Taxonomy & naming
Hemigrammus cylindricus was described by Mary D. Durbin in 1909, with the type locality recorded as Tumatumari on the Potaro River in the Potaro-Siparuni region of Guyana, within the broader Essequibo drainage. The authority record is held in Eschmeyer's Catalog of Fishes and the Catalogue of Life. The species remains placed in Hemigrammus, one of the most species-rich genera in the family Characidae, though the limits of that genus have been discussed repeatedly in the literature.
A point worth noting is that the ETYFish Project places etymological notes for this species under the Acestrorhamphidae page, reflecting the long-running debate over the circumscription of Characidae and the provisional placements of various small-characid genera; the Catalog of Fishes retains Hemigrammus cylindricus within Characidae as understood here. Any future molecular phylogenetic revision may reshuffle generic boundaries, but the species epithet and authority are stable.
FishBase also records a distribution note from Eigenmann (1918) referencing the upper Amazon near Igarapé Préto in Brazil, suggesting the species may range across a wider part of the Guiana Shield drainages than the type locality alone implies. Museum material is held at the Academy of Natural Sciences, Philadelphia (ANSP 170200).
Morphology
The species reaches a reported maximum length of approximately 4.5 in, making it one of the larger members of its genus. The most diagnostically useful character is the body form: Eigenmann (1918) described it as 'subcylindrical, only slightly compressed' — a notably un-hemigrammus-like profile that gives rise to both the scientific name and the English common name 'Cigar-shaped Tetra'.
Beyond this cylindrical cross-section, detailed colour and meristic descriptions are sparse in the accessible literature. Most Hemigrammus species carry a lateral stripe, a caudal spot, or a combination of both, and share the large eye, deeply forked caudal fin, and scaled caudal peduncle that characterise the genus. Published figures for H. cylindricus specifically are not widely reproduced in the hobby or secondary literature, and the species is rarely encountered in the aquarium trade, so photographic documentation remains limited.
Habitat
The type locality is Tumatumari, a site on the Potaro River in the Potaro-Siparuni region of central Guyana. The Potaro is a major tributary of the Essequibo, draining interior Guiana Shield terrain characterised by rocky rapids, deep pools, blackwater streams, and gallery forest. FishBase classifies the species as benthopelagic and freshwater, with a tropical distribution across northern South America.
A distributional note in the literature also places the species near Igarapé Préto in the upper Amazon basin of Brazil, which, if confirmed, would indicate the species occupies multiple Guiana Shield drainages. Water conditions typical of these habitats — warm, soft, and mildly acidic — are likely representative, though no detailed water-chemistry measurements specific to confirmed H. cylindricus localities are available in the published record. FishBase notes a temperature reference of 73 °F and a pH of 6.5 as indicative parameters.
Feeding
FishBase lists the diet of Hemigrammus cylindricus as insectivorous, consistent with the feeding ecology of most small characids from the Guiana Shield region. In the wild these fishes typically take small aquatic invertebrates — insect larvae, microcrustaceans, and surface-fallen insects — as the primary prey, supplemented opportunistically by whatever invertebrate prey is available at the water surface or in the water column.
No detailed gut-content studies for this species appear to have been published. In aquarium care, small-characid insectivores of this type generally accept high-quality flake and micro-pellet foods readily, and are brought into best condition with regular supplements of live or frozen foods such as Daphnia, brine shrimp, and bloodworm.
Mating
No specific mating behaviour has been described for Hemigrammus cylindricus in the published literature or in hobbyist accounts. The species is presumed to follow the typical characid pattern: egg-scattering open spawning with no parental care, in which ripe males pursue females through fine-leaved vegetation or over open substrate, releasing sperm concurrent with the females' egg deposition.
Adhesive or semi-adhesive eggs are likely deposited among fine-leaved plants or over the substrate, as is typical for the genus. Males in spawning condition generally show heightened activity and may exhibit minor intensification of any colour markings present. No courtship display specific to this species has been documented.
Breeding
Captive breeding of Hemigrammus cylindricus is not documented in the hobbyist or scientific literature available, reflecting the species' rarity in the trade. By analogy with other Hemigrammus species of similar size, conditioning would likely involve raising temperature slightly into the mid-to-upper 20s Celsius, offering a high-quality live-food diet, and providing fine-leaved plants or spawning mops as egg-deposition sites.
Eggs in egg-scattering characids of this type are typically small, adhesive, and photosensitive; adults should be removed after spawning to prevent egg predation. Fry would be expected to hatch within 24–48 hours at warm temperatures and require microscopic first foods such as infusoria or rotifers before graduating to newly hatched brine shrimp nauplii. Actual breeding data for this species remain absent from the record.
In the aquarium
Hemigrammus cylindricus is essentially absent from the mainstream aquarium trade and has not been established in hobbyist collections to any documented extent. Its large size for the genus (nearly 4.5 in) and the rarity of any commercial collection from the Potaro drainage of Guyana mean that aquarium-keeping notes are non-existent in English-language hobby literature.
If the species were to become available, the general framework for a large Hemigrammus would apply: a spacious tank with a long swimming corridor, soft and mildly acidic water in the 72–81 °F range, a school of six or more individuals, and fine-leaved vegetation for structure. Its insectivorous diet maps well onto standard live and frozen invertebrate foods. Any keeper working with this species would be working in a near-total information vacuum and contributing new knowledge to the record.
Conservation
FishBase cites an IUCN assessment of Least Concern (LC) for Hemigrammus cylindricus, dated 12 November 2020, reflecting the species' apparent broad presence across the Essequibo system and its tolerance of typical Guiana Shield freshwater habitats. The Potaro River and its tributaries remain among the less-degraded large drainages of the Guiana Shield, with limited agricultural or industrial pressure compared to lowland Amazonian rivers.
Nonetheless, the species is documented in relatively few museum collections, and the combination of its limited representation in the trade, sparse published literature, and restricted type locality warrants continued monitoring. The annotated checklist of Guyana's freshwater fishes (ANSP 170200 specimen records) provides one of the more accessible modern distributional references. No targeted conservation programme for H. cylindricus is known to exist.