Taxonomy & naming
Copella arnoldi was originally described by the British ichthyologist Charles Tate Regan in 1912 on the basis of specimens that reached Europe through the ornamental-fish trade. The parenthetical authority reflects a subsequent generic reassignment: Regan placed it in a genus that has since been redefined, and the name Copella Myers, 1956 is now accepted for the group. Eschmeyer's Catalog of Fishes recognises the valid name as Copella arnoldi (Regan, 1912).
Copella belongs to the family Lebiasinidae, a lineage of small, slender-bodied, mostly Neotropical fishes that includes the pencilfish genus Nannostomus and the pyrrhulinas. Within Lebiasinidae, Copella is characterised by its splash-spawning reproductive strategy, which appears unique within the family. The genus name Copella is a diminutive of Copeina, itself a name derived in honour of the American ichthyologist Edward Drinker Cope; the relationship to Cope is thus indirect, mediated through the older genus name.
Some older aquarium literature lists this species under the genus Copeina or as Copeina arnoldi; these are synonyms and the name Copella arnoldi is the current accepted form. In hobbyist usage it is sometimes grouped loosely with 'tetras', reflecting its South American origin and small size rather than any formal family-level affinity with the characid tetras.
Morphology
Copella arnoldi is a slender, torpedo-shaped fish with a noticeably elongated body plan suited to life near the surface in still or slow-moving water. Males reach approximately 1.5 in standard length; females are consistently smaller, typically around 1.5 in SL. In aquarium conditions standard lengths of 2 in have been reported for well-grown adults, the larger figures likely reflecting total length. The body is laterally compressed but narrow in cross-section, and the caudal fin in males shows distinctive extension of the upper lobe — a feature that plays a role in the splash-spawning behaviour.
Colouration is subdued but attractive. The body is a warm brownish-gold to olivaceous above and paler below, with a diffuse, dark lateral stripe running from behind the eye to the caudal peduncle. Scales reflect a faint metallic sheen in good light. Males typically display richer colouration, a more pronounced red or orange tint in the fins, and the elongated upper caudal lobe. Females are plainer and more rounded in the belly, particularly when in spawning condition. Overall, the species is not gaudy — its appeal lies in behaviour rather than colour.
Habitat
In the wild, Copella arnoldi inhabits the lower Amazon basin and associated coastal drainages of Pará and Amapá in Brazil, as well as Guyana, Surinam, French Guiana, and the mouth of the Orinoco basin in Venezuela. It is a fish of slow-moving streams, minor tributaries, and seasonally flooded forest margins — shallow, densely vegetated waters with abundant overhanging riparian vegetation. Blackwater and tannin-stained conditions are characteristic, and the fish are typically found within the top 8–12 in of the water column.
The presence of overhanging leaves above the waterline is a non-negotiable habitat element, as it provides the out-of-water spawning substrate the species depends on. Water chemistry across this range tends toward soft and acidic, consistent with the Guiana Shield and lowland Amazon forest: pH 6.0–8.0 has been recorded, with moderate hardness (5–12 dH) and temperatures of 77–84 °F (77–84 °F). The species is absent from fast-flowing, open, or deeply shaded water bodies without marginal leaf litter and riparian canopy.
Feeding
Copella arnoldi is a micropredator that forages at or near the water surface. The natural diet is dominated by small invertebrates: mayfly (Ephemeroptera) larvae, ants, terrestrial insects that fall onto the water surface, small worms, and aquatic crustaceans. The upward-angled mouth and body posture reflect a surface-oriented feeding strategy, and the fish is capable of short aerial leaps to intercept prey just above the waterline — a behaviour that likely co-evolved with the out-of-water spawning habit.
In the aquarium, Copella arnoldi readily accepts a variety of small live and frozen foods: Artemia nauplii, Moina, grindal worms, micro-worms, Daphnia, and small bloodworm. It will also take good-quality flake food sized appropriately for its small mouth, though a diet based primarily on live and frozen invertebrates keeps the fish in better condition and colour. Feeding at the surface or just below it is typical, and competition from mid-water or bottom-dwelling tankmates that claim surface food quickly can disadvantage this species.
