Taxonomy & naming
Chaetostoma marginatum was described by Charles Tate Regan in 1904 in the Transactions of the Zoological Society of London (volume 17, part 3, no. 1, page 249, Plate 13, fig. 1), from syntype specimens (BMNH 1901.8.3/27–28 and ?1902.5.27.44) collected at Salidero, northwestern Ecuador — now interpreted as near Bulún (Pulún), approximately 1°05′N, 78°40′W, Río Bogotá drainage, Esmeraldas Province, elevation 164–361 ft. The original epithet was spelled marginatus (masculine), but was subsequently corrected to marginatum (neuter) to agree with the gender of the genus Chaetostoma, which is neuter. The synonym Chaetostomus marginatus reflects the original orthography. The Catalog of Fishes (Eschmeyer, CAS) accepts Chaetostoma marginatum Regan, 1904 as the valid name.
Chaetostoma sits in tribe Chaetostomini within subfamily Hypostominae of Loricariidae. The genus currently contains more than 50 valid species distributed across Pacific- and Atlantic-slope drainages of Colombia, Ecuador, Peru, and Venezuela. Molecular phylogenetic work by Lujan et al. (2015) placed C. marginatum within the western Chaetostoma clade. No L-number has been assigned to this species.
Morphology
Adults reach approximately 7.5 in standard length (SL), making C. marginatum among the larger Chaetostoma species — though it remains notably smaller than the common plecos (Hypostomus, Pterygoplichthys) with which it is sometimes confused in trade. The body is stocky and somewhat rounded in cross-section compared to the more flattened whiptail relatives, with a broad depressed head, a prominently rounded rubber-like snout (giving the 'rubbernose' trade name), and a robust caudal peduncle.
The ventral surface bears no bony plates on the abdomen — a key generic character of Chaetostoma, distinguishing it from Ancistrus (which also lacks abdominal plates) but with additional interopercular odontodes behind the gill-cover. The snout lacks the marginal plates present in the related C. platyrhynchus. Colouration is brown to dark grey above, with variable spotting, banding, or marginal patterning along the fins — the 'marginatum' pattern of pale-edged fins providing one identification clue in fresh material.
Sexual dimorphism is distinct. Males develop a notably larger and broader head, a slimmer ventral profile, and disproportionately large pelvic fins. The enlarged pelvic fins in males appear functional during spawning: by inverting them over the egg clutch, the male prevents sperm from being swept away by current (PlanetCatfish). Females are broader ventrally when gravid, with a rounder abdominal profile.
Habitat
The type locality on the Río Bogotá in Esmeraldas Province places C. marginatum in a Pacific-slope drainage of northwestern Ecuador. The species is also recorded from Colombia and Ecuador in the Río Chimbo and Río San Juan systems — all Pacific-slope rivers draining the western Andes cordillera into the Pacific Ocean or coastal lowland basins. This is an Andean-foothills to lowland zone, with rivers characteristically cold, highly oxygenated, fast-flowing over rocky substrates, and low in turbidity.
Chaetostoma in general are rheophilic — preferring strong current and high dissolved oxygen. At the type locality elevation of 164–361 ft, temperatures are moderate for tropical fish: likely 68–75 °F year-round, with cool-season lows reaching below 68 °F. Water chemistry in Pacific-slope Andean drainages is typically hard and circumneutral to slightly alkaline due to carbonate dissolution from volcanic and sedimentary geology, contrasting with the acidic blackwater of much of the Amazon basin. The species clings to exposed boulders and bedrock surfaces using its strongly developed suctorial mouth, rasping periphyton in well-lit riffle zones.
Feeding
Chaetostoma marginatum is a specialised periphyton and epilithic algae grazer. The rubbernose's broad, flexible snout and strong oral disc are adaptations for adhering to irregular rock surfaces in swift current, where the fish scrapes diatoms, green algae, cyanobacteria, and associated biofilm. The genus name alludes to this feeding organ: Greek chaite (bristly hair) + stoma (mouth), referring to the fringe of sensory bristles around the lips.
In aquaria the diet should replicate this herbivorous emphasis. Abundant algae growth on rocks and glass — particularly brown diatoms and green spot algae — is valuable as a base. Commercial algae wafers, spirulina tablets, blanched courgette and cucumber are accepted and important for sustained nutrition. Unlike wood-specialist plecos, C. marginatum has no significant wood-rasping habit and does not require driftwood as a dietary component, though it will rest on wood and scrape biofilm from it.
