Taxonomy & naming
Hypostomus soniae was formally described by Hollanda Carvalho and Weber in 2005 in the Revue Suisse de Zoologie (volume 111, number 4, pages 953–978) as one of five new species assigned to the Hypostomus cochliodon group from the middle and lower Amazon system. The Catalog of Fishes (Eschmeyer, CAS) recognises Hypostomus soniae as a valid species within the sprawling genus Hypostomus Lacépède, 1803, which currently encompasses well over 130 described species and remains one of the most taxonomically dynamic genera in Loricariidae.
In the ornamental trade the fish is most widely known by its DATZ registration number, L137, under which it circulated for years before receiving a formal scientific description — a common trajectory for Hypostomus species exported from Brazil during the 1990s and early 2000s. The common name 'Blue-Eyed Red Fin Pleco' alludes to two of its most commercially appealing features: the pallid, somewhat iridescent eye and the suffusion of reddish or orange pigment in the unpaired fins.
Hypostomus belongs to the subfamily Hypostominae and to the tribe Hypostomini (sensu Armbruster, 2004), the core group of suckermouth armoured catfishes characterised by dentaries bearing numerous slender bicuspid teeth, a ventrally directed mouth with fleshy lips, and an extensive bony exoskeleton of overlapping scutes. Within the cochliodon group, species share unusually shaped, spoon-like teeth adapted for scraping hard substrates — a morphological and ecological bridge between the more typical Hypostomus and the more derived wood-eating Panaque lineage.
Morphology
Adults reach approximately 6.5 in standard length (SL), placing H. soniae firmly in the mid-size bracket of the genus — neither as compact as the dwarf 'L-number' species that colonise fast rapids, nor as large as the river-channel giants that exceed 12 in. The body follows the typical Hypostomus bauplan: strongly depressed head and anterior trunk, tapering toward a slender caudal peduncle, with all flanks armoured in keeled bony scutes.
The coloration that earns this species its hobby names is genuinely distinctive within the genus. Background body colour is a pale tan to greyish-brown, densely covered with dark spots of variable size. The dorsal, caudal, and pectoral fins carry a wash of reddish to orange-red pigment along the leading rays and membranes, particularly vivid in fresh or well-conditioned specimens. The eyes are noticeably pale — described as blue-eyed in the trade, though this is more accurately a pale-grey or silvery iris rather than true blue — which contrasts with the darker head.
The teeth are of the cochliodon type: fewer in number than in typical Hypostomus, with a more spoon-like curvature adapted for scraping hard substrates including wood. The mouth is bordered by fleshy, papillated lips. Odontodes are present on the body scutes, with sexually mature males developing more elongated and numerous interopercular odontodes and a denser coat of body odontodes during breeding condition.
Habitat
The type locality of Hypostomus soniae is recorded as temporary pools on the riverside of the Rio Tapajós, between Vila Nova and Urua, Pará State, Brazil. The Tapajós is a major southern tributary of the Amazon, known for its exceptional clarity, warm temperatures, and soft, acidic waters — characteristics shared across much of its drainage network. The river's lower course runs through ancient Precambrian shield terrain, with sandy or rocky substrates, scattered driftwood, and minimal turbidity.
The reference to 'temporary pools on riversides' in the type locality description is notable, suggesting the species is capable of occupying marginal and seasonally variable microhabitats beyond the main river channel — a trait consistent with the facultative air-breathing capacity documented in Hypostomus generally. During low-water seasons, portions of the Tapajós floodplain are isolated as ephemeral pools; the ability to access atmospheric oxygen via the highly vascularised stomach lining confers a survival advantage in such oxygen-depleted environments.
Water parameters from the broader Tapajós system typically reflect soft, mildly acidic Amazonian blackwater conditions: temperatures of 75–82 °F, pH values in the weakly acidic range, and very low conductivity. Rocky or gravel substrates with submerged wood accumulations are the preferred microhabitat for Hypostomus species of this type.
Feeding
Hypostomus soniae belongs to the cochliodon group, members of which possess modified spoon-shaped teeth that indicate a greater capacity for scraping hard substrates — including wood — than the standard bicuspid teeth found in most Hypostomus. In the wild, the diet likely consists of periphyton (algae, diatoms, and associated microbial mat), biofilm, detritus, and wood-surface material scraped from submerged roots, branches, and boulders. The shift toward spoon-like teeth in the cochliodon group parallels the full wood-eating dentition of Panaque and Panaqolus, though H. soniae is not considered a strict xylophage.
In the aquarium, the species performs well on a mixed diet skewed toward plant-based matter: spirulina-enriched algae wafers, blanched vegetables (courgette, cucumber, spinach, sweet potato), and commercial sinking pellets formulated for herbivorous catfishes. Access to driftwood or cork bark is beneficial both nutritionally and behaviourally — the fish rasp biofilm from wood surfaces even where it is not their primary caloric source. Animal protein should be offered sparingly; occasional frozen bloodworm or Daphnia can be given as enrichment without forming the dietary base.
