Taxonomy & naming
Tridensimilis magnus was described in 2026 by André L. C. Canto, Alexandre Datovo, Willian M. Ohara, Mário C. C. de Pinna and Fernando R. V. Ribeiro, in a paper in Neotropical Ichthyology that both introduced the new species and revised the taxonomic limits of the genus Tridensimilis. It belongs to Trichomycteridae, subfamily Tridentinae, a group of small, sand-dwelling catfishes currently comprising five genera and, with this description, roughly a dozen species — most known only from their original, decades-old descriptions.
The holotype, MZUSP 131529 (1.2 in SL), was collected from igarapé Branco, a tributary of rio Aruri Grande in the rio Jamanxim drainage of the rio Tapajós basin, Itaituba Municipality, Pará State, Brazil. Paratypes were taken from multiple tributaries across the upper and middle Tapajós and Teles Pires drainages. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) records Tridensimilis magnus Canto, Datovo, Ohara, de Pinna & Ribeiro, 2026 as the currently valid name.
The description also revisited the generic placement of Tridentopsis tocantinsi, a species known only from its holotype (missing for decades). The authors found that its radiographed skeleton shows an apparently separated upper caudal plate — a condition otherwise unique to Tridensimilis — along with ventrally exposed eyes and an absent nasal barbel, characters shared with Tridensimilis rather than Tridentopsis. They stopped short of a formal generic transfer pending further study, but flagged the placement as questionable.
Morphology
Tridensimilis magnus is diagnosed principally by size: the type series reaches 1.34 in standard length, exceeding the previous maximum recorded for the genus (about 0.91 in in other Tridensimilis) and for every other tridentine genus — 1.06 in in Tridens, 0.98 in in Tridentopsis, 0.71 in in Rhinotridens, and 0.67 in in Miuroglanis. The paper's title describes it as the largest translucent catfish species of Tridentinae, reflecting the see-through, lightly pigmented body typical of the subfamily.
Other diagnostic features include 39–41 post-Weberian vertebrae (versus 34–38 in most other tridentines and 45–52 in Tridens); maxillary barbels long enough to surpass the opercular odontophore; externally visible rictal barbels reaching the base of the interopercular odontophores; 7 or 8 dorsal-fin rays; and a pelvic-fin base aligned with the 13th or 14th vertebra rather than the 11th. As in other tridentines, the eyes sit laterally on the head and are oriented more ventrally than dorsally, and the basipterygia (pelvic girdle elements) are fused at the midline.
Habitat
The type locality, the rio Aruri Grande in the rio Jamanxim drainage of the Tapajós basin, is a clear-water stream roughly 30 ft wide and 19.7 in deep with slow current, bordered by extensive riparian forest. At the time of collection, water temperature was 77 °F, pH 5.9, conductivity 27.4 μS/cm, and dissolved oxygen 5.88 mg/L. Substrate at the collecting sites was predominantly coarse yellow sand, and specimens were taken almost exclusively on sandbanks and small beaches during the dry season.
Tridensimilis magnus shares its habitat with three other trichomycterids — Ochmacanthus reinhardtii, Stegophilus panzeri and Hyaloglanis nheengatu — and has been recorded across several tributaries of the upper and middle Tapajós and Teles Pires rivers, an area the authors suggest may hold further endemic diversity.
Feeding
No gut-content or diet data have been published for Tridensimilis magnus, but the description records a striking behavioral first for the subfamily: live specimens were observed capturing atmospheric air at the water surface, filling the digestive cavity with bubbles — the first record of this air-gulping behavior in any tridentine. Its function (respiration, buoyancy control, or something else) was not established.
Activity patterns varied by site: at the type locality and at igarapé Miaçú, specimens were collected during the day, remaining associated with sandy substrate and presumably concealed for camouflage; at other sites along the Tapajós, individuals were captured at dusk or early night swimming in the water column, suggesting they move up from the sand to forage as light fades. The authors compare this pattern to other psammophilous (sand-dwelling) Amazonian fishes such as Gymnorhamphichthys rondoni and Mastiglanis asopos, which combine cryptic daytime concealment with nocturnal activity.
Mating
No courtship or spawning behavior has been documented for Tridensimilis magnus; the description is a taxonomic and morphological study and does not address reproduction.
Breeding
Breeding biology is entirely undescribed for this species, and it has never been kept or bred in captivity.
In the aquarium
Tridensimilis magnus is not part of the aquarium trade and has no husbandry record. At a maximum of 1.3 in standard length it is among the smallest catfishes described, and its ecology — buried in fine sand by day, moving into open water at dusk — would make it an extremely demanding subject even if it were ever collected for the hobby. Any care guidance would be pure speculation; none is offered here.
Conservation
Tridensimilis magnus has not been formally assessed by the IUCN Red List. The describing authors recommend a preliminary Least Concern classification, citing a wide extent of occurrence, apparent local abundance, and no observed decline in range or population — their own suggestion from the description paper, not a formal IUCN listing.
The species' broader range in southwestern Pará State faces substantial pressure from illegal logging, agricultural expansion and gold mining, but the smaller tributaries where Tridensimilis magnus has been recorded drain areas that are comparatively well protected, falling within the Amazon National Park and Jamanxim National Park.