Taxonomy & naming
Hypostomus tenuis was described by Marinus Boeseman in 1968 in his comprehensive monograph on Surinamese Hypostomus (Zoologische Verhandelingen, Leiden, No. 99, p. 64, Plate 14, figs. 3–4). The holotype, RMNH 16198, is the only known specimen. The type locality was recorded by Boeseman as 'near Paramaribo, Suriname', but this locality has subsequently been shown to be erroneous by Weber (in Reis et al. 2003, Check List of the Freshwater Fishes of South and Central America); the actual provenance of the holotype is unknown.
The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination Hypostomus tenuis Boeseman, 1968. The species was also assigned to the segregate genus Squaliforma by Isbrücker et al. in their reclassification work (2001–2012), as Squaliforma tenuis; current consensus, following Armbruster (2004) and later authors, restores the species to Hypostomus.
Within Loricariidae, Hypostomus tenuis is placed in subfamily Hypostominae. Its slender, elongate body and remarkably long caudal peduncle distinguish it immediately from more robust Hypostomus relatives, though the functional or ecological significance of this body form is not well understood given the absence of field data.
Morphology
The species reaches at least 7.5 in standard length (SL) based on the holotype, making it a moderately large Hypostomus. Its most immediately striking character is extreme slenderness: a very elongate body with a remarkably long, thin caudal peduncle — the feature that inspired both the Latin epithet and the common descriptor 'slender suckermouth'. The body is armoured in typical loricariid fashion with overlapping bony scutes.
Full morphological details — fin ray counts, scute row counts, keeling, and coloration — are established in Boeseman's (1968) original description. As only one specimen is known, natural variation within the species cannot be assessed.
Sexual dimorphism has not been documented. Broader genus patterns involve elaboration of interopercular odontodes in breeding males, but this cannot be confirmed for H. tenuis from a single unsexed specimen.
Habitat
The habitat of Hypostomus tenuis is unknown. The original type locality — 'near Paramaribo, Suriname' — has been identified as erroneous (Weber in Reis et al. 2003), and no subsequent collection records have surfaced to establish where the species actually occurs. The IUCN assessment (2021) tentatively assigns it to the Suriname River basin, but this assignment is constrained by the same provenance uncertainty that affects the type locality.
Without confirmed field data, it is not possible to characterise the water chemistry, substrate, current regime, or microhabitat of this species. Analogies with other Surinamese Hypostomus from Boeseman's 1968 series — which represent rivers across the Guiana Shield — provide only the broadest inference: tropical, freshwater, likely blackwater or clearwater drainages with acidic to neutral, soft water typical of Guiana Shield rivers.
Feeding
Diet has not been studied for Hypostomus tenuis. Within the genus, most species are opportunistic grazers of algae and periphyton (aufwuchs) from hard substrates, supplemented by detritus, fine organic matter, and invertebrates. Without dentition analysis of the holotype being available in secondary sources consulted, the specific dietary guild of H. tenuis cannot be precisely placed.
In aquarium contexts — were the species ever to become available — a generalist Hypostomus diet would be a reasonable starting point: algae wafers, vegetable matter, sinking pellets, and driftwood as a behavioural and dietary supplement.
Mating
No information on mating behaviour exists for Hypostomus tenuis. The species has never been recorded in aquaria or observed in the wild. General Hypostomus mating patterns involve male territoriality centred on cave or crevice sites, with males displaying odontode elaboration to rivals and potential mates. Whether H. tenuis follows this pattern cannot be confirmed from available evidence.
Breeding
Breeding is entirely undocumented for Hypostomus tenuis. No aquarium reproductive records exist. The species' Endangered status and representation by a single museum specimen mean that breeding biology remains entirely unknown.
Hypostomus species generally practise cave-spawning with paternal brood care, with the male guarding and fanning eggs through incubation. This pattern is expected but unconfirmed for H. tenuis.
In the aquarium
Hypostomus tenuis is not known in the aquarium hobby. Given that the species is represented by a single holotype, its wild population — if it still exists — has never been located, and no live specimens have entered trade or captive culture.
Any hypothetical husbandry for a slender, 7.5 in Hypostomus from a Guiana Shield drainage would call for a spacious aquarium (at least 55 US gal for a single adult), soft acidic water with stable temperature in the tropical range, quality driftwood and rocky retreat structures, and strong filtration. These are generalisations from genus biology; no species-specific recommendations are possible.
Conservation
The IUCN Red List assessed Hypostomus tenuis as Endangered (EN) under criterion B1ab(iii) in January 2021. The EN listing reflects a restricted extent of occurrence combined with continuing decline in the quality of freshwater habitat in the assigned range. The Suriname River basin faces incremental pressures from bauxite mining, logging, and agricultural runoff that degrade water quality and stream structure.
The conservation situation of H. tenuis is complicated by a fundamental scientific problem: the type locality is erroneous (Weber in Reis et al. 2003), meaning that even the river basin the species inhabits has never been confirmed from field evidence. The species is known from one specimen whose exact origin is unknown. This provenance uncertainty makes it impossible to conduct targeted population surveys, to define a meaningful extent of occurrence, or to assess whether the species still exists in the wild. Until a second specimen is collected from a verified locality, the true conservation status of H. tenuis cannot be fully resolved.