Mating
The courtship behaviour of Copella arnoldi is as specialised as its spawning. Males display to females near the surface beneath candidate overhanging leaves, using lateral spreading of the fins and erect posture to advertise quality. Pairs select a leaf — typically broad-leaved riparian plants that project over the water — whose underside will serve as the egg substrate; the critical feature is that the leaf surface must be above the waterline and accessible to a jumping fish.
Once a site is selected, the male and female leap simultaneously or in rapid succession, pressing their bodies against the underside of the leaf for a brief moment during which eggs and milt are released. A small batch of eggs is deposited and fertilised in each leap. The pair repeat this sequence until the full clutch of roughly 100–200 eggs has been laid, with individual egg batches adhering to the damp leaf surface. The male's extended upper caudal lobe is thought to assist in the physical act of clinging briefly to the leaf during the egg-laying leap, though the exact mechanics are difficult to observe in the wild.
Breeding
After spawning is complete, the female's role ends. The male remains below the egg cluster and, approximately once per minute for the next 36–72 hours, leaps and splashes the eggs with vigorous beats of his caudal fin, coating them with water droplets to prevent desiccation. This parental behaviour is one of the most precisely documented in freshwater fishes and is essential: unattended eggs rapidly dry out and die. Clutch size is typically 100–200 eggs. At temperatures in the range of 77–82 °F (77–82 °F) the eggs hatch in approximately 36–72 hours, and the newly hatched fry drop into the water below.
In the aquarium, replicating this spawning requires an overhanging surface above the water: a broad-leaved floating plant such as Echinodorus or a piece of slate or plastic sheeting positioned a few centimetres above the waterline works well. A tight-fitting cover glass is essential, both to create a humid air space that prevents the eggs from drying and to prevent the jumping fish from escaping. The tank should be kept lightly stocked and well-conditioned with live and frozen foods before attempting to breed. Fry are tiny on hatching and initially require infusoria or commercial liquid fry food before graduating to freshly hatched Artemia nauplii. Growth is reported to be slow.
In the aquarium
Copella arnoldi is best suited to a quiet, species-specific or single-species-dominant setup, or a community of similarly small, peaceful, surface-oriented fishes. A tank of 23.5–31.5 in length is adequate for a small group of six to ten, provided the water surface is partially covered by broad-leaved floating plants or overhanging terrestrial plants, which both replicate the natural environment and serve as spawning sites. The air space above the water should be warm and humid — the standard approach is a tight cover glass with a small gap — and surface agitation should be gentle.
Water conditions should match the Amazonian origin: soft, slightly acidic to neutral water (pH 6.0–7.5, 5–12 dH) at 77–84 °F (77–84 °F). The fish is sensitive to poor water quality and does not thrive in hard, alkaline conditions. Regular partial water changes with dechlorinated soft water are important. A tight-fitting lid is non-negotiable: Copella arnoldi is an accomplished jumper and will exit through any gap. Tankmates should be small, non-boisterous, and ideally surface-tolerant species that will not compete aggressively for the top of the water column. Avoid large or nippy companions entirely.
Conservation
Copella arnoldi is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment dated 11 September 2020. The species has a broad distribution across the Guiana region and lower Amazon basin, occupying a range that encompasses multiple countries and large areas of relatively intact lowland forest habitat. No major population-level threats have been identified that would warrant a higher category, and it remains common within its range.
That said, the species' dependency on intact riparian vegetation and overhanging leaf substrate makes it sensitive to the type of habitat degradation — bank clearance, channelisation, and deforestation — that is ongoing at scale across the Amazon and Guiana lowlands. As with many small, habitat-specialist fishes, the apparent security of an LC assessment can mask local declines where river margins are degraded. In the aquarium hobby the species has been kept and bred since the early twentieth century, and captive-bred stock is widely available, meaning collecting pressure on wild populations from the ornamental trade is likely low.