Taxonomy & naming
Chaetostoma thomsoni was described by the British ichthyologist Charles Tate Regan in 1904 in the Transactions of the Zoological Society of London, volume 17, part 3, number 1, page 250, plate 14 (figure 2), as part of his monograph of the Loricariidae. The syntypes — BMNH 1902.5.15.28–30 (3 specimens) — are held at the Natural History Museum, London. The type locality is Villeta, Colombia, a town on the Río Negro (a right-bank Magdalena tributary) in Cundinamarca department.
The Catalog of Fishes (Eschmeyer, CAS) recognises the valid name as Chaetostoma thomsoni Regan 1904. The synonyms are Chaetostoma thomasi (an erroneous subsequent spelling) and Chaetostomus thomsoni (the original generic placement prior to standardisation of the genus name to Chaetostoma).
Chaetostoma Tschudi, 1846 is the third most species-rich genus in Loricariidae, with approximately 49 valid described species (Zeise et al. 2025 in Zoosystematics and Evolution) and a considerable number of undescribed forms circulating in the hobby. The genus is placed in subfamily Hypostominae, tribe Ancistrini (Armbruster 2004), and is the dominant genus of the 'rubbernose' or 'bulldog' pleco group — recognised by the broad rounded snout, naked abdomen, interopercular odontodes, and absence of fleshy tentacles (distinguishing it from Ancistrus). Regan named the species in honour of Mr Kay Thomson, who collected the original specimens near Villeta.
Morphology
Chaetostoma thomsoni reaches approximately 4 in (4 in) standard length — PlanetCatfish gives this figure; ScotCat records 4 in — making it a compact, medium-small member of its genus. The body plan is typical Chaetostoma: strongly depressed, broad and flat across the pectoral girdle, with a wide rounded snout bearing the circular ventral sucker mouth and tapering to a narrower caudal peduncle.
Colouration is dark brown to grey-black on the dorsum, with variable pale cream to buff markings. The species is described as the 'fin-banded rubbernose' because of banding or reticulation on the fins; body spotting or reticulation is present but the pattern varies geographically and with condition. The ventral surface is pale. The abdomen is naked — unplated — as in all Chaetostoma species, a consistent generic character that distinguishes the genus from Hypostomus in which the belly is often partially plated.
Sexual dimorphism follows the pattern typical of Chaetostoma. Males have a broader and larger head and lips, slimmer ventral sides, and — notably — disproportionately large pelvic fins. PlanetCatfish notes that these oversized pelvic fins may play a functional role in fertilisation: when the male inverts them over the egg mass in fast current, they may prevent sperm from being swept away. Females are wider-bodied when gravid, particularly visible from above. Adult males also typically show better-developed interopercular odontodes (cheek bristles).
Habitat
Chaetostoma thomsoni is endemic to Colombia, recorded from the Magdalena, Sinú, Cesar, and Cauca basins (Maldonado-Ocampo et al. 2005). Within the Santander department it has been found in the rivers Charalá, Oro, Chicamocha, and Santacruz, and in ravine El Aburrido of the Lebrija River system (Castellanos Morales et al. 2011). The type locality — Villeta on the Río Negro, ca. 5°N, 74.5°W — sits in the central Andean foothills at roughly 2625 ft elevation, where the Magdalena basin transitions from highland to upper-lowland character.
Like other Chaetostoma, C. thomsoni is a rheophile — a current-loving species that clings to boulder and rock surfaces in fast-flowing sections of rivers and streams. ScotCat gives a temperature range of 68–72 °F (a notably cool requirement reflecting Andean foothill conditions), and FishBase and PlanetCatfish record 68–72 °F with pH 6.0–7.5. This relatively cool preference distinguishes C. thomsoni from many Amazonian plecos and is a critical care parameter: sustained temperatures above 77 °F are likely to cause chronic stress.
Microhabitat is characterised by exposed rock surfaces in well-oxygenated, clear, fast-moving water. The Magdalena and its Andean tributaries are diverse in water chemistry — conductivity ranges from low to moderate depending on whether streams originate in crystalline basement or sedimentary formations — but consistent in their provision of current, oxygen, and substrate for periphyton growth.
Feeding
Chaetostoma thomsoni is primarily an algae grazer and biofilm scraper. The broad sucker mouth and multi-cusped teeth are well-suited to rasping soft algae and periphyton from rock and glass surfaces. In the wild, the diet consists of epilithic algae, diatoms, and the bacterial and fungal biofilm communities that colonise rock surfaces in nutrient-limited but light-rich Andean stream sections.
In the aquarium, ScotCat describes C. thomsoni as a great algae eater, noting that the algal food must be supplemented with algae wafers and vegetable items such as cucumber, zucchini (courgette), and lettuce. Tablet and worm foods are also accepted. This broadly herbivorous but opportunistically omnivorous diet is typical of the genus. High-quality spirulina-based wafers form a good staple, supported by blanched vegetables offered on a regular schedule and occasional protein foods (frozen bloodworm, Artemia, Daphnia) perhaps once or twice per week.
