Bio Facts: Tetra, Cardinal
South America: the upper Orinoco and Negro Rivers of Venezuela and Brazil
Blackwater rivers characterized by an acidic pH, low mineral content and the presence of humic acids.
Adult length: 1.25 in. (3 cm); coloration: striking iridescent blue line with the body below this line being vivid red in color; appearance: mature females are usually bigger and noticeably stockier than males.
In the wild – 1 year; in captivity – several years
Less than 1 year
In the wild, they eat small crustaceans and worms; in the Zoo, they are fed a scientifically developed, commercially available high quality flake fish and pelleted food.
Cardinal tetras are a shoaling species by nature, and will fare much better when in the company of its own kind. A micropredator, it feeds mainly on small invertebrates.
The cardinal tetra, in the wild, swims upstream in large numbers to parts of its native river habitat that are completely enclosed above by rainforest canopy. Such waters are subject to heavy shading by the rainforest trees, and virtually no sunlight reaches them. Here, the fishes spawn in large aggregations.
In the aquarium, a single pair can be conditioned for breeding, but the breeding aquarium not only needs to contain water with the correct chemical parameters cited above: the breeding aquarium needs to be heavily shaded to mimic the low light conditions of the fish’s native spawning grounds. If the fishes are ready to spawn, the male, which will be the slimmer of the two fishes in outline, will pursue the female into fine-leaved plants: her fuller outline, which usually indicates the presence of ripe eggs within her reproductive tract, should be readily apparent at this point. If the female is ready, she will allow the male to swim alongside her, and together, the pair will release eggs and sperm.
Apart from the stringent requirements with respect to water chemistry, one of the major difficulties mitigating against success in captive breeding of the species is the nature of the newly laid and fertilized eggs. The eggs of the cardinal tetra are photosensitive, and will die if exposed to bright light. Consequently, after spawning, the fishes should be removed and the aquarium covered to darken it, thus providing the developing eggs with the conditions necessary for development.
If the eggs are fertile, and kept in darkened surroundings, they will hatch in approximately 3 days at 82°F (28°C). Free swimming fry remain photosensitive for at least the first 7 days of life, and need to be introduced to increasing light levels on a gradual basis. During this time, they are approximately 4 mm in length, and require infusoria, or liquid fry food. Newly hatched brine shrimp and other similar live foods such as sifted Daphnia can be fed to the growing fry at between 7 and 14 days of age. Growth continues at a modest rate, and the fishes assume full adult coloration only after a period of approximately 8 to 12 weeks, depending upon quality of food and aquarium water.
The blue stripe serves as a signal to conspecifics, keeping shoals together in the dim blackwater conditions typical of their natural habitats. It is also thought to assist in confusing would-be predators. The stripe contains reflective pigment cells known as iridophores, which reflect light and can be seen as blue or green depending on the angle the light hits the stripe. At night when there is no light to be reflected the fish appear brown or grey. It takes around 15 minutes for the stripe to start reflecting light in the morning. The red stripe on the other hand appears black in the attenuated light and serves to camouflage the fish.
The cardinal tetra’s appearance is similar to that of the closely related neon tetra, with which it is often confused; the neon’s red coloration extends only about halfway to the nose, and the neon’s blue stripe is a less vibrant blue.
Paracheirodon axelrodi is also often called the red neon tetra. Cheirodon axelrodi (the original name) and Hyphessobrycon cardinalis are obsolete synonyms. The fish’s common name, cardinal tetra, refers to the brilliant red coloration, reminiscent of a cardinal’s robes. The species name, axelrodi, honors ichthyologist Herbert R. Axelrod.
The characteristic iridescence of this tetra is a structural color, caused by refraction of light within guanine crystals that develop within specialized cells called iridocytes in the subcutaneous layer. The exact shade of blue that is seen will depend upon the viewing angle of the aquarist relative to the fish - if the aquarist changes viewpoint so as to look at the fish from the substrate upwards, the color will change hue, becoming more deeply sapphire blue and even indigo. Change the viewpoint to one above the fishes, however, and the color becomes more greenish.
Normally aquarists prefer to buy tank bred fish, but some Brazilian ichthyologists believe that fish keepers should continue to support the sustainable Cardinal fishery of the Amazon basin, since thousands of people are employed in the region to capture fish for the aquarium trade. It has been suggested that if those fishermen lost their livelihood catching Cardinals and other tropical fish, they might turn their attention to engaging in deforestation.
Jacksonville Zoo History:
Cardinal tetras have been part of our animal collection most of the time since their arrival in 2004.