Paracheirodon axelrodi
Cardinal tetra
(Also: Neon; Neon tetra; Red neon; Scarlet characin)

Cardinal tetras live in Brazil, Colombia, and Venezuela, in the upper Orinoco and Negro River drainages. (Froese, et al., 2004; Lundie, 2004)

Paracheirodon axelrodi individuals prefer slow moving, middle layer water in shoals. They prefer a slightly acidic pH (5.8) and a temperature of 24°C. Cardinal tetras do not migrate and are generally found in open water. (Froese, et al., 2004; Lundie, 2004)

Cardinal tetras are small fish, males grow to about 2.5 cm in length in the wild, but can attain lengths of 5 cm in an aquarium. This species is characterized by a horizontal neon blue stripe and deep red markings. The neon stripe of Paracheirodon axelrodi becomes iridescent because of external lighting. As the angle of light that strikes them changes, the color of the neon stripe turns from green to blue or vice versa. During the night, when no light hits the stripe, cardinal tetras are a transparent, brownish color. Cardinal tetras, as well as many other tetras, have a small adipose fin on their dorsal side at the tail end of their bodies. Cardinal tetras have a single row of dentary teeth. (Butler, 2003; "Tempting Terrific - The Tetras", 2004; "Poisson : Paracheirodon axelrodi", 2004; Butler, 2003; Bydzovsky, 2000; Fink and Weitzman, 1983; Froese, et al., 2004; Lundie, 2004)

Sometimes cardinal tetras are confused with neon tetras, Paracheirodon innesi. Cardinal tetras, however, have a longer red band dorsally. ("Poisson : Paracheirodon axelrodi", 2004)

Paracheirodon axelrodi exhibit some sexual dimorphism. Females are slightly larger and wider than males. Females have larger stomachs as well. Mature male cardinal tetras have bony pelvic fin hooks. (Fink and Weitzman, 1983; Lundie, 2004; Fink and Weitzman, 1983; Lundie, 2004)

Eggs hatch within 24 to 30 hours of fertilization. Fertilized fish eggs, in general divide by discoidal meroblastic cleavage. Because fish eggs are telolecithal (have a large percentage of yolk) cleavage can only take place in a small part of the egg. (Gilbert, 2003)

Female cardinal tetras release their eggs during the rainy season. The eggs become fertilized by the milt (sperm) of males in close proximity. Mating takes place at twilight during the rainy season. The male embraces the female while swimming. Fertilization is external. As the female scatters about 500 eggs into the water, males fertilize the eggs. ("Tempting Terrific - The Tetras", 2004; Norris and Chao, 2002)

Female cardinal tetras may release about 500 eggs. Once the eggs have been laid, they sink and some adhere to plants. Because cardinal tetra eggs are light sensitive, the only eggs that can develop into adult fish are released in a shaded river habitat. To breed cardinal tetras successfully in captivity, the pH should be between 5.5 and 6, the temperature should be at 24°C and the light should be dim. Fry are independent within 3 to 4 days of hatching. Both males and females reach sexual maturity by approximately 9 months of age. (Froese, et al., 2004; "Cardinal Tetra", 2002; "Tempting Terrific - The Tetras", 2004; "Poisson : Paracheirodon axelrodi", 2004; Butler, 2003; Froese, et al., 2004; Norris and Chao, 2002; "Cardinal Tetra", 2002)

Parent cardinal tetras take no care of their eggs or young. After eggs are released, some parent cardinal tetras may even eat some of their spawn. ("Tempting Terrific - The Tetras", 2004)

Cardinal tetras are able to live longer in captivity than in the wild. In the wild, they are expected to live about 1 year. In captivity, the life expectancy is about 5 years, although individuals may live as long as 10 years. ("Poisson : Paracheirodon axelrodi", 2004; "Cardinal Tetra", 2002)

Paracheirodon axelrodi individuals are active during the day, and live in schools of several thousands in natural habitats. At least 10 of this species should be kept together in an aquarium in order for them to feel safe. In cases of fewer than 10 individuals living together in a tank, cardinal tetras tend to develop sickness due to stress. ("Poisson : Paracheirodon axelrodi", 2004; Lundie, 2004)

Cardinal tetras are known to make small scale seasonal migrations upstream or downstream depending on the height of water during a particular season. (Bydzovsky, 2000)

The home range of these fish has not been reported.

Communication has not been described in this species. However, tactile communication occurs during mating. It is likely that these fish use tactile and visual cues as part of their communication, since they seem aware of the number of other cardinal tetras in a tank in captivity. ("Poisson : Paracheirodon axelrodi", 2004; Froese, et al., 2004; "Cardinal Tetra", 2002)

Little is known about how cardinal tetras perceive their environment. They use their eyesight and tactile cues and are able to perceive sound through the water.

The diet of P. axelrodi consists of very small crustaceans, mesofauna, eggs, algae, detritus, and some other types of prey. Most types of small crustaceans eaten are cladocera (small, spherical moinids, daphnids, and macrothricids), as well as some copepods (benthonic Harpacticidae). Rotifera and Thecamoebae are included in the mesofauna that cardinal tetras eat. The types of algae that they eat include unicellular diatoms (Navicularia and Pinnularia), and some green algae (Chlamydomonas, Conjugatophyta, and Volvocaceae). Cardinal tetras sometimes feed on dead fish, eating the detritus of their muscular, proteinous, and membraneous tissues. Paracheirodon axelrodi individuals may also eat ants, Diptera larvae or pupae, mites, newly hatched shrimp, fungus, pieces of fruit, and fish larvae. ("The food spectrum of the cardinal - tetra (Paracheirodon axelrodi, Characidae) in its natural habitat", 2004)

The bright, neon, lateral stripe of cardinal tetras makes it difficult for predators to single out and attack an individual. Their schooling behavior also helps to protect individuals from predators. Although little is known on the specific predators of cardinal tetras, they likely fall prey to larger fish and other small to medium aquatic predators as adults, fry, and eggs. (Sharpie, 2004)

When kept in an aquarium with larger fish, such as angelfish, cardinal tetras may become their prey. (Butler, 2003)

In relatively few cardinal tetras (6.3% of 80 dissected), existence of parasitic nematodes was found in the coelom, or stomach. Cardinal tetras serve as important predators of their small, invertebrate and zooplanktonic prey. They are important food sources, in all life stages, for larger predators. ("The food spectrum of the cardinal - tetra (Paracheirodon axelrodi, Characidae) in its natural habitat", 2004)

Cardinal tetras are often kept and enjoyed as a beautiful aquarium fish. Because of their peaceful nature, cardinal tetras are highly recommended for tanks with more than one species of fish. Most of the cardinal tetras sold come straight from South America because of the difficulty of breeding them in nature; between the years 1977 and 1981, an average of 12 to 17 million cardinal tetras were exported annually. (Bydzovsky, 2000; Froese, et al., 2004; Lundie, 2004)

Paracheirodon axelrodi has no known negative affect on humans.

Cardinal tetras are common in their native range, they are not considered threatened.

Paracheirodon axelrodi was first described by Schultz in 1956. Paracheirodon axelrodi is sometimes also called cardinal tetra, red neon, or roter neon. Cheirondon axelrodi or Hyphessobroycon cardinalis are synonyms of P. axelrodi. ("Poisson : Paracheirodon axelrodi", 2004; Butler, 2003)