The Cardinal Tetra (Paracheirodon axelrodi)

Jun 17, 2012 Features / Columnists, Interesting Creatures in Guyana

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The cardinal tetra, (Paracheirodon axelrodi), is a freshwater fish of the characin family of the order Characiformes. It is native to the upper Orinoco and Negro Rivers in South America.
They can grow to about three centimetres (1.25 in) total length, and have a striking iridescent blue line characteristic of the Paracheirodon species laterally bisecting the fish, with the body below this line being vivid red in colour. It is in fact from this attribute that the name ‘cardinal tetra’ was derived.
However, the species can exist in a number of different colour forms or phenotypes such as a ‘gold’ and ‘silver-blonde’ form in the Rio Negro drainage, which have less blue in the longitudinal stripe. The normal form from the Rio Negro drainage has a blue stripe which extends to the adipose fin, while the Orinoco drainage phenotype has a stripe which stops posteriorly of the adipose.
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. They are said to be excellent aquarium fishes.
In the wild, they are able to swim upstream in large numbers to parts of their 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, but also the breeding aquarium needs to be heavily shaded to mimic the low-light conditions of the fish’s native spawning grounds.
If the fish are ready to spawn, the male, which will be the slimmer of the two 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 fish 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 three days at 28°C. The offspring called fry remain photosensitive for at least the first seven days of life, and need to be introduced to increasing light levels on a gradual basis. During this time, they are approximately four 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 seven and 14 days of age. Growth continues at a modest rate, and the fish assume full adult coloration only after a period of approximately eight to 12 weeks, depending upon quality of food and aquarium water.
Although the cardinal tetra is a very popular aquarium fish it is less widespread than the neon tetra because until recently it was difficult to breed in captivity.
The fish is also effectively an annual species, and has a lifespan of just a single year in nature, but lives for several years in captivity.
Given the origins of the cardinal tetra, namely blackwater rivers whose chemistry is characterised by an acidic pH, low mineral content and the presence of humic acids, the species is adaptable to a wide range of conditions in captivity.
The species will feed upon a wide range of aquarium foods, though again, conditioning fish of this species for breeding will usually require the use of live foods.
Aquarium furnishings should be planned with some care. Live aquatic plants, as well as providing an additional biological filtration component that assists with nitrate management in the aquarium, provide an environment that resembles at least part of the wild habitat, and fine-leaved plants such as Cabomba are usually the plants of choice. Other plants such as Amazon Swordplants and Vallisneria are equally suitable for an aquarium housing the cardinal tetra.
(Source: Wikipedia – The Free Online Encyclopedia)

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