Strong current oxygenating the food surface is important: in low-flow environments, rubbernoses often feed less enthusiastically and may show signs of respiratory stress. Fast, well-oxygenated water keeps them active and feeding.
Mating
Courtship behaviour in Chaetostoma has been observed in aquaria for several congeners, and the pattern is consistent with what is inferred for C. marginatum from its morphology and genus-level reproductive mode. Males become territorial and advertise their reproductive condition by displaying enlarged interopercular odontodes — the spine-like projections behind the gill-cover that are substantially more developed in males than females.
A receptive female is coaxed by the male toward a chosen spawning site: a flat-bottomed rock, a broad piece of slate, a smooth cave ceiling, or the underside of a large stone in shallow, fast-flowing water. The male positions himself inverted beneath the spawning substrate in preparation for fertilising the egg clutch. The enlarged pelvic fins documented in males of this species appear to play a direct role in this inverted fertilisation posture, preventing sperm dispersal in current. Multiple males competing for territories in small enclosures can produce aggressive interactions; providing visual barriers and multiple flat-rock spawning sites reduces conflict.
Breeding
Chaetostoma marginatum, consistent with its genus, is reported to spawn in shallow, fast-flowing water on the underside of a flat object. The male, oriented upside-down, guards an adhesive clutch of eggs by covering it with his body, continuing to fan and protect the clutch until hatching. This inverted guarding posture is well-documented for Chaetostoma generally (PlanetCatfish; Seidel & Evers, Wels Atlas).
In the aquarium, spawning is most reliably triggered by recreating natural cues: strong current (powerhead or spray bar directed across a flat stone), cool water temperature in the 68–72 °F range, and excellent water quality with frequent partial changes. A broad flat stone or slate slab, slightly elevated to create a shallow cave cavity beneath, is the preferred site. Clutch sizes in Chaetostoma typically number several dozen eggs. Fry emerge as miniature adults and begin grazing immediately on fine algae and spirulina. The male continues guarding until fry are free-swimming and dispersed. No registered breeding reports for C. marginatum specifically are recorded on PlanetCatfish as of 2025.
In the aquarium
Chaetostoma marginatum at up to 7.5 in SL suits tanks of 40 US gal or more. The most critical environmental requirement is high dissolved oxygen delivered by strong current: this is not a species that does well in slow, warm, sluggish aquaria. A powerhead, spray bar, or wavemaker providing vigorous surface agitation and internal circulation is essential. Without adequate oxygen and flow, rubbernoses become lethargic, cease feeding, develop fin issues, and may decline and die within weeks.
Temperature should remain in the 68–75 °F range — cooler than most tropical community tanks, which makes C. marginatum a better fit for species set up at lower temperatures: hillstream-biotope tanks, cold-tolerant South American community tanks, or dedicated Andean-river setups with Astyanax or similar cool-tolerant schooling fish. pH 7.0–7.8 and moderate hardness reflect the Pacific Andean drainage chemistry.
Substrate should include smooth boulders, flat slates, and large river pebbles arranged to create current shadows and resting ledges. The fish will spend much of its time pressed flat against rock surfaces. Driftwood may be added for aesthetics and biofilm but is not dietary. Avoid tankmates requiring warm, still water. Compatible species include other cool-water loricariids, hillstream loaches, cold-adapted tetras, and small Corydoras that tolerate similar temperatures.
Sexual dimorphism makes pair selection straightforward in adult fish: select a male with a notably wider head and larger pelvic fins paired with a broader-bellied, smaller-headed female.
Conservation
The IUCN Red List assessed Chaetostoma marginatum as Least Concern in 2014 (assessment published 7 October 2014), noting the species' presence across multiple Pacific-slope drainages in Colombia and Ecuador and no documented population decline meeting threatened thresholds. The assessment encompasses the Bogotá, Chimbo, and San Juan river basins — a total distribution covering several hundred kilometres of Pacific-slope Andean watersheds.
Pacific-slope rivers of Ecuador and Colombia face significant pressures from deforestation, agricultural expansion (particularly banana, cacao, and palm oil production in lowland zones), artisanal gold and mineral mining in foothill rivers, and water extraction for irrigation. Sedimentation degrades the rocky riffle microhabitats that rheophilic Chaetostoma require, smothering the epilithic algae that form the basis of their diet. The genus Chaetostoma contains numerous narrow-range endemics, several of which face more acute threats than C. marginatum; the relatively broad distribution of this species provides some resilience. Captive breeding remains limited, and most specimens in trade are wild-collected from Ecuador.