Taxonomy & naming
Nannostomus mortenthaleri was formally described by Hans-Joachim Paepke and Ronald Arendt in 2001, published in Verhandlungen der Gesellschaft für Ichthyologie. The type locality is a small tributary of the Rio Nanay at Alvarenga, near Iquitos, Loreto, Peru (approximately 3°31'10"S 74°25'40"W). It was originally proposed as a subspecies, Nannostomus marginatus mortenthaleri, but is now treated as a full species. Eschmeyer's Catalog of Fishes records the valid name as Nannostomus mortenthaleri Paepke & Arendt, 2001.
The species name honours Martin Mortenthaler, an Austrian fish exporter and the owner of Aquarium Rio Momon in Iquitos, who collected the holotype material and brought the fish to scientific attention. The genus Nannostomus (family Lebiasinidae, subfamily Pyrrhulininae) contains the pencilfish — slender, horizontally striped characoids of tropical South America that display a distinctive nocturnal colour change, losing their longitudinal stripes at night in favour of a blotched or bar pattern. N. mortenthaleri is most similar in body plan to N. marginatus but is immediately distinguished by far more vivid red coloration suffusing the entire body rather than being restricted to the fin bases.
Morphology
Nannostomus mortenthaleri is a small, elongate pencilfish reaching approximately 1 in standard length, with some aquarium records citing up to 1.5 in total length. The body is slender and nearly cylindrical in cross-section, as is typical of the genus. It carries the pencilfish family's characteristic horizontal banding: a bold dark lateral stripe runs from the snout tip to the base of the caudal fin, underlain by a narrower pale stripe below, and additional dark lines run along the upper body.
What sets this species apart is the colour ground: the body between and above the dark stripes is suffused with intense coral to brick-red, with red also saturating the unpaired fins. Males display the most vivid coloration, with deeper red overall and more intensely pigmented fins; females tend to be plumper-bodied, particularly when gravid, and somewhat paler. Like other Nannostomus species, it shows a pronounced diurnal rhythm in colour: the horizontal stripe pattern is most vivid by day, while at night the fish shifts to a mottled or blotched pattern that disappears again by morning.
Habitat
Nannostomus mortenthaleri is endemic to the Rio Nanay drainage in Loreto, Peru, making it one of the most geographically restricted pencilfish known. The Rio Nanay is a blackwater tributary of the Amazon that drains heavily leached, nutrient-poor soils; its waters are characteristically dark with dissolved tannins and humic acids, very soft, and strongly acid. The species has been collected from small forest tributaries within this system at the type locality near Iquitos.
Reported water parameters from collection sites and established husbandry references indicate temperatures of approximately 75–84 °F, pH in the range of 4.0–7.0 (likely toward the acid end in prime habitat), and low hardness (2–8°dH). These are classic Amazonian blackwater conditions: dim, tannin-stained water filtered through leaf litter and root tangles in flooded forest streams. The species' extremely restricted range — a single river basin — is the root cause of its precarious conservation status.
Feeding
Nannostomus mortenthaleri is a micropredator in the wild, subsisting primarily on small zooplankton, tiny invertebrates, and other minute animal prey encountered in the water column and at the surface of leaf litter. Its small mouth and slender body are adapted for picking off fine prey items rather than pursuing larger food, and in nature it likely feeds heavily on copepods, ostracods, and small insect larvae.
In the aquarium it accepts a broad range of small live and frozen foods: newly hatched brine shrimp, Daphnia, Cyclops, Grindal worms, and small bloodworm. It will also take finely crushed high-quality dried flake and micro-pellet foods. Because of its small mouth, food particle size matters — even fine-ground flake is preferable to whole pellets. Feeding small amounts multiple times a day suits its foraging style and improves condition and colour.
Mating
Nannostomus mortenthaleri is an egg-scattering open spawner. Males are territorial and notably aggressive toward one another: in the confines of an aquarium, dominant males will harass and stress subordinates and females if space and cover are insufficient, requiring dense planting or a larger tank to diffuse this aggression. Courtship consists of male display behaviour, with fin-spreading and colour intensification directed at females.
Spawning occurs among fine-leaved vegetation, with the pair releasing adhesive eggs that settle or cling among plant leaves and substrate debris. There is no parental care; adults will consume eggs given the opportunity. Because males are highly territorial, breeders often condition fish in groups with plenty of plant cover, or use a single well-conditioned pair in a dedicated spawning setup, removing adults after spawning is observed.
Breeding
Breeding Nannostomus mortenthaleri in captivity is considered achievable but requires attention to water quality and space. Conditioned pairs or trios (one male, one or two females) are placed in a small, well-planted tank with soft, acid water mimicking blackwater conditions — pH 5.0–6.5, very low hardness, and temperature in the upper part of the species' range (around 79–82 °F) to promote spawning. Java moss, fine-leaved plants, or a spawning mop provides the egg-laying substrate.
Eggs are small and adhesive; adults should be removed after spawning to prevent predation. At typical breeding temperatures, eggs hatch in approximately one week. Newly hatched fry are very small and require the finest first foods: infusoria or commercial liquid fry food initially, then vinegar eels or newly hatched brine shrimp nauplii as the fry grow. Adult coloration develops gradually over the first weeks, with full red pigmentation appearing around eight weeks of age according to hobbyist records.
In the aquarium
Nannostomus mortenthaleri is kept by specialist pencilfish enthusiasts and experienced soft-water aquarists rather than as a general community fish. Its small size and delicate constitution demand stable water chemistry with low hardness and moderately acid pH; the strong blackwater conditions of its Rio Nanay homeland are worth replicating with peat filtration, botanicals, or RO water. It is sensitive to poor water quality and wide parameter swings.
A group of six or more is ideal for natural behaviour — males display their best coloration when rival males are present — but the tank must be densely planted or structured with ample visual breaks to limit territorial aggression between males. Tankmates, if any, should be similarly small, gentle species that will not outcompete it at feeding or stress it with boisterous movement. It is often kept in a species-only aquarium or with other small, soft-water characoids and tiny Corydoras. Wild-caught specimens are best avoided given the species' Critically Endangered status; captive-bred fish are occasionally available from specialist breeders and should be the only source sought.
Conservation
Nannostomus mortenthaleri is assessed as Critically Endangered (CR) on the IUCN Red List (assessment 24 April 2014, criterion B1ab(v)). The assessment identifies a single population with a continuously declining trend driven primarily by collection for the international ornamental fish trade. From the moment this spectacularly coloured pencilfish became known to the hobby following its 2001 description, demand was intense; large-scale collection from its restricted Rio Nanay range inflicted heavy pressure on what was already a spatially limited wild population.
The combination of a single known population, a restricted endemic range, and direct take for the trade is precisely the threat profile that warrants a Critically Endangered listing. There are no large, buffering outlier populations to absorb localised collecting pressure. The practical conservation response for aquarists is straightforward: do not purchase wild-caught specimens, and support the availability of captive-bred stock. Captive breeding is feasible and, where it succeeds, removes any justification for further wild collection. Monitoring of the Rio Nanay population and enforcement of sustainable export limits, where enforced at all, remain the primary in situ measures.