Taxonomy & naming
Ancistrus megalostomus was described by N.E. Pearson in 1924 in Indiana University Studies (volume 11, number 64), from syntypes taken at Huachi — the confluence of the Bopi and Cochabamba rivers in Bolivia, at an elevation of approximately 2234 ft (2,235 feet). The Catalog of Fishes (Eschmeyer, CAS) treats Ancistrus megalostomus Pearson, 1924 as a valid species.
The species epithet megalostomus derives from the Greek megas (large) and stoma (mouth), alluding to a relatively wide oral disc. No common name has been recorded in either the scientific or hobbyist literature.
The type locality places this species in the Beni River system — a major western tributary of the Madeira River, itself one of the principal upper-Amazon drainages. Within the family Loricariidae, Ancistrus belongs to the tribe Ancistrini of subfamily Hypostominae (Armbruster, 2004). The genus is among the most speciose in the family, with more than 70 valid species distributed across tropical South America and Panama.
A taxonomic remark on FishBase notes that no later record of the species exists beyond the syntypes; the teeth were described by Muller and Weber (1992, pp. 113, 115, pl. 1c). No subsequent collecting has extended the range or added to the type material.
Morphology
Maximum reported size is 3.5 in standard length (SL), based on the syntypes (Fisch-Muller, 2003). As with other Ancistrus, the body is encased in loricariid bony scutes arranged in longitudinal rows. The generic name Ancistrus refers to the small hooks (Greek agkistron, hook) formed by the interopercular odontodes — in mature males of the genus these develop into conspicuous, branched fleshy tentacles that fan out around the snout and over the upper lip; females may develop shorter, unbranched tentacle rows along the snout margin.
Beyond what can be inferred from the original description and the genus-level characters, detailed morphological data for A. megalostomus are not available from the published record. The species is known only from preserved syntypes, and no photographs of living specimens appear in the literature or hobbyist databases.
Habitat
The type locality — Huachi, at the junction of the Bopi and Cochabamba rivers in Bolivia — lies at roughly 2234 ft elevation in the Beni River basin, a high-gradient Andean piedmont drainage that ultimately feeds the Madeira and Amazon systems. At this altitude, rivers typically carry cool, well-oxygenated water with strong seasonal flow variation driven by Andean snowmelt and tropical rainfall patterns.
No field data (temperature, pH, conductivity, or substrate) have been published for the collection site or for the species since its description. The ecological context can only be inferred from the geography: high-altitude Bolivian tributaries of the Beni tend toward cool, clear, moderate-current waters over rocky substrates — conditions common to other altitude-tolerant Ancistrus in the Madeira drainage.
The species has not been recorded since the original 1924 collection. Whether this reflects genuine rarity, inaccessibility of the type locality, or lack of targeted surveys is unknown.
Feeding
No feeding observations exist for Ancistrus megalostomus in the wild or in captivity. By analogy with other members of the genus — and based on the ventral, suckermouth morphology shared across Ancistrus — the species is presumed to be an aufwuchs and biofilm grazer, rasping algae, detritus, and the microorganisms colonising rock and wood surfaces with its bicuspid teeth.
Ancistrus species generally exploit a broader diet than many other loricariids, accepting soft algae, decaying organic matter, and incidental invertebrates, with the oral disc and teeth well suited to scraping hard substrates in fast-flowing rocky streams.
Mating
No observations of mating behaviour exist for this species. Ancistrus as a genus are cave-spawners, and in species where behaviour has been documented, males establish and defend a cavity — a crevice, undercut rock, or hollow wood — and court females with flared tentacles and body posturing. The branched snout tentacles of male Ancistrus are thought to function in species recognition and mate assessment, though the precise signal they convey remains debated in the literature.
Breeding
No breeding data exist for Ancistrus megalostomus. The species has not been kept in captivity; no captive breeding attempts or observations have been reported on any database or in the hobbyist literature. PlanetCatfish notes no breeding records on file.
The genus-level expectation, based on the many better-known Ancistrus, is paternal cave-brooding: after spawning, the male remains in the cavity to fan and guard the adhesive egg clutch until fry hatch and disperse. Whether A. megalostomus follows this pattern cannot be confirmed from the available record.
In the aquarium
Ancistrus megalostomus has not been kept in captivity, and no aquarium husbandry information exists in the scientific or hobbyist literature. The species is not available in the ornamental trade, and PlanetCatfish carries no images or keeper records.
Given the absence of field and captive data, any specific husbandry recommendation would be speculative. Hobbyists interested in altitude-tolerant Bolivian Ancistrus have other, better-documented species available. If A. megalostomus were ever collected and maintained, its high-altitude origin would suggest preference for cool, well-oxygenated conditions with moderate current and rocky or woody substrate — but this inference is drawn from geography alone, not observation.
Conservation
The IUCN Red List assessed Ancistrus megalostomus as Least Concern in 2014, on the basis that no evidence of major threats sufficient to trigger a threatened category was identified. The assessment notes the restricted distribution in the Beni River system of Bolivia.
The absence of any post-1924 field record is conspicuous. The IUCN assessment acknowledges that the species is known only from syntypes collected at Huachi; whether living populations persist in the Bopi–Cochabamba confluence or surrounding Beni tributaries has not been established by modern surveys. The Bolivian Andes and Andean piedmont face pressures from deforestation, agricultural expansion, and hydroelectric development in some reaches of the Beni drainage, though the specific impact on the type locality has not been assessed.