Taxonomy & naming
Wolfgang Staeck described Apistogramma similis in 2003 in the third volume of his Cichliden-Lexikon (Teil 3: Südamerikanische Zwergbuntbarsche, Dähne Verlag), the page-76 account that remains the only formal treatment of the species. The holotype (MTD F 27038, a fish 1.5 in in standard length) came from a forest brook at the Laguna La Brava in the río Yata system, Department of Beni, Bolivia, collected by Staeck himself in 2001; paratypes are split among the Dresden museum (MTD), the Naturhistorisches Museum Wien (NMW), the Zoologisches Forschungsmuseum Alexander Koenig in Bonn (ZFMK), and the Berlin museum (ZMB). The Catalog of Fishes and FishBase both list the name as valid, and there are no synonyms — the species has never been split, lumped, or renamed.
Where it sits inside the genus is the more interesting question, and the sources broadly agree. Apistogramma is conventionally divided into a handful of major lineages; similis belongs to the regani lineage (the Apistogramma regani species-group), and within it to the commbrae complex, the cluster of small, lateral-band dwarves of the upper Madeira and Paraguay drainages that includes Apistogramma inconspicua, Apistogramma linkei, and Apistogramma commbrae. This places it apart from the agassizii lineage, whose members carry a metallic patch at the corner of the mouth and reduced cephalic pores — features the regani-group fishes lack. The species epithet is a direct nod to this company: Staeck coined 'similis' because the new fish so closely resembles Apistogramma inconspicua and Apistogramma linkei, its near-neighbours in the same Bolivian waters, that telling them apart in the field is genuinely difficult.
Morphology
This is a very small cichlid even by Apistogramma standards. The 1.5 in standard-length holotype is among the largest measured specimens, and like its commbrae-complex relatives — Apistogramma inconspicua males run to roughly 1 in SL — most individuals are well under 1.5 in, with females smaller still. FishBase carries no maximum-length figure, a reflection of how thin the published data are.
In body plan and pattern similis is a generic 'regani-type' dwarf: a slender, slightly compressed little fish whose defining mark is a continuous dark lateral band running from the snout to the tail base, without the discrete mid-body lateral spot or the bold vertical bars seen in showier congeners. The lateral band, dark caudal-peduncle marking, and rather muted brown-grey ground colour are exactly the combination that makes it so hard to separate from Apistogramma inconspicua and Apistogramma linkei — the resemblance that gave the fish its name. Breeding males develop the modest red and blue flush on the fins and gill covers typical of the group; the diagnostic separation from its look-alikes rests on fine details of squamation, coloration, and proportions given in Staeck's original description rather than on any single field mark. As a harem-spawning Apistogramma it is sexually dimorphic — males larger and more colourful, females smaller and, when guarding fry, turning the bright yellow with black markings characteristic of the genus — though the species-specific particulars are not well documented in the literature.
Habitat
Apistogramma similis is endemic to the lowland Bolivian Amazon, in the upper Madeira basin. Its type locality is a forest brook feeding the Laguna La Brava in the río Yata system of northern Beni — the Yata is a north-flowing tributary that joins the río Mamoré near the Brazilian border (the broader river-mouth region around 12.3°S, 66.1°W; the type locality itself is approximate, as no precise coordinates were published). Pascal Hablützel's 2012 survey near Santa Ana del Yacuma (around 13.7°S, 65.4°W) recorded the species for the first time outside that type locality and reported it as widespread in rivers and ponds throughout the lowlands of the río Mamoré sub-drainage west of the river's main course. FishBase and the Catalog of Fishes give the distribution simply as Bolivia; there is no sourced record from Brazilian territory, so the range is treated here as Bolivian only.
