Taxonomy & naming
Scissor macrocephalus was described by Albert Günther in 1864 based on a single holotype, giving the species the slim diagnostic foundation that has followed it ever since. It is currently placed in the monotypic genus Scissor within the family Characidae, though its exact position within that family remains uncertain. GBIF and related aggregators flag the species as incertae sedis within Characidae, and some sources note a possible affinity with Oligosarcus and the subfamily Stethaprioninae, but no comprehensive revisionary work has confirmed or resolved these suggestions.
The Integrated Taxonomic Information System (ITIS) records the species as valid under Taxonomic Serial Number 641714, within the order Characiformes. Eschmeyer's Catalog of Fishes is the governing authority for the valid name Scissor macrocephalus Günther, 1864. Because the species has been referred to by various workers in different generic contexts over the decades, the parenthetical convention does not apply here — Günther placed it in Scissor, and it has remained there formally, however tenuously.
The species has no established common name in the aquarium hobby or scientific literature, reflecting its complete absence from the trade and from modern fieldwork.
Morphology
The most remarkable feature of Scissor macrocephalus is the size of its head relative to the body. According to the original description and ETYFish Project notes drawn from that material, the head length is approximately 3.3 times the remainder of the body, and the head is roughly two-thirds as high as it is long — proportions that are striking even among the morphologically diverse Characiformes. This large-headed build is paired with a wide-cleft mouth, the feature that directly inspired the genus name.
Beyond these defining characters, detailed morphological data for S. macrocephalus are sparse, as all available information derives from the holotype. No live coloration records exist. Body shape and fin counts were recorded in the original description but have not been verified or expanded upon in modern revisions, and the specimen's condition in the type repository limits what can be re-examined today. The species is described as a freshwater pelagic fish on FishBase, which implies a body plan suited to open-water swimming rather than benthic life, but this inference rests on general classification rather than direct observation.
Habitat
The type locality for Scissor macrocephalus is given tentatively as Suriname, South America, though this attribution carries uncertainty — FishBase records the distribution with a question mark, reflecting doubt about the provenance of the holotype. No confirmed locality data or georeferenced collection record exists, and the species has not been collected again since the original description.
Given its placement in South American freshwaters and the FishBase notation of a freshwater pelagic lifestyle, the species presumably inhabited open-water reaches of a lowland river or associated wetland. Without additional specimens or field observations it is not possible to characterize the habitat with any precision: water chemistry parameters, depth range, associated vegetation, and co-occurring species are all unknown. The possibility that the holotype's label data were in error compounds this uncertainty.
Feeding
No dietary data exist for Scissor macrocephalus. The species has never been observed feeding in the wild or in captivity, and its gut contents have not been reported in the literature.
The wide-cleft mouth, proportionally large relative to the head and body, is suggestive of a predatory or at least gape-limited feeding strategy, but this remains speculative in the absence of stomach-content analysis. Many large-headed, wide-mouthed characids feed on smaller fish or invertebrates, and the broad oral gape of S. macrocephalus may well reflect a similar niche — but without collection material or observation records that inference cannot be stated with confidence.
Mating
Nothing is known about the mating behaviour of Scissor macrocephalus. The species has never been maintained in captivity, and no field observations of courtship or spawning have been recorded.
By analogy with other open-water characids within Characidae, the species may be an egg-scattering broadcast spawner releasing adhesive or semi-adhesive eggs among fine-leaved vegetation or into the water column, with no parental care — the typical mode across the order. This is inference from family-level patterns rather than species-specific data, and it should be read as such.
Breeding
Scissor macrocephalus has never been bred in captivity, and reproductive biology details — clutch size, incubation period, fry development — are entirely unknown. The species has not been held in aquaria and appears never to have been the subject of husbandry trials.
Given that the species is known solely from a single museum specimen of uncertain provenance, there is no realistic pathway to captive breeding in the near term. Any future breeding data would require first re-collecting live individuals from the wild, which in turn requires confirming the type locality and conducting targeted fieldwork.
In the aquarium
Scissor macrocephalus is not kept in the aquarium hobby and has never entered the ornamental fish trade. No husbandry information, care requirements, or compatibility notes exist for the species.
Its complete absence from the hobby is a direct consequence of never having been re-collected since Günther's 1864 description. Even the type locality remains uncertain, making targeted collection impossible without prior survey work. The species is effectively a museum specimen species — known to science on paper but entirely outside the lived experience of aquarists.
Conservation
Scissor macrocephalus has not been assessed by the IUCN Red List. FishBase records its conservation status as Not Evaluated. Given that the species is known from a single holotype of uncertain origin and has not been recorded in over 160 years of subsequent collecting in South America, its status in the wild is completely unknown.
The absence of an IUCN assessment does not imply that the species is secure — it more likely reflects the difficulty of evaluating a taxon for which no population data, no confirmed range, and no recent records exist. Whether Scissor macrocephalus persists in a specific South American river system, represents a vagrant or mislabelled specimen, or is synonymous with a described species under another name are questions that only fresh fieldwork and molecular analysis could resolve. For now, it occupies that uncertain category of fishes that science knows of but cannot find.