Family Characidae or American tetras ( Characidae )
Group of uncertain status ( Incertae Sedis )
Genus Nematobrycon — Nematobrycon.
Two emperor tetras are commonly kept in aquariums: the typical “palmeri”, which is found quite often, and the extremely rare “red-eyed” tetra. Between them, like a kind of mirage, there is also a third species, Nematobrycon amphiloxus — the black emperor tetra. In terms of body and fin shape, except for the caudal fin, it does not differ from palmeri. The overall coloration is black. The head and the whitish stripes running from it along the belly and the nape to the dorsal fin are lighter. During disputes, males become completely black, including all fins, with only their bright blue eyes sparkling. In a calm environment, the fish have a black body with light contours along the back and belly. Fins may be light gray or even colorless and transparent.
In its color scheme, lacortei (Nematobrycon lacortei) is similar to palmeri (Nematobrycon palmeri), but the colors differ noticeably. While palmeri displays a blue-green overall tone, lacortei, especially males, exhibits a yellowish-reddish hue. The snout and gill covers are tinted reddish, which deepens into a relatively broad dark blue-violet stripe with diffuse borders that extends along the body to the tail. On the caudal fin, this stripe turns into a narrow black line extending onto the middle rays that project far beyond the tail fork.
A highly remarkable feature of its coloration is a body patch that shines brightly under side lighting, starting before the middle of the body and reaching the caudal peduncle. It resembles the iridescent sheen of a thin oil film on water. On the sides of this fish almost the entire spectrum is present: red, yellow, sky blue, deep blue, and violet, though these colors are of a delicate, soft tone.
The overall background coloration of the female is lighter and somewhat yellowish. Ruby-red eyes are found only in the male, and even then not entirely: a thin yellowish-greenish rim runs along the edge of the eye. Females have eyes of the same color as the females of palmeri and amphiloxus, that is, yellow-green. Young males also have such eyes until maturity. The redness of the eye helps hobbyists sex the fish at the very beginning of their maturation.
Now a few comparisons among these three species. Adult males of all three species differ in the shape of the caudal fin. In Nematobrycon palmeri, the caudal “trident” is most distinctly expressed. The edges of the lobes are black and elongated to such an extent that their streamers sometimes meet each other. The central ray is black, long, and pointed.
Nematobrycon lacortei has more modestly sized ornaments. A streamer is present only on the upper lobe, and the central fork tooth is shorter. The lower lobe of the caudal fin lacks a streamer, but features a whitish edging. Nematobrycon amphiloxus has an even simpler tail with equal, relatively short pointed tips on the lobes, without streamers, but with a thin and long central projection.
— Lacorte’s tetra, Rainbow tetra, Red-eyed tetra — Nematobrycon lacortei [Weitzman & Fink, 1971]. Basins of the San Juan and Río Atrato rivers in Colombia. 5 cm, female smaller. They differ from the well-known emperor tetra (N. palmeri) by mother-of-pearl inserts along the lateral line and ruby eyes; the longitudinal stripe is only faintly sketched, and the colorful body pattern is less clearly defined. The head is brownish. The eyes of males are bright ruby-red, while in females they are green.
From the eye, a broad violet or dark blue stripe runs along the body, transitioning into a narrow black line on the middle rays of the caudal fin. The back is copper-brown, and the fish’s flanks have a silvery-brownish color that shimmers under incoming light. A blue sheen and irregularly scattered spots resembling the spangle pattern on galvanized sheet metal are visible on the sides.
A distinctive feature of the species is the unusual scales on the sides of the fish. It looks as though some of the scales have been stripped away, and this pattern is unique to each individual and even to each side of the body. In diffused light, it displays an astonishingly beautiful, blue-sheened, spot-illuminated pattern.
Fins are yellowish and transparent. The anal fin features a yellow border and a black submarginal line along the lower edge. Its rays are not very long, but there are many of them, and the base of this fin extends almost to the caudal. In males, the dorsal fin is sickle-like with a streamer, and the caudal fin forms a trident. Males are larger than females; females are lighter, more petite, with a rounded abdomen.
Peaceful, schooling; when kept singly, they become aggressive. They exhibit hierarchical behavior, with males competing for territory, yet the fish do not assume an inclined body posture; instead, they hover horizontally in the water column like “normal” fish should. Overall territory control is maintained by the most impressive, dominant male, who considers the entire aquarium his domain. Therefore, a spacious aquarium is required.
Females roam throughout the tank in small shoals or individually. At the same time, priority in mating always belongs to the “king”, even in a tiered harem setup. While the leader inspects his domains, subordinate males out of his sight line also court passing females.
The greatest ornamental effect is achieved in large aquariums surrounded by related species (neons, pencilfishes, pyrrhulinas), as well as small armored catfish (Otocinclus, loricariids) and Corydoras catfish. Dense vegetation and floating plants are recommended. Diffused lighting. Soft to medium-hard water.
Nematobrycon are quite picky eaters and may refuse offered food, so the more varied their diet, the better. The best food for adult fish is planktonic crustaceans, especially cyclops and glassworms (chaoborus). Vegetable food is essential, otherwise they will nibble young leaves. The fish readily pick food off the bottom. Fish reach sexual maturity at 9 months of age. The female scatters up to 160 eggs among fine-leaved plants. Lifespan is up to 6 years. pH 6.0-7.0; dH 2-12°; T 20-27°C.