Unlike the wood-eating Panaque and Panaqolus, Chaetostoma does not require driftwood as a dietary component, though wood surfaces will be rasped for biofilm. Plant matter in the tank — particularly soft-leaved species — may be grazed, so robust plant choices (Anubias, Java fern, Bolbitis) are preferable for planted tank setups with this species.
Mating
Chaetostoma thomsoni has not been bred in captivity in any documented report on PlanetCatfish as of this writing. What is known about Chaetostoma mating behaviour comes principally from the more thoroughly studied C. milesi and closely related species bred in cooler, fast-flowing tank setups.
Males become territorial around spawning sites, typically the underside of a flat rock or a similar sheltered substrate. The sexual dimorphism is functionally illuminating: the male's oversized pelvic fins appear adapted for a specific hydrodynamic purpose during fertilisation in fast current. When a male spawns under fast-flowing water, inverting his large pelvic fins over the adhesive egg mass may deflect the current sufficiently to allow sperm to reach the eggs before being swept away — an unusual adaptation that highlights just how specialised fast-water rheophile spawning behaviour can be.
Conditioning adults for breeding in a dedicated tank with strong flow, smooth flat stones, clean cool water (64–72 °F), and regular changes should increase the probability of spawning attempts. Male-to-female ratio and separation of multiple males are important considerations given the territorial nature of breeding males.
Breeding
No captive breeding report for Chaetostoma thomsoni specifically exists in the published hobby literature. The broader biology of the genus, however, gives a clear picture of what breeding likely involves. Chaetostoma species are rock-spawners: eggs are deposited on the underside of a smooth flat stone or the undersurface of a horizontal object in fast-flowing water. The adhesive clutch is guarded by the male, who positions himself inverted against the eggs for the incubation period.
PlanetCatfish describes the general Chaetostoma breeding mode for C. thomsoni: spawning occurs in shallow, fast-flowing water on the underside of a flat object; the male, upside-down, guards the adhesive clutch by covering it with his body. Incubation duration at typical temperatures is not recorded for this species but would be expected to fall in the range of 5–10 days based on congeners. Fry are relatively well-developed on emergence and begin grazing biofilm quickly.
For the aquarist attempting breeding, the key conditions are strong directional flow across a spawning site, clean cool water (68–72 °F), and smooth flat stones or slate pieces tilted slightly to allow water to pass beneath. A dedicated breeding tank of 20–30 US gal with a powerful powerhead or spray-bar and a minimal footprint will provide better breeding conditions than a community aquarium.
In the aquarium
Chaetostoma thomsoni is one of the more challenging rubbernose plecos to maintain, primarily because of its cool-temperature requirement. While many Colombian Loricariidae adapt to a broad thermal range, C. thomsoni's Andean origin suggests it performs best at 68–72 °F — a temperature range at the lower end of most tropical aquarium setups and well below the 75–79 °F routinely maintained for most community fish. Keeping the species at 75 °F or above long-term risks chronic stress, suppressed immunity, and shortened lifespan.
A dedicated tank for C. thomsoni (or a tank shared with other cool-water species such as hillstream loaches, danios, or temperate Ancistrus forms) is preferable. The aquarium should provide strong water movement — a powerhead or internal filter rated for at least twice the tank volume per hour is a minimum — over a substrate of smooth, rounded stones with flat pieces available as resting and potential spawning sites. The fish will cling to vertical glass and stone surfaces and is an effective grazer of algae on all exposed surfaces.
Size at up to ~4 in SL makes C. thomsoni manageable in a tank from about 20 US gal, though 120+ litres is preferred if multiple specimens are kept. Males are territorial with each other; one male with one or two females in a well-furnished tank is workable. Water quality requirements are high: the species originates from clear, oxygen-rich mountain streams, and low nitrate (under 15 mg/L), zero ammonia and nitrite, and regular 25–30% weekly water changes are non-negotiable. Avoid pairing with fish requiring warm conditions; optimal tankmates are other cool-water species of similar size.
Conservation
The IUCN Red List assessed Chaetostoma thomsoni as Least Concern (LC) in 2014/2016 (Jimenez-Segura, Mojica & Alonso 2016), noting a wide distribution, abundance, and no known major threats. The species is recorded from the Magdalena, Sinú, Cesar, and Cauca basins across a substantial area of Colombia's Andean foothills.
The Magdalena basin has undergone substantial environmental change through deforestation, agricultural runoff, sedimentation, and decades of hydroelectric and irrigation development. However, C. thomsoni appears sufficiently widespread and adaptable within its preferred fast-flowing habitat that these pressures have not yet driven a measurable population decline. The IUCN note that the assessment needs updating (Scotcat cites 'needs updating IUCN 2016'), and a more recent survey-based assessment would be valuable.
The broader Magdalena ichthyofauna is of high conservation priority: the basin harbours several hundred endemic freshwater fish species, many of them unstudied or poorly understood. Chaetostoma thomsoni benefits from the relative resilience of its fast-flowing rocky-stream habitat to some land-use pressures (riffles and rapids are less affected by organic pollution than slow-water pool habitats), but increased sediment loads and water abstraction for agriculture represent ongoing concerns.