Taxonomy & naming
Ancistrus mullerae was described by Bifi, Pavanelli, and Zawadzki in 2009 as one of three new Ancistrus species from the Río Iguaçu drainage of Paraná State, Brazil (Zootaxa 2275: 41–59). The paper provided a morphological characterisation and comparisons with congeners from the Río de la Plata and the coastal river systems of southern Brazil. The Catalog of Fishes (Eschmeyer, CAS) recognises Ancistrus mullerae Bifi, Pavanelli & Zawadzki, 2009 as a valid species.
The species belongs to subfamily Hypostominae within Loricariidae. Among the congeners it most closely resembles in the same southern Brazilian region — A. abilhoai, A. agostinhoi, A. formoso, A. multispinis, and A. taunayi — A. mullerae is distinguished by a combination of meristic and morphometric characters: mandibular teeth row width 16.5–21.0% of head length (vs. 13.4–15.5% in A. abilhoai), occipital depth 15.4–18.8% SL (vs. 18.8–22.0% in A. agostinhoi), orbital diameter 11.9–18.4% head length (vs. 8.3–8.7% in A. formoso), and interopercular spine counts of 12–22 (vs. 24–35 in A. multispinis and 22–28 in A. taunayi). No L-number has been assigned to this species.
Morphology
Ancistrus mullerae shows one of the more marked instances of sexual size dimorphism recorded in the genus: males reach approximately 5 in SL, while females top out at around 3 in SL — a difference of more than 1.5 in between sexes in the same population. This is the reverse of the situation seen in many other catfish groups, where females are typically the larger sex; in Ancistrus, and particularly here, it is the male that attains a substantially larger adult size.
Beyond body length, the species shares the diagnostic Ancistrus characters: bony loricariid armour of overlapping odontode-bearing scutes, a ventrally positioned sucking mouth, and the fleshy tentacles on the snout. In males these tentacles are large and branched; in females the tentacles are shorter and less elaborately branched, limited mainly to the snout margin. Interopercular spines — 12 to 22 in this species — can be erected laterally. The maxillary barbel is short and not free from the lower lip, a character that distinguishes A. mullerae from some congeners with a longer, free barbel.
Habitat
Ancistrus mullerae is known from the lower Río Iguaçu basin in Paraná State, Brazil. The Iguaçu is a major left-bank tributary of the Río Paraná, notable for the Iguazú Falls system and for a high level of endemic ichthyofauna, having been isolated from other drainages for a geologically extended period. The lower Iguaçu, below the falls, has its own fish assemblage distinct from the upper basin, and the Ancistrus fauna of this stretch appears to include multiple endemic species.
Detailed water chemistry and microhabitat data specific to A. mullerae are not available in accessible sources. The Iguaçu system in its lower reaches flows over basaltic substrate; waters tend toward soft to moderate hardness with slightly acidic to neutral pH, reflecting regional geology and the absence of strongly calcareous substrate. The general Ancistrus microhabitat preference for hard substrate surfaces — rock, submerged wood, boulder surfaces in riffles — is expected to apply.
Feeding
As with other members of the genus, Ancistrus mullerae is presumed to be primarily an algae and biofilm grazer, using the ventral sucking mouth and fine rasping teeth to scrape aufwuchs from hard surfaces. The tooth morphology — with a mandibular tooth row width between 16.5 and 21.0% of head length — is consistent with the general Ancistrus feeding apparatus adapted for biofilm and algae removal rather than wood excavation.
In captivity, the feeding approach used for other Ancistrus applies: algae wafers, spirulina-based foods, blanched vegetables (courgette, cucumber, peas), and biofilm grazing from smooth rocks and driftwood surfaces. Occasional supplementation with invertebrate-based protein foods is acceptable but should not dominate the diet.
Mating
Courtship behaviour has not been described specifically for A. mullerae in the accessible literature. The general Ancistrus pattern is expected to apply: males establish cave territories, display branched snout tentacles and erected interopercular odontodes toward rivals and potential mates, and court females to the spawning cavity. Given the pronounced size difference between sexes — males substantially larger than females — the male's physical ability to block the cave entrance with erected cheek spines while sheltering a smaller female inside is geometrically effective.
The dimorphism in adult size may be related to the demands of prolonged paternal cave-guarding, which in larger-bodied species can involve extended fasting and physical contests with rival males or predators. Data specific to courtship behaviour in this species are sparse.
Breeding
Ancistrus mullerae is a cave spawner with paternal brood care, consistent with all described Ancistrus species. The male guards the adhesive egg clutch after spawning, fanning the eggs to maintain oxygenation and removing any that fail to develop. Fry are guarded until yolk absorption is complete.
Specific breeding data for A. mullerae — clutch size, incubation duration, fry size at hatching — are not documented in accessible sources for this species. Aquarium breeding reports from hobbyists have not been confirmed in the primary or hobbyist literature consulted. General Ancistrus breeding husbandry (tight-fitting caves, varied diet, water-change conditioning, temperatures in the tropical range) is the reasonable starting framework.
In the aquarium
Ancistrus mullerae is not commonly encountered in the ornamental trade. It is an endemic of the lower Iguaçu basin and has not been documented as a regular export species. Hobbyists working with southern Brazilian Ancistrus should be aware that multiple morphologically similar species occur in the Paraná drainage and its tributaries; accurate identification to species level is difficult without reference to the 2009 original description.
The male's adult size of up to 5 in SL means this is a somewhat larger Ancistrus than the tank-bred 'common bristlenose' most hobbyists encounter, and tank volume should reflect that — a minimum of 25–30 US gal for a single animal, more for a pair or group. Cave provision is essential. Water conditions typical of the Paraná drainage — soft to moderately hard water, pH slightly acidic to neutral, temperatures 72–79 °F — are a reasonable target. The pronounced sexual size dimorphism makes sexing straightforward in adults.
Conservation
The IUCN Red List assessed Ancistrus mullerae as Least Concern in November 2018. No major threats were identified at the time of assessment. The species' estimated population doubling time of 1.4–4.4 years indicates moderate resilience.
The Iguaçu basin as a whole has experienced significant hydrological modification from hydroelectric dams; the Iguazú Falls system itself represents a long-standing natural barrier, but the basin's endemic fish communities downstream are vulnerable to altered flow regimes and barrier effects from infrastructure. The species' restriction to the lower Iguaçu basin means its total range is geographically limited, and changes to that system could have population-level effects not reflected in the current Least Concern status.