Taxonomy & naming
Ctenocheirodon pristis was described by Luiz R. Malabarba and Daniel C. Jerep in 2012, in the peer-reviewed journal Copeia (volume 2012, issue 2), as both a new genus and a new species within the subfamily Cheirodontinae of the family Characidae. The type locality is in the upper Rio Tocantins drainage, Brazil. The valid name as recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is Ctenocheirodon pristis Malabarba & Jerep, 2012, the authors not parenthesised because the species has always been placed in the genus they created for it.
The subfamily Cheirodontinae — one of the major radiations within Characidae — comprises small, slender characids broadly distributed across South America. Ctenocheirodon is monotypic: only C. pristis is known from this genus. Its placement within Cheirodontinae is supported by shared morphological characters of the clade, while the unusual projecting procurrent caudal-fin rays distinguish it from all other genera in the subfamily.
Morphology
Ctenocheirodon pristis is a very small fish, with FishBase recording a maximum standard length of approximately 1.5 in in males and 1.5 in in females. The body plan is typical of Cheirodontinae — slender and laterally compressed — with a dorsal fin that FishBase records as carrying 11 soft rays. As in many cheirodontines, the species lacks the robust body depth of larger characids.
The genus-defining character is dramatic for so small a fish: in males, 16–19 ventral procurrent caudal-fin rays are elongated and project from the ventral caudal peduncle, forming a pronounced keel or serrated ridge on the underside of the caudal peduncle. This structure — analogous to the saw-toothed crest the species name references — appears to be the most readily observed diagnostic feature in the field. Whether females display this modification at lower intensity or lack it entirely is part of the original diagnosis, consistent with the description noting the feature most prominently in males.
Habitat
Ctenocheirodon pristis is endemic to the upper Rio Tocantins drainage in central Brazil. The Tocantins is one of the major rivers of the Brazilian interior, draining the Cerrado savanna and transitional zones before meeting the lower Pará system north of the Amazon mouth. The species is recorded from the upper portion of this drainage.
FishBase characterises the species as benthopelagic, occurring in clear water over sand and gravel substrates with slow current and little or no submerged vegetation. This habitat description is consistent with the oligotrophic, clear-water streams and side channels typical of the upper Tocantins headwaters in the Cerrado biome, where water is typically warm, well-oxygenated, and moderately acidic to neutral. The IUCN notes that the species occurs in permanent rivers and streams as well as seasonal and intermittent lakes within its range, suggesting some tolerance for fluctuating water bodies typical of the seasonally dry Cerrado landscape.
Feeding
No detailed diet studies specific to Ctenocheirodon pristis appear in the available literature. The Cheirodontinae are generally understood to be microinvertebrate and zooplankton feeders: species in the subfamily commonly consume small crustaceans, microcrustaceans, aquatic insect larvae, and in some genera algae or plant material. Given C. pristis's small size and benthopelagic habit over bare sand and gravel in clear water, a diet dominated by small invertebrates and zooplankton drifting in the water column or resting on the substrate would be consistent with the ecology of the subfamily.
In an aquarium context, if C. pristis is ever maintained by hobbyists, very small live and frozen foods — newly hatched brine shrimp, small Daphnia, micro-worms — would be appropriate starting points, supplemented by fine-particulate prepared foods.
Mating
No mating behaviour has been documented in the literature for Ctenocheirodon pristis. It is a member of Cheirodontinae, a subfamily in which open egg-scattering over fine substrates or vegetation is the predominant reproductive strategy and no parental care has been recorded. The pronounced sexual dimorphism in the ventral procurrent caudal-fin rays — strongly developed in males but not females — suggests these structures may play a role in courtship or male-male signalling, though this has not been formally investigated.
As with other small cheirodontines, spawning would be expected to involve scattering adhesive or semi-adhesive eggs among fine-leaved plants or over the substrate, with adults offering no subsequent care for eggs or larvae.
Breeding
Ctenocheirodon pristis has not been documented in captive breeding. As a newly described monotypic genus from a relatively remote part of the upper Tocantins basin, it does not appear to have entered the ornamental fish trade or been held in aquaria in any recorded capacity. The following is inferred from Cheirodontinae biology rather than documented observation.
Egg-scattering over fine vegetation or substrate would be expected, with adhesive eggs distributed in small numbers. Incubation at tropical temperatures typical of the upper Tocantins (approximately 73–82 °F) would likely produce larvae within a few days. Fry at this size class would require very fine first foods — infusoria-grade or rotifers — before advancing to newly hatched brine shrimp. No specific captive breeding data are available, and any aquarist attempting this species should treat it as an undocumented breeding challenge.
In the aquarium
Ctenocheirodon pristis does not appear to be available in the ornamental fish trade. As the sole species of a monotypic genus described only in 2012 from the upper Tocantins — a basin remote from major ornamental fish collecting routes — it has not been documented in hobbyist or commercial aquaculture contexts as of the available sources.
If the species were to become available, its small size (under 1.5 in SL), habitat in clear, slow, warm Cerrado water, and cheirodontine membership would suggest care consistent with other small, peaceful characids from similar environments: soft to moderately hard, slightly acidic to neutral water in the low-to-mid 20s Celsius, a school of several individuals, and a diet of small live and frozen invertebrates. Its most distinctive feature — the male's serrated ventral caudal keel — would make a striking display in a species or biotope aquarium for the dedicated specialist.
Conservation
Ctenocheirodon pristis is assessed as Least Concern (LC) on the IUCN Red List, with the assessment conducted by Flávio Lima on 4 March 2021 and published in 2023. The justification cites a relatively wide distribution within the upper Rio Tocantins and characterises the species as moderately common and resilient for a cheirodontine. Population trend is recorded as stable.
Despite the Least Concern status, the IUCN assessment notes threats from water abstraction for irrigation and dam construction in the Rio Paranã basin, a major Tocantins tributary within the species' range. The Tocantins basin has seen significant hydroelectric development, including the Tucuruí dam — one of the largest in the world — further downstream, and ongoing damming in the upper basin presents long-term risks to the clear-water habitat the species occupies. No formal conservation measures targeting C. pristis are reported.