Taxonomy & naming
Acestridium dichromum was formally described by Retzer, Nico & Provenzano in 1999, published in Ichthyological Exploration of Freshwaters (volume 10, number 4, pages 313–326). The type locality is Pozo Lucas, a tributary of the Río Orinoco approximately 4 mi south of San Fernando de Atabapo, Amazonas State, Venezuela (4°02'N, 67°42'W).
The genus Acestridium belongs to subfamily Hypoptopomatinae — a group of small, often flattened loricariids that are sometimes informally called 'otocinclus relatives' or 'dwarf whiptails'. Acestridium itself is a small, slender genus whose members are superficially reminiscent of farlowellas or twig catfishes in body shape, though they are not closely related. The Catalog of Fishes (Eschmeyer, CAS) accepts Acestridium dichromum as a valid species.
The epithet dichromum — from Greek for 'two colours' — directly references the species' most remarkable trait: an active colour change between green and brown depending on background and light conditions.
Morphology
Adults reach approximately 2.5 in standard length (SL). The body is extremely elongate and slender, with a depressed profile and a long, tapering caudal region that gives the fish a twig-like silhouette in repose — a form that provides camouflage among aquatic stems and leaf petioles in its blackwater habitat.
The most remarkable feature of A. dichromum is its capacity for rapid colour change. At rest on green vegetation, the fish adopts a vivid mid-green coloration; when moved to darker substrates or under stress, it shifts to brown within minutes, earning it the informal name 'chameleon catfish' among hobbyists who have kept it. The mechanism involves chromatophores in the skin responding to background brightness.
Females in good condition are noticeably broader and more robust than males, particularly visible when viewed from above. The difference in body depth becomes pronounced once animals are well-acclimated and well-fed.
Habitat
Acestridium dichromum is distributed across the upper Río Orinoco and upper Río Negro basins, including the Casiquiare Canal corridor that connects these two great river systems — an area straddling Amazonas State (Venezuela) and adjacent Amazonas State (Brazil). GBIF occurrence records confirm presence in both Venezuela and Brazil.
The type locality and field observations place this species firmly in blackwater habitats: shallow, tannin-stained waters with extremely low mineral content (conductivity under 100 µS/cm), soft substrates of leaf litter and fine silt, and dense growth of aquatic and semi-aquatic vegetation. Water temperatures at collection sites fall in the range of approximately 77–84 °F. pH at occupied sites is strongly acidic, typically between 4.5 and 5.5.
Microhabitat centres on vegetation — the fish is observed mainly on the surfaces of aquatic plants and semi-aquatic stems, where it grazes biofilm and algae. It is not a typical rock or wood surface grazer in the manner of larger loricariids.
Feeding
In the wild, A. dichromum is a biofilm and algae grazer, feeding on the thin layer of micro-organisms, diatoms, and fine organic matter that colonises submerged plant surfaces, roots, wood, and leaf litter. The microhabitat focus on vegetation rather than rock suggests a preference for the soft biofilm that builds up on plant surfaces in slow-moving, nutrient-poor blackwater.
In captivity, the species accepts fine flake food, algae-covered stones, Spirulina and Chlorella tablets, and Artemia nauplii. Providing surfaces colonised by natural algae and biofilm — flat stones left under good lighting, or mature driftwood — will support the species' natural grazing behaviour. A varied diet that includes both algal and fine animal matter is preferable to a single food source.
Mating
Sexual dimorphism is the primary cue for identifying potential pairs: females in good condition are distinctly broader and deeper-bodied than males, a difference that becomes clear when animals are viewed from above. The degree of dimorphism is linked to acclimation and diet — recently imported or stressed animals may not show the difference clearly.
Detailed courtship behaviour has not been documented in published accounts. As with other small loricariids in the Hypoptopomatinae, pair conditioning in clean, warm, soft, and acidic water is the standard approach taken by hobbyists attempting to induce spawning.
Breeding
Captive breeding of Acestridium dichromum has not been conclusively documented in published hobby literature, though the species has been kept successfully in specialised collections. Eggs observed in captive fish are relatively large, greenish-oval, and approximately 0.5 in in diameter — consistent with a low-clutch-size reproductive strategy common in small loricariids. Males are reported to guard egg clutches.
The combination of very soft, acidic blackwater conditions, stable temperature, and high water quality appears essential to any breeding attempt. Stress from water chemistry outside the natural range likely suppresses reproductive behaviour. Patience and strict parameter control are required.
In the aquarium
Acestridium dichromum is one of the more challenging small loricariids in the hobby, not because of size or aggression, but because of its strict requirement for blackwater chemistry. Water must be extremely soft (conductivity below 100 µS/cm), strongly acidic (pH 4.5–5.5), warm (77–84 °F), and impeccably clean — any significant deviation in chemistry or water quality quickly causes deterioration.
A minimum tank volume of around 10 US gal is workable for a small group, provided filtration is gentle (the species is not adapted to strong currents) and water changes are performed with appropriately conditioned soft, acidic water. Hard tap water, even briefly, is harmful. Reverse-osmosis water softened with peat or blackwater extract is the practical approach for most keepers.
Tankmates must be chosen carefully. The species is not suited to general community tanks. Other blackwater specialists of similar size — small Apistogramma, pencilfish (Nannostomus), and similar Amazonian or Orinocan species sharing its water chemistry requirements — are appropriate if the tank volume allows. Boisterous or large tankmates will stress the fish.
Plants and fine-leaved vegetation, driftwood, and leaf litter complement the setup and encourage natural behaviour. The species uses plant surfaces actively for grazing and perching. Its colour-change ability is best observed in a tank with varied background colours.
Conservation
The IUCN Red List assessed Acestridium dichromum as Least Concern (LC) in October 2020, noting an absence of known major threats and distribution across the upper Orinoco and Río Negro systems — a large and relatively intact region of the Neotropical ichthyofaunal zone.
The upper Orinoco and Casiquiare corridor remain among the less impacted large-river systems in South America, though deforestation and associated changes to water chemistry and vegetation in headwater areas are a background concern for specialists of intact blackwater habitats. The species' strict requirements for very soft, acidic, tannin-rich water make it inherently vulnerable to any catchment-scale changes that alter the chemical character of these rivers.