Taxonomy & naming
Uwe Römer, Joscha Beninde, Fabrice Duponchelle, Carmen Rosa García Dávila, Antonia Vela Díaz and Jean-François Renno described Apistogramma paulmuelleri in 2013 in the Senckenberg journal Vertebrate Zoology (63(1): 15–34; doi 10.3897/vz.63.e31410). Their account rests on 28 specimens taken in August 2011 from a nameless forest quebrada — a stream — in the catchment of a tributary of the Río Amazonas, about 50 mi south of Iquitos in Departamento Loreto, Peru, near the road between Iquitos and Nauta at roughly 04°24′S / 73°34′W. The holotype is a 1.5 in-standard-length male, MUSM 41805, held at the Museo de Historia Natural de la Universidad Nacional Mayor de San Marcos in Lima; the paratypes are divided among MUSM, the Senckenberg collection in Dresden (MTD) and the California Academy of Sciences (CAS). Eschmeyer's Catalog of Fishes lists the name as valid, in Cichlidae: Cichlinae.
The genus name pairs the Greek apisto ("uncertain, inconstant") with gramma ("line" or "mark") — a nod to the variably developed, often incomplete lateral line that puzzled early workers on these dwarf cichlids. The epithet honours the late Professor Emeritus Dr. Paul Müller (1940–2010), a biogeographer at the University of Trier whose work on the origins of the Neotropical fauna, Apistogramma among it, shaped the field. The describers set the species in the Apistogramma eunotus complex, within the Apistogramma regani group of the Apistogramma regani lineage; at a glance it closely resembles Apistogramma regani itself, which is much of why it waited so long for a name.
That long pre-description history left a tangle of trade labels. The fish carries the code A52 in the DATZ "A-number" system and has circulated as Apistogramma sp. "Masken," Apistogramma sp. "Apache," Apistogramma sp. "Carapintada," Apistogramma sp. "Peru (II)," Apistogramma sp. "Peru-regani" and Apistogramma sp. cf. regani (Belém). Several of those — "Apache" and "Carapintada" especially — were also hung on other, non-conspecific Apistogramma populations, so they are unreliable as identifiers; stock of uncertain origin sold under them is best treated as Apistogramma cf. paulmuelleri rather than assumed to be this species.
Morphology
This is a true dwarf. Römer and colleagues give a maximum standard length of about 1.5 in — body length excluding the tail, with the holotype male at 1.5 in SL and females smaller. Well-fed aquarium fish can look bigger: the German exporter Aquarium Glaser has cited commercial males to roughly 3 in and females near 2 in in total length, but those numbers mix a different measure (total rather than standard length) with pampered captive stock, so the conservative scientific value is the one to trust for wild animals.
The colours are understated rather than gaudy. Adult males carry metallic bluish lines and bright red marks on the cheeks and gill covers, ivory-white sides to the head below the cheeks, and often a yellow-to-orange wash on the lower belly and across the chin and branchiostegal membranes between the gill covers; orange-red blotches sit in the centres of the lower-flank scales, lining up into a set of horizontal stripes. The rounded caudal fin usually runs slightly longer in its upper lobe than its lower, giving a faintly lopsided tail, and its central half is crossed by faint rows of dusky spots while the rest stays transparent with a green-to-blue sheen. The dorsal fin is low — only about half to two-thirds the body's height — without the dramatic extensions some congeners flaunt.
The diagnostic marks, following the original description, gather at the tail base: a vertical black blotch covering the full height of the caudal peduncle, set against up to four distinct abdominal stripes, a narrow mid-ventral stripe present in both sexes, and a lateral band about two-thirds of a scale wide that fades out before it reaches that peduncle spot. Females are plainer, the lateral band often shrinking to a row of up to six oval spots depending on mood. Sexual dimorphism runs to the usual Apistogramma pattern — males larger, more colourful, and carrying more extended finnage than females.
Habitat
On current evidence Apistogramma paulmuelleri is a narrow-range upper-Amazon endemic, known with confidence only from its type locality and a few nearby points in Loreto, Peru. The describers treated it as apparently endemic to the Loreto region and present at low abundance, with all localities lying east of the Río Itaya and west of the Amazon north of Nauta; the Norwegian field collector Tom Christoffersen, who took the species in the same Iquitos–Nauta corridor, frames the range as the middle and lower Río Ucayali drainage. FishBase summarises it simply as the Amazon basin in Peru.
