considered at risk.
Because the species is barely present in the aquarium trade — importers report handling only isolated specimens — collection pressure for the ornamental market is negligible. The main long-term watch items for the Rio Negro and Orinoco systems generally are the broader pressures facing Amazonian freshwater habitats, such as damming and water-quality change, rather than any threat specific to this species.