— Palmeri, Emperor tetra — Nematobrycon palmeri [Eigenmann, 1911]. Synonym: Nematobrycon amphiloxus. Colombia, San Juan and Atrato river basins. 5 cm. Unlike most characins, Nematobrycon lack an adipose fin, and the caudal fin contains elongated central rays in addition to two lobes, giving it a unique three-lobed trident shape. The base body coloration is golden-yellow with a violet-blue and green luster; the belly is yellowish-silvery. The eyes are blue. In N. lacortei the eye iris is bright red, whereas Inpaichthys kerri is smaller, has modest fins, and unlike Nematobrycon possesses an adipose fin.
The upper head and back are olive-brown. The belly and lower part of the head are white. On the flank, from the lips to the tail, run two broad iridescent stripes: the upper stripe ranges from pale green to greenish-blue, while the lower stripe is dark brown to black; closer to the tail, the stripe narrows and turns into a black “streamer” extending far beyond the deep notch of the relatively large caudal fin. The gill covers, entire back, and caudal area above the black lateral stripe gleam turquoise in reflected light. Often a thin dark pink stripe is also present in the caudal area above the anal fin.
All fins have a faint yellowish tint, with some fin rays being creamy-white. The front three or four dorsal fin rays are saber-shaped, elongated, and blackish. The dorsal fin is yellow with reddish-brown rays. The long, relatively narrow anal fin is yellowish-brown with a blackish submarginal line and a yellow border along the edge.
The caudal fin is yellowish-lemon with reddish-brown streaks; its upper and lower rays are elongated and colored black or dark brown, making the tail resemble a trident. Some specimens also develop two streamers on the caudal fin. Pectoral and pelvic fins are light yellow with whitish-yellow edges. The pectoral fins are set quite low on the body and move in a vertical plane, so when the fish suddenly stops while flutter-fanning all paired fins, it looks as though it is “treading water” in place.
Males differ from females by the middle projection of the caudal fin, strongly elongated outer and central rays, a dorsal fin elongated into a streamer, and yellow fins. The full spectrum of coloration in males can be observed at around one year of age. The female is smaller, plumper, less intensely colored, has a silvery iris, and the middle caudal rays are only slightly extended.
In males, the third ray of the dorsal fin is the longest, whereas in females the fourth or fifth rays are longest. The full color spectrum of the male becomes evident at one year of age. The black emperor tetra, Nematobrycon amphiloxus, is considered a dark melanistic color form of palmeri.
Peaceful, easygoing, rarely falling ill. Fish usually hold a head-down posture at an angle of about 45° across all water levels. Females and young males swim freely throughout the tank in small groups, while males hold territories, usually near plant thickets. Keep them in a school with a predominance of females; they exhibit hierarchical behavior with males contesting territory. Males, especially the school leader, show strong fidelity to a specific territory and chase away other fish.
The size of claimed territory depends on tank stocking levels and the individual disposition of each fish. Their skirmishes pose no danger to other species; other fishes are generally ignored unless they provoke a territorial defense response. During aggressive displays, the front of the male’s body takes on a purple tinge, and they hold their body head-down in the water. A female is courted and invited into the chosen patch for spawning. During courtship, the male darts fussily around the female, quivering all fins.
Males establish a strict hierarchy among themselves in the aquarium. When kept singly or in small groups in large tanks, dominant males can turn into true tyrants, vigorously defending their chosen territory. In a small tank, an alpha male will suppress all other conspecific males. Other fish species with territorial habits that claim space among plants may also get targeted. In a spacious aquarium with abundant vegetation, clashes between males are much less severe.
When a new male appears (or an old one is removed), rank sorting begins and continues until the hierarchy is re-established. A defeated male clamps his trembling fins, turns flank to the victor, and shows full submission. The male at the top of the social ladder avoids petty brawls; however, if his position is challenged directly, he acts with equal firmness.
The finest colors of these fish emerge in aquariums with lush vegetation and driftwood against dark substrate under diffused overhead light (floating plants). Regular water changes improve fish vitality. Water chemistry is not particularly critical, though soft, slightly acidic water is preferred. Live and plant foods are essential; otherwise, they will chew on tender young leaves. At 7–9 months of age, the fish begin breeding.
By one month of age, fry develop a dark longitudinal stripe. In the third week, they adopt the head-down slanted posture typical of N. palmeri and maintain a relatively sedentary lifestyle until nearly two months old. Then they begin taking interest in one another, engaging in typical juvenile fish chases. The first reliable sex indicator in juveniles is the lengthening of the middle black caudal rays (appearance of the “trident”) in males and the emerging rounded abdomen in females. An excellent choice for novice aquarists. Lifespan is around 8 years. pH 6.0-7.5; dH 5-20°; T 23-26°C.
Identification: Emperor Tetra — Nematobrycon palmeri.
Paracheirodon axelrodi, Cardinal Tetra, Paracheirodon simulans.
Moenkhausia grandisquamis, lepidura, oligolepis, pittieri, sanctaefilomenae. 