Mating
No field observations of mating behaviour in Hypostomus soniae are on record, and PlanetCatfish lists no breeding reports for the species as of its most recent update. The following account is therefore based on general Hypostomus reproductive biology, which is relatively well-documented for commoner species.
Males of Hypostomus become territorial around cave or crevice sites, particularly as the breeding season approaches (often correlated with simulated wet-season rainfall and slight temperature reduction in captive settings). The interopercular odontodes of conditioned males lengthen noticeably, and body odontodes increase in density — these secondary sexual characters are thought to function both in male–male rivalry and in assessing mate quality. Females ready to spawn show a plumper abdominal profile and less developed odontode ornamentation.
Courtship involves the male displaying at and around a selected cavity, with the female investigating before committing to enter. In species with documented aquarium breeding, spawning occurs inside the cavity with the male fertilising eggs in situ.
Breeding
No captive breeding of Hypostomus soniae has been reported on PlanetCatfish or in the broader hobby literature as of the available records. This is unsurprising given the species' relatively low representation in the hobby — PlanetCatfish listed 97 registered keepers at its last update, and the fish remains an uncommon import.
Based on the general Hypostomus reproductive model, the species would be expected to be a cave-spawner with paternal brood care: the male selects and defends a suitable cavity (a hollow in driftwood, a ceramic tube, or a rocky crevice), the pair spawns inside, and the male subsequently fans, cleans, and guards the adhesive egg clutch until hatching. Incubation periods across the genus typically span 5–10 days depending on temperature. Fry emerge able to graze biofilm almost immediately.
Keepers wishing to attempt breeding should prioritise a species-specific setup with multiple oversize driftwood pieces and caves, soft acidic water matching Tapajós conditions (pH 6.0–7.0, temperature 75–81 °F), and a conditioning diet rich in plant matter with periodic feedings of live or frozen invertebrates. A seasonal cycle of slightly cooler, more frequent water changes followed by a gradual temperature increase — simulating Amazonian wet-season onset — may serve as a spawning trigger.
In the aquarium
Hypostomus soniae is an uncommon but periodically available import that appeals to keepers interested in L-number catfishes. At 6.5 in SL it is a manageable size relative to some of the larger Hypostomus species, but it is still substantially larger than the dwarf plecos often marketed to community tank hobbyists. A minimum tank footprint of 100 × 15.5 in (approximately 40–50 US gal for a single specimen) is advisable, with more space needed if other pleco-sized catfishes are to be housed alongside it.
Decoration should include substantial driftwood — both for the fish's biofilm-scraping behaviour and as shelter. Caves of appropriate size (clay pots, ceramic tubes, or hollowed wood sections large enough for the fish to turn around inside) provide security and may be needed for any future breeding attempt. Flat rocks and a sandy substrate suit the species' natural riverbottom associations.
Water chemistry should approximate soft, warm Amazonian conditions: temperature 75–82 °F, pH 6.0–7.2, low hardness (ideally below 10 °dH). Regular partial water changes of 25–30% weekly maintain the water quality this fish demands. Filtration should be efficient; like all Hypostomus, this species produces significant waste and is sensitive to accumulated nitrates.
Hypostomus soniae is reported to be generally peaceful toward dissimilar tankmates — tetras, small cichlids, and other mid-water species present no compatibility issues. Intraspecific and inter-pleco aggression is common in the genus; two large Hypostomus in a modest tank will clash over cave sites. The species is not destructive to robust plants (Anubias, Java fern), but fine-leaved plants or substrate plantings are likely to be uprooted or damaged by the fish's excavation of the substrate.
Because the species originates from the Tapajós — a river with exceptional water clarity and quality — it is probably more demanding in terms of water quality than some Hypostomus from turbid whitewater drainages. Overfeeding and under-filtering are the most common sources of captive losses.
Conservation
The IUCN Red List assessed Hypostomus soniae as Least Concern in 2018, reflecting a distribution centred on the Tapajós drainage in Pará, Brazil, with no evidence of significant population decline at the time of assessment.
The Tapajós basin faces intensifying pressure from deforestation and agricultural expansion in the arc of deforestation that encroaches on Pará State, as well as from large-scale hydroelectric development. The Tapajós river has been the subject of proposed dam projects that, if constructed, would inundate extensive stretches of clear-water habitat. Hypostomus species tied to specific drainage systems, rocky substrates, and particular water chemistry profiles can be vulnerable to such alterations even when their overall range appears broad. The LC assessment should therefore be read in the context of ongoing habitat change in the Brazilian Amazon rather than as a statement of permanence.