The habitat is small and intimate — not a great river but a forest brook only a few hundred metres long. When the type stretch was sampled in August 2011 it ran 10–26 ft wide and up to about 31.5 in deep, carrying clear, light-brown water shaded by riparian forest at around 394 ft elevation. The water is soft and acidic. The describers call the species adapted to very clear water, while Christoffersen's measurements from the same corridor — taken near the end of the dry season in 2005, 2006 and 2009 — record a blackwater influence: pH between about 4.7 and 5.3, conductivity from roughly 10 to 70 µS/cm, and temperatures of about 80–86 °F. Together these read as a clearwater-to-blackwater forest stream — nutrient-poor, ion-poor, tea-stained by leaf litter — rather than a turbid whitewater system, with the dry-season figures probably marking the warm end of its yearly range. Fishes found alongside it include the cichlid Laetacara flavilabris, the sailfin tetra Crenuchus spilurus, several Aequidens (logged by the IUCN assessors as Aequidens and Cichlasoma), two Rivulus killifish and an undescribed Hyphessobrycon.
Feeding
No dedicated diet study exists for Apistogramma paulmuelleri, so its trophic ecology is read from the genus and from its microhabitat. Apistogramma are micropredators of the leaf-litter and sediment interface: they sift and pick small benthic invertebrates — insect larvae, microcrustaceans and the like — from the substrate and detritus rather than grazing plants or hunting other fish. FishBase puts the species at an estimated trophic level near 3.3, in keeping with a small invertebrate-eating carnivore.
In a shallow, ion-poor forest brook that makes it one of the small mid-level predators of the litter community, taking the tiny animals that themselves feed on rotting leaves and biofilm, and in turn serving as prey for larger fishes. Submerged wood and leaf litter set both where it forages and where it shelters. Its modelled population resilience is high — minimum doubling time under about 15 months — and its fishing vulnerability low, as you would expect of a tiny, fast-maturing fish of no food-fishery interest.
Mating
Like the rest of its genus, Apistogramma paulmuelleri is a small territorial cichlid whose social life reorganises around breeding. The describers report the pattern typical of the genus's monogamous species — a pair forms and shares a territory — but note that polygamy turns up too, less often, as in some congeners; in captivity a single male will hold a territory and tend a small harem of females where space and cover allow. Pair formation runs on colour and posture, and this is an unusually fluent signaller: the original description singles out a reversal of the dark band pattern during aggression and courtship, the fish flipping the contrast of its vertical bars and lateral markings in a rapid, mood-driven display. That strong colour-change is a big part of why the old German trade name "Masken" ("masks") stuck. As in many dwarf cichlids, a female guarding eggs or fry can turn intensely aggressive toward her mate at close quarters, sometimes forcing his removal in a small tank.
Breeding
Apistogramma paulmuelleri is a cave-spawning substrate brooder, not a mouthbrooder. The female fastens her eggs to the ceiling or wall of a crevice or cavity and leads the guarding of both clutch and fry, while the male defends the wider territory; the describers watched a few wild females shepherding small free-swimming young. No precise wild clutch count is published, but congeneric cave-spawning Apistogramma typically lay on the order of several dozen to a couple of hundred eggs, and this fish sits in that range rather than producing the large open clutches of bigger cichlids. Spawning is reported to be straightforward in suitably soft, acidic water — Christoffersen, who kept the wild form, found it bred easily at around pH 5.5. Once past the egg stage the fry grow on without much trouble, retreating to cover under the female's watch at any threat.
In the aquarium
Apistogramma paulmuelleri is uncommon in the hobby — it comes through specialist importers as wild-caught Peruvian stock, held by only a handful of dwarf-cichlid breeders at a time — but that reflects its narrow range and recent description, not real difficulty. German importers have even pitched it as close to an ideal "beginner's Apistogramma," relatively hardy and forgiving of new-keeper mistakes, and anyone who has kept the eunotus complex or the similar Apistogramma regani will find it familiar.
