Nannostomus mortenthaleri · Coral Red Pencilfish

Overview

Bright red between the upper two dark flank stripes is a striking feature of male Nannostomus mortenthaleri. This small pencilfish is generally peaceful toward other species, yet male rivalry is more intense than in most congeners. Its size should not be used to justify a cramped aquarium.

Distribution & habitat

It appears restricted to the Nanay basin and the slightly more westerly Tigre system in Loreto, Peru. The type locality is a Nanay tributary near Alvarenga village.

Habitats include sluggish streams, small rivers, swamps, flooded forest, and floodplain lakes in blackwater regions. Shallow, nearly still water contains vegetation, submerged branches, and leaves, with low hardness and conductivity. Tigre collection sites had thick bank vegetation and leaf-and-branch substrates alongside other small characins, pencilfishes, and dwarf cichlids.

Aquarium setup

A mixed-sex group needs an aquarium at least 90 cm long. Dense plants, wood, and branches must interrupt sight lines and provide retreats that subordinate males can actually use. Increasing numbers in a bare aquarium does not solve aggression.

A dark substrate and floating plants help reduce nervousness. Dried leaves encourage microorganisms that may supplement fry feeding. Keep filtration gentle, typically with an air-driven sponge. Stable water conditions are necessary, so introduce the fish only after the system has matured.

Diet & feeding

Tiny invertebrates and zooplankton form its natural diet. Correctly sized dry foods are accepted in captivity, but give daily small live or frozen foods such as Artemia nauplii, Moina, and small worms.

Behaviour & compatibility

Prefer a species aquarium or a restrained community of tiny peaceful characins and small callichthyid or loricariid catfishes. It generally occupies the middle and upper water and can act as a dither fish for dwarf cichlids. It does not actively hunt fry, which is different from guaranteeing complete fry safety.

Groups are loose rather than tightly coordinated schools. Territorial males spar frequently by day, and confinement can lead to serious injuries or death. Ten or more help distribute pressure, but only alongside adequate space and effective visual barriers.

Sexual dimorphism

Adult males are slimmer and more colorful. A white mark at the front of the dorsal-fin base appears relatively early. Bright red occupies the area between the upper two dark lateral stripes, and the pelvic and anal fins are largely red. Females are fuller-bodied and less intensely colored.

Breeding

Aquarium breeding has succeeded, with occasional fry surviving unassisted in mature planted tanks. For greater output, prepare aged water and abundant fine spawning yarn, moss, or other finely divided plants. Lighting is unnecessary, and a small air-powered sponge can be used.

Introduce a conditioned pair or several pairs. With multiple pairs, separate the spawning media into distinct areas, such as opposite ends with open space between. Remove adults after two to three days; fry may appear a day or two later. Alternatively, inspect yarn daily and move egg-bearing material to a separate hatching container, replacing it with fresh media.

Begin with Paramecium or fine fry food around 5–50 μm. Add newly hatched Artemia and microworms once they can be swallowed. Adult patterning develops after roughly eight weeks.

Notes

Originally described as a subspecies of Nannostomus marginatus, it was subsequently recognized as a separate species. Confusion with Nannostomus rubrocaudatus is common: that species has more uniform purplish red over the rear body, a red front to the dorsal, and transparent distal fin areas. Here, the anterior dorsal base is white, the outer portion reddish, and nonbreeding body red is concentrated between the upper two dark stripes.

Females provide another comparison. The corresponding interstripe area is reddish in this species but mainly silver in N. rubrocaudatus, whose pelvic and anal fins have red bases and clear tips. Night coloration becomes much paler in N. mortenthaleri, but the dark longitudinal stripes remain visible. That daily change alone is not evidence of disease.