Taxonomy & naming
Acestridium triplax was described by Marco S. Rodríguez and Roberto E. Reis in 2007 in Neotropical Ichthyology (volume 5, number 4, pp. 429–434), from specimens collected in the igarapé Mutum, a small forest stream tributary to the rio Aruã → rio Branco → rio Aripiuns → lower rio Tapajós, in the Município de Juruti, Pará state, Brazil (2°36′45″S, 56°11′37″W). The Catalog of Fishes (Eschmeyer, CAS; species id 70028) recognises the valid combination Acestridium triplax Rodríguez & Reis, 2007.
The genus Acestridium was established by Haseman in 1911 and diagnosed by Schaefer (1991) on characters including ten principal caudal-fin rays; plates at the anterior snout margin forming a conspicuous spatulate projection; an extremely slender and depressed head and body; longitudinal ridges on the dorsal surface of the head and trunk; the dorsal fin positioned posteriorly (beyond the pelvic fin and just anterior to the anal-fin origin); absence of an adipose fin; and dermal plates anterior to the first dorsal-fin radial that are expanded with a ventromedial keel.
The species epithet triplax derives from the Greek tri (three) and plax (plate), directly referencing the diagnostic character that distinguishes A. triplax from all other members of the genus: the presence of one series of middle abdominal plates between the two lateral abdominal plate rows (versus middle plates absent in all congeners). The genus name Acestridium is derived from the Greek agkistron (hook), Latinised.
Morphology
Acestridium triplax reaches 2 in standard length — moderate for the genus, which comprises very small to small catfishes. The body is extraordinarily slender and depressed, with the extreme elongation and flat profile typical of Acestridium, resembling a stick or twig even more than the related Farlowella. Longitudinal ridges run along the dorsal surface of the head and trunk, and the dorsal fin is placed unusually far posterior on the body, originating beyond the pelvic fins and just anterior to the anal-fin origin.
The defining diagnostic character of the species is the presence of a single series of middle abdominal plates running between the lateral abdominal plate rows — a three-row abdominal plate arrangement unique among Acestridium. All other species in the genus lack this middle plate series. Additionally, A. triplax is distinguished from Acestridium dichromum (its closest relative by morphology) by: a small, rounded snout-tip spatulate projection (versus large and oval in A. dichromum); a thick pectoral-fin spine that is unserrated or only slightly so on the inner face (versus very thick and strongly serrated); a small posterior-most lateral abdominal plate (versus large); and a preanal plate that is approximately rectangular with short lateral projections (versus cross-shaped with large projections).
No adipose fin is present, consistent with the Acestridium genus diagnosis. Dorsal soft rays number 8; anal soft rays 6.
Habitat
Acestridium triplax is known from small forest streams (igarapés) in the lower Tapajós drainage, Amazon basin, Pará state, Brazil. The type-locality igarapé Mutum is described as a shallow stream — 1–3 ft deep and approximately 16–33 ft wide — with a sandy bottom, clear to slightly blackwater coloration, moderate current, and dense aquatic and marginal vegetation. Depth range in the database is 0–3 ft, consistent with this small-stream, shallow-water existence.
This microhabitat profile — clear or blackwater forest streams, sand substrate, shallow, vegetated margins — is characteristic of many Amazonian Hypoptopomatinae. The extremely cryptic, stick-like body of Acestridium presumably provides camouflage among the leaf litter, fallen twigs, and submerged root mats common in such environments. The species is unlikely to occur in larger river channels or turbid whitewater streams.
Feeding
Acestridium triplax is an aufwuchs and biofilm grazer. Its ventral mouth and fine teeth are adapted for scraping the thin microbial and algal film from hard and semi-hard surfaces — submerged wood, plant stems, roots, and smooth stones. At 2 in SL, the mouth is very small, and only fine food particles or surface biofilm are practical targets.
In the aquarium, sinking algae wafers, spirulina-based foods, and the natural biofilm of a mature, well-lit tank with submerged driftwood and plants would form the appropriate diet. Soft vegetable matter (cucumber, blanched courgette) may also be accepted. Protein-rich foods are not a natural component of the diet. Given the species' small size and delicate nature, competition at feeding time from larger or more aggressive tankmates should be minimised.
Mating
No courtship observations for Acestridium triplax are documented in the available literature, and the species is not established in the hobby. The reproductive biology of the genus is poorly characterised overall.
By inference from related hypoptopomatines, breeding males may develop odontodes on the snout, cheeks, or pectoral spines during the reproductive season, and females are likely distinguishable by a fuller body profile when gravid. The extreme slenderness of this genus may make sex determination at other times of year difficult without close examination.
Breeding
No captive breeding records for Acestridium triplax exist in the available literature; PlanetCatfish lists no breeding reports. The species is not known in the trade. Reproductive mode for the genus is uncharacterised.
Given the habitat preferences and body plan of Acestridium, spawning in dense vegetation, root tangles, or crevices of submerged wood within shallow stream margins seems plausible — consistent with the shelter-seeking behaviour implied by its cryptic form. Captive breeding, if ever attempted, would likely require careful replication of the type-locality conditions: shallow, clear to slightly tannin-stained water, sandy substrate, dense plant cover, and minimal disturbance.
In the aquarium
Acestridium triplax is not in the ornamental trade and has no established aquarium record. PlanetCatfish notes the species is not reported in trade. Its extreme slenderness, small size, and specialised habitat requirements would make it a challenging subject even for an experienced keeper of small loricariids.
Should specimens ever become available, care conditions should replicate the type-locality as closely as possible: a small, mature aquarium with a sandy substrate, submerged driftwood and root-like structures providing cover, dense planting, soft and slightly acidic to neutral water, and gentle flow. The species would be best kept as a species tank or with extremely peaceful, similarly-sized companions. Its biofilm-grazing diet requires an established, well-matured tank rather than a newly set-up aquarium. Photographic documentation of its maintenance and behaviour would be a genuine contribution given how little is known.
Conservation
The IUCN Red List assessed Acestridium triplax as Least Concern in November 2018. The species is known from the lower Tapajós drainage in Pará state, within the Brazilian Amazon.
The Tapajós basin is increasingly subject to deforestation pressure and is a flashpoint for infrastructure development, including the controversial Tapajós hydroelectric complex and associated road networks. Small forest-stream specialists like A. triplax are potentially sensitive to the sedimentation, temperature increases, and loss of riparian cover that accompany deforestation, even if they do not appear in river-channel surveys affected by dam construction. Ongoing land-use change in the western Pará region warrants monitoring of such microhabitat-dependent species.