Taxonomy & naming
Nannostomus harrisoni was described by Carl H. Eigenmann in 1909 from material collected in the Demerara River drainage of what is now Guyana. The valid name as listed in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is Nannostomus harrisoni Eigenmann, 1909. The genus Nannostomus belongs to the family Lebiasinidae, a group of small, elongate Neotropical characoids often called pencilfish or pencilfishes because of their slender, rod-like body form.
The specific epithet harrisoni is a patronym honouring Sir John Burchmore Harrison (1856–1926), who served as Government Geologist of Georgetown, British Guiana, and facilitated Eigenmann's collecting expeditions in the region. The genus name Nannostomus derives from the Greek nannos (dwarf) and stoma (mouth), a reference to the very small gape that characterises all pencilfish. Within Nannostomus, the species belongs to a clade of Guyanese pencilfish and shares its restricted drainage with no other member of the genus.
Morphology
Harrison's pencilfish is a slim, laterally compressed fish with the characteristic pencil-like profile of the genus. FishBase records a maximum standard length of 2 in, placing it at the smaller end of the Nannostomus range. The body is elongate, the snout bluntly pointed, and the mouth small and upturned, adapted for picking tiny food items from the water column and the surface film.
The colour pattern is typical of the genus: a broad, dark lateral stripe runs from the snout through the eye and along the midflank to the caudal peduncle, underlined by a paler gold stripe. The flanks are silvery to pale tan, and the fins are largely hyaline with a faint reddish tinge in the caudal lobes and sometimes in the dorsal. Sexual dimorphism is moderate; females tend toward a fuller abdomen when ripe, and males may show slightly more intense red in the tail. Colouration can intensify or fade rapidly depending on stress level, and fish maintained in good condition in well-planted, dimly lit tanks show far richer hues than stressed individuals.
Habitat
Nannostomus harrisoni is endemic to the Demerara River basin in Guyana, South America. Confirmed localities include tributaries and streams such as the Haiama, Kuruabaru, Haiakwa, and Haianari drainages. The species occupies sluggish, low-energy watercourses — small tributaries, swampy backwaters, and flooded forest margins — that carry dense aquatic and marginal vegetation, submerged woody debris, and deep accumulations of leaf litter.
These habitats are characteristically dark, tannin-stained, and soft. FishBase records water conditions of pH 5.5–7.0 and temperatures of 75–82 °F with hardness up to 4 dH; Seriously Fish cites pH 5.0–7.0, temperatures 72–82 °F, and hardness 18–179 ppm for the drainage, reflecting the range across seasons and localities. Light penetration to the forest floor is low, and the fish cruise at mid-water to just below the surface among plant stems and overhanging vegetation rather than in open water.
The species' distribution within Guyana appears localised even by pencilfish standards. There is no record of it outside the Demerara system, and within that basin its presence seems concentrated in particular slow-water habitats rather than distributed uniformly across the drainage.
Feeding
Harrison's pencilfish is a micropredator adapted to picking tiny invertebrates and zooplankton from the water column, the surface film, and submerged plant surfaces. The natural diet is presumed to consist of minute crustaceans, worm larvae, insect eggs and early-instar larvae, and microzooplankton — the same food guild exploited by other small Nannostomus species in similar habitats.
In captivity the species accepts a good range of small live and frozen foods: newly hatched Artemia (brine shrimp nauplii), Moina, micro worms, grindal worms, and small Daphnia. Dry foods in the form of finely crushed flake or micro-pellets are taken once the fish is settled, but the diet should be varied and weighted toward small live or frozen invertebrates for best condition and colouration. The small gape is a practical constraint: food particles must be genuinely tiny, and large flake or whole bloodworm will simply be ignored or spat out.
Mating
Nannostomus harrisoni is an egg-scattering open spawner with no parental care, following the pattern common to the genus. Courtship consists of males pursuing and displaying alongside females, adopting an angled or oblique body posture to show off their colouration, a behaviour seen across pencilfish. Spawning takes place among fine-leaved aquatic plants or at the surface near floating vegetation rather than over open substrate.
As with most Nannostomus species, males appear to compete loosely for access to females in a diffuse, non-territorial way, and the species does best when kept as a small group rather than in strict pairs. Condition and water chemistry have a pronounced effect on spawning readiness: females that are visibly full-bodied and males that are actively displaying are the most reliable spawners, and water changes with slightly cooler, soft water can trigger spawning activity.
Breeding
Breeding Harrison's pencilfish follows the standard Nannostomus protocol. A densely planted tank with fine-leaved plants such as Java moss, hornwort, or synthetic spawning mops provides the necessary spawning medium. The eggs are adhesive and small, deposited individually or in small groups among plant material near the surface. Adults will eat their own eggs and should be removed from the spawning tank once a clutch is detected — Seriously Fish notes that fry become visible within one to two days of removing the adults, indicating a short incubation period at typical temperatures.
The fry are tiny at hatching and require very fine first foods: infusoria, paramecia, rotifers (Brachionus), or commercial micro-fry products. Newly hatched Artemia nauplii can be introduced once the fry are large enough to handle them. Growth is slow and the fish remain delicate for the first few weeks; clean, soft, warm water with minimal flow is important during this period. Raising pencilfish fry requires patience and attention to water quality more than complexity of technique.
In the aquarium
Harrison's pencilfish suits a mature, well-planted nano or species tank of 10–15 US gal or larger. It is a peaceful and somewhat retiring fish that does best in the company of its own kind — a group of six to ten allows natural schooling and reduces individual stress — or with other small, non-competitive Neotropical species of similar temperament. Large or boisterous tankmates will outcompete it at feeding and prevent it from displaying properly.
Water chemistry should favour the soft, acidic conditions of its natural habitat: pH 5.5–7.0, hardness well below 10 dH, and temperature 75–82 °F. Subdued lighting, a dark or leaf-litter substrate, floating plants to diffuse light further, and the addition of dried Indian almond leaves or alder cones all help replicate the tannin-rich environment of the Demerara tributaries and bring out the best colouration. Gentle filtration — a sponge filter is ideal — maintains water quality without creating current the fish finds stressful. The species feeds near the surface and mid-water; feeding small live or frozen foods daily in quantities the fish consume within a few minutes keeps them in peak condition.
Conservation
Nannostomus harrisoni is assessed as Least Concern (LC) on the IUCN Red List; the assessment was published as part of the 2022.2 version of the Red List (IUCN taxon ID 49830644). The species is collected for the commercial aquarium trade and presumably also taken incidentally in small-scale subsistence fisheries in Guyana, though neither pressure has driven it to a threatened category. Its Demerara River basin range, while restricted compared with more widespread Neotropical fish, encompasses intact stretches of flooded forest and slow-water habitat that remain in reasonable condition.
The principal long-term risk for a species endemic to a single drainage system is habitat degradation through deforestation, agricultural runoff, and changes to the hydrology of the basin. Guyana retains a high proportion of intact tropical forest relative to much of South America, which provides some buffer, but the localised distribution of N. harrisoni within the basin means that localised habitat loss in its known tributaries could be disproportionately damaging. Wild-caught specimens do appear in the ornamental-fish trade, and aquarists keeping or breeding the species contribute to reduced demand for wild collection.