Tank: a single pair does well over a small footprint, reported anywhere from about 45 × 12 in (a roughly 40-litre tank) to 60 × 12 in (about 15 US gal); a harem of one male and two or three females wants at least 31.5 in of length so each female can hold her own ground. Give it fine sand to sift, broken sight lines from driftwood and slow-growing plants such as Java fern (Microsorum pteropus), and shaded caves — flowerpot halves, pipe sections or cave ornaments, whichever hollow the female chooses as a spawning site. A layer of dried leaf litter (oak or catappa, replaced before it fully decays) softens and tints the water and grows the infusoria, copepods and nematodes that feed newly free-swimming fry.
Water: the key variable. The type-stream corridor is ion-poor, soft and acidic — pH about 4.7–5.3 in the dry season, conductivity in the tens of µS/cm, 81–86 °F. Aquarium tolerance is wider (pH 4.0–6.5, up to about 3 dH, 72–88 °F), but working near the natural end — pH 5.5–6.5 for routine care, 5.0–6.0 to reliably trigger spawning — brings better colour, calmer fish and better fry survival than neutral or hard tap water. Where the supply is alkaline, use reverse-osmosis water cut with a little mineralised water, or conditioned with dried leaves. Keep flow gentle with an air-driven sponge filter — it holds biological filtration without the current these still, leaf-littered streams never have — and lean on small, regular water changes, since accumulated organic waste kills appetite, dulls colour and feeds the bacterial infections that claim most captive dwarf cichlids.
Tankmates: small mid- to upper-water fish that won't contest the bottom — pencilfish (Nannostomus), hatchetfish (Carnegiella) and small tetras that share the soft, acidic conditions. Avoid other bottom-dwelling cichlids, and other Apistogramma, for both the territory competition and the risk of hybridising with look-alike regani-group forms; add small Corydoras or Otocinclus only with care, since Corydoras may disturb spawning caves and some dwarf catfish dislike the low-conductivity water this fish needs.
Sexing and mistakes: near adult size males are larger, brighter and more elaborately finned, while females flush bright yellow on the underside of the head and belly when spawning-ready or guarding — a reliable cue even in dim light. The species is honest about its difficulty: more demanding than a standard community fish, less finicky than some cave-dwelling Apistogramma. The usual missteps are an immature tank (it reacts badly to ammonia or nitrite), water that is too hard or too alkaline, and crowding the male in with a guarding female, who can do real injury to a mate she cannot drive off.
Conservation
The IUCN Red List assesses Apistogramma paulmuelleri as Data Deficient (assessed 25 April 2014, published 2016; assessors Ortega Torres & Chuctaya, reviewer Tognelli). The reasoning is plain: the fish was newly described, is known only from its type locality, and comes with no data on population size, trend or threats — the trend is logged as unknown and the assessment itself flagged as needing an update. The Red List notes that it enters the international ornamental-fish trade, and the describers reported that parts of the type stream had been degraded since 2009 by removal of riparian vegetation. So the documented pressures on this particular fish are localised habitat disturbance and hobby collection, not any measured decline; the species is not CITES-listed.
"Data Deficient" is a statement about missing information, not a clean bill of health. A fish known from one small forest brook, present at low abundance, with the very vegetation that shades and stabilises its stream already being cleared, is exactly the kind of narrow-range upper-Amazon endemic that can be lost quietly before anyone notices — which is why the assessors call explicitly for research on its distribution, population and threats. For wider context, the standard taxonomic baseline for Amazon cichlids remains Kullander's treatment of the family in the Check List of the Freshwater Fishes of South and Central America (Kullander & Ferraris, 2003). Most western-Amazon dwarf cichlids face habitat change — deforestation, riparian clearing, agriculture and the steady expansion of roads like the Iquitos–Nauta highway that runs past this species' home stream — alongside ornamental collection, rather than outright population collapse. The honest reading for Apistogramma paulmuelleri is narrower still: we do not yet know enough to assign a threat category, the forest around it is under real and rising pressure, and the prudent course is to treat a single-stream endemic in a clearing landscape as vulnerable in fact even while it stays Data Deficient on paper.