The biotope is the classic flooded-lowland mosaic of the Llanos de Moxos: small forest brooks, oxbow lakes, seasonal floodplain pools, and the margins of larger white-water rivers such as the río Yacuma and río Rapulo. Hablützel collected similis specifically in oxbow lakes and rivers, working dip-nets through the leaf litter and submerged vegetation where dwarf cichlids shelter. These are shallow, structure-rich, soft-bottomed waters at low elevation (the IUCN assessment places the species at about 492 ft above sea level); it is a benthic fish of the litter zone rather than open water, so the depth range is effectively the shallows. No in-situ depth, pH, or temperature readings were recorded for similis at the collection sites, and FishBase publishes none. The figures used here are basin-typical for the lowland Mamoré drainage it occupies: across the wider Mamoré–Madre de Dios system surface temperatures average roughly 75–81 °F and pH spans about 5.1 to 7.9 (Goulding et al.), while a documented forest-stream biotope in the same lowlands — home to the close relatives Apistogramma linkei and Apistogramma erythrura — ran 70–82 °F, pH 6.6–7.2, and 110–220 µS/cm, the soft-acidic to near-neutral white- and clear-water envelope typical of these meandering streams and oxbows. The wider region experiences a strong wet–dry seasonal cycle that connects and isolates these water bodies over the year.
Feeding
No dedicated diet study exists for Apistogramma similis, but its feeding ecology can be read confidently from its body plan, its genus, and its habitat. Like essentially all Apistogramma, it is a micro-predator of the substrate and litter zone — a small benthic invertivore that picks and sifts at the bottom for aquatic insect larvae, micro-crustaceans, worms, and other tiny invertebrates among the leaf litter and detritus. FishBase places the genus around trophic level 3.3, consistent with a carnivore working low in the food web. In the flooded forest brooks, oxbow lakes, and pond margins it inhabits, that means living off the dense interstitial fauna of the litter and the fine organic ooze of these shallow, slow waters, and in turn serving as prey for the larger characins, cichlids, and catfishes that share the Mamoré floodplain.
Mating
Specific courtship observations for similis have not been published, so its mating system is inferred from the well-established pattern of the regani-group and of cave-spawning Apistogramma generally. The Cichlid Room Companion characterises it as polygynous — a harem brooder — which is the dominant arrangement in the genus: a single male holds and patrols a territory overlapping the smaller territories of several females, each of which defends a spawning cave of her own. Pair bonds are brief and centred on the cavity; the male's role is largely to court receptive females, fertilise their clutches, and guard the outer perimeter of the harem against rivals and predators, while the females do the close brood care. As in its relatives, spawning is bound up with finding and defending a small, dark, enclosed cavity, and the sexes are strongly size- and colour-differentiated during the breeding period. The fine details — display repertoire, the number of females a male holds, the triggers for spawning — remain undocumented for this particular species.
Breeding
Breeding has not been studied in the wild for Apistogramma similis; what can be said follows from its classification as a substrate cave-spawner in the harem-brooding Apistogramma mould, as recorded on the Cichlid Room Companion. The female selects and cleans a small, concealed cavity — a hollow under a root or branch, a crevice, or a snail shell in nature, and an inverted pot or coconut shell in aquaria — and attaches a modest clutch of eggs (Apistogramma clutches typically number in the dozens to low hundreds) to its ceiling, where the male fertilises them. Care is essentially maternal: the female tends and fans the eggs and then leads and guards the free-swimming fry, taking on the bright yellow-and-black guarding livery typical of the genus, while the male defends the broader territory. The young pass through the usual cichlid sequence of adhesive eggs, yolk-bearing wrigglers, and then a tight, parent-shepherded school of fry foraging over the bottom. No clutch counts, egg or fry sizes, or developmental timings have been published for similis specifically, and the figures above are the genus-level expectation rather than measured values for this species.
In the aquarium
Apistogramma similis is only occasionally encountered in the hobby — it is not a trade staple and is unlikely to turn up at an ordinary fish shop. The most common route to obtaining it is through specialist cichlid societies, European importers of Bolivian dwarves, or keepers who have bred it from wild-caught stock. Confusion with Apistogramma bitaeniata (a Peru/Brazil species from the same bitaeniata group, broadly popular in the trade) is a real risk when buying unlabelled fish, but similis is a regani-lineage species and the two are not closely related; for husbandry purposes the distinction matters less than correctly identifying the animal before you plan the tank. If provenance is uncertain, treat the fish as the soft-water, warm-water Bolivian lowland fish it is and you will not be far wrong.
