Taxonomy & naming
Trachelyichthys epachrus was described in 2026 by Bruno B. Calegari, Alany Akama, Carl Ferraris Jr., and Roberto E. Reis, as part of a revision that expanded the genus Trachelyichthys from two recognised species to six. Before this study, the genus — erected by Mees in 1974 — had gone more than four decades without a new species, its two nominal members, T. decaradiatus and T. exilis, each known from only a handful of specimens. Trachelyichthys belongs to the family Auchenipteridae (the driftwood catfishes), subfamily Auchenipterinae, and current phylogenetic work places it in the tribe Auchenipterichthyini, as sister to the genus Trachelyopterichthys.
In the genus's phylogeny, T. epachrus — referred to as 'Trachelyichthys "brown Madeira"' in earlier molecular work — is placed among the earlier-diverging Trachelyichthys lineages, alongside T. gaiophanes from the upper rio Negro. The species is diagnosed from every other member of the genus by a single distinctive skull character: the posterior process of the epioccipital bone is a simple, unbifurcated spine that does not contact the parapophysis of the fifth vertebra, whereas in all its relatives that process is forked and sutured to the vertebra. This osteological feature, confirmed on cleared-and-stained specimens, gives the species its name.
Morphology
Trachelyichthys epachrus is one of the smallest known species in its genus, reaching a maximum of 1.6 in standard length among the four known specimens — a paratype, slightly larger than the 1.56 in SL holotype, a mature male. The body is roughly cylindrical from the head through the abdomen, increasingly compressed toward the caudal fin, with a broadly rounded head and a short, rounded snout. As in all Trachelyichthys, there is no adipose fin and the skin bears no scales.
Colouration is uniform light to medium brown over the entire body, without the dark spots or blotches seen in most congeners — a pattern shared only with T. gaiophanes. The posterior nuchal plate is expanded laterally and ventrally, another trait shared with T. gaiophanes. The dorsal fin has 4 soft rays on three free pterygiophores; the pectoral fin has 5 rays with a strongly serrated spine; the anal fin is unusually long for the genus, with 42–45 total rays, more than any other Trachelyichthys species. The caudal fin is truncate, with the dorsal lobe slightly longer than the ventral lobe.
Habitat
Trachelyichthys epachrus is known from only two localities in the middle rio Madeira basin of Amazonas, Brazil: the rio Marmelos above the mouth of igarapé Escondido, and a stream near the Nove de Janeiro Indigenous Reserve on the road to Apuí. Both are small blackwater tributaries, a notable detail because the rio Madeira as a whole is a sediment-rich whitewater river with headwaters in the Andes. The species description singles out T. epachrus as inhabiting small blackwater environments embedded within an otherwise whitewater drainage, one of several disjunct blackwater pockets across the Amazon basin that Trachelyichthys species appear to depend on.
Like other driftwood catfishes, it is presumed to occupy submerged wood and root tangles during the day, though this has not been directly documented for the species.
Feeding
No dietary data have been published for Trachelyichthys epachrus specifically. The only ecological study within the genus — Freitas & Zuanon (2022), which found a diet predominantly carnivorous and tending toward piscivorous — was based on specimens now recognised as a different species, T. displicatus, from the Santarém region. Whether T. epachrus shares that diet is plausible for a genus given to carnivory, but it remains unconfirmed for this species.
Mating
Nuptial male Trachelyichthys epachrus develop the anal-fin modification typical of the subfamily Auchenipterinae: the anterior anal-fin rays form a genital tube that functions as an intromittent organ, delivering sperm directly into the female for internal fertilisation — a reproductive strategy shared by essentially the whole family and rare elsewhere among catfishes. The species description notes no corresponding modification of the maxillary barbel or the dorsal fin in males, unlike some of its congeners.
No further reproductive data — timing, egg number, or courtship behaviour — have been published for this species.
Breeding
Trachelyichthys epachrus has never been bred, or even kept, in the aquarium. As in other Auchenipteridae, internal fertilisation is presumed to let the female store sperm and lay fertile eggs some time after mating, with eggs deposited adhesively on wood or other submerged surfaces and little or no parental care — a family-level pattern, not one confirmed directly for this species.
In the aquarium
Trachelyichthys epachrus is known from only five museum specimens and has never appeared in the aquarium trade. No husbandry information exists for this species.
Extrapolating from the driftwood-catfish family in general, and labelled clearly as inference rather than documented care: a small, soft-water aquarium with tannin-stained water, subdued lighting, and dense submerged wood for cover would likely suit a fish of this size and build. Like all auchenipterids it is scaleless, so careful netting and cautious, copper-free medication would be standard precautions if the species were ever kept.
Conservation
Trachelyichthys epachrus has not been formally assessed by the IUCN Red List, having only been described in 2026. The authors recommend a category of Least Concern under IUCN criteria, on the grounds that the species occurs in an area that has not been densely sampled and may prove more widespread through the middle rio Madeira basin than the five known specimens suggest; no specific threat has been identified, though data on abundance and population trend are unavailable. This is a recommendation from the species description, not a formal Red List assessment.