A pair or a harem trio (one male, two females) is comfortable in an aquarium of 15–20 US gal; a single female needs far less space, but the male's territory requirements mean anything under 15 US gal makes keeping a functioning harem difficult without aggression problems. Use fine sand as the substrate — these are bottom-picking micro-predators that work the litter zone all day. Leaf litter (dried catappa, oak, or beech leaves), a handful of twisted roots or driftwood pieces, and a scattering of coconut-half caves or small ceramic tubes provide the visual breaks and spawning sites the fish depend on. Dense planting is an asset: Echinodorus, Helanthium, or floating plants like Salvinia dim the light and make females feel secure enough to come out and feed. Keep lighting moderate to low.
Water chemistry should match the lowland Mamoré biotope: temperature 75–82 °F, pH 5.5–7.5, and soft to moderately hard water (below roughly 150 µS/cm conductivity is ideal; the documented forest-stream congener habitat ran 110–220 µS/cm). These are white- and clear-water fish accustomed to variable but broadly soft conditions, and they will tolerate pH approaching neutral — but spawning success and colour expression are generally better in the soft, slightly acidic end of that range. Avoid calcareous substrate and hard tap water without RO dilution; limestone gravel and pH above 7.5 are the most common keeper mistakes.
Tankmates should be chosen carefully. Small, peaceful characins from the same drainage — like pencilfish (Nannostomus species) or small tetras — work well as dither fish: their presence in mid-water reassures the cichlids that no serious predator is lurking, which brings the dwarves out into the open. Avoid any boisterous or fin-nipping species, and do not mix multiple Apistogramma species in a standard display tank unless the tank is large and heavily broken up. Corydoras or small Ancistrus catfishes make acceptable bottom-sharing companions provided the cave density is high enough that the females have exclusive spawning territories.
Sexing is straightforward once you know the genus pattern: males are noticeably larger (up to roughly 1.5 in SL) and carry better colour; females are smaller, more plainly marked at rest, and turn vivid yellow with black facial and fin markings when guarding a brood. Like other harem-brooding Apistogramma, Apistogramma similis is not a difficult species to spawn once conditions are right — a drop in pH, a water change with slightly cooler, soft water, and plenty of caves will often trigger spawning — but because it is rarely kept, first-hand accounts from successful breeders are sparse. Broods are modest in size (dozens of eggs rather than hundreds), the female handles all close parental duties, and the male should ideally be separated or given enough space to avoid harassing a brooding female in a small tank. Fry accept Artemia nauplii and microworms from first feeding.
Conservation
The IUCN Red List assesses Apistogramma similis as Least Concern (assessed 23 April 2014 by J. Sarmiento and F. Carvajal; published 2016). The rationale is the ordinary one for a small, widely distributed lowland fish: Hablützel's work shows it is not a single-locality endemic but a species widespread across the rivers and oxbow lakes of the río Mamoré lowlands, and there is no evidence of decline or of any acute, range-wide threat. The assessment notes that part of its range falls within the Yata RAMSAR wetland site. It carries no CITES or CMS listing and is harmless to people.
The pressures on its habitat are the diffuse ones facing the whole Bolivian Amazon lowland — deforestation and agricultural expansion in the Llanos de Moxos, cattle ranching, and the hydrological changes that come with road-building and dam projects on the Madeira system — but none is documented as a specific danger to this fish. The more honest caveat is how little is actually known: a species described only in 2003 from one hobbyist-collected locality, with no field studies of its biology, could be carrying undescribed diversity or a patchier real distribution than the current 'widespread in Bolivia' summary implies. As with many of the small, look-alike dwarves of this basin, the conservation story is less about a fish in trouble than about a fish we have barely begun to study.