Family: Characidae, Hemigrammus Clade.
About the Cardinal Tetra
Species Type: Freshwater Fish
Care Level: Moderate. May tolerate only a narrow range of water parameters, have specific dietary requirements including frozen or even live foods, may have behaviors that severely limit potential tankmates or may require a specialized aquarium setup.
Origin: Amazon Basin, northern tributaries of the Rio Negro in Brazil and Peru [Brazilian form], Orinoco basin in Columbia and Venezuela [Columbian form]. Occurs in forest streams and creeks in slow or standing water, well shaded by vegetation and frequently having aquatic plants.
Compatibility/Temperament: Very peaceful; suitable in a species tank or combined with similar peaceful characins, rasbora, small catfish and loaches. Good tankmate for rarer gouramis, discus and dwarf cichlids that require higher temperatures. They should be kept in groups of six or more.
Two forms of the cardinal tetra are known, one from the Rio Negro basin [the "Brazilian" form] and one from the upper Rio Orinoco basin in Columbia. In the late 1990's, Dr. Jacques Gery suggested that they might differ morphologically, and be distinct species, sub-species or variants; more recent studies are suggesting the latter. They are outwardly recognizable in three ways. On the Columbian form, the neon line ends at the adipose fin, the red colouration does not extend as far under the belly so there is slightly more white, and the fish is chunkier in general build. By contrast, the neon line on the Brazilian form extends below the adipose fin and is straighter in appearance, the red extends slightly further down on the belly, and the fish is more slender and thus appears longer than the Columbian form. In the accompanying photos, the Columbian form is shown on the left [green background]and the Brazilian form on the right [brown background].
In their natural habitat cardinals are found in small slow-moving streams and still waters having a pH from 3.4 to 4.5, some locations a bit higher but never above 6, and less than 1 degree of hardness. The water is always shaded by dense trees and either land vegetation or aquatic plants are always present. They congregate in groups amid aquatic plants, branches, or under overhanging land vegetation. They may be an annual fish in nature, only living one season due to shortages of food.
In the aquarium, they thrive best and can live more than 10 years when kept in soft, acidic water, and while they will somewhat adapt to other water conditions provided they are acclimated properly, their life span will be shorter. When maintained in harder water they can develop calcium blockage of the kidney tubes. It is also believed they have a light phobia, and the aquarium should not be brightly lit. The majority of the cardinal tetras available in the trade are wild-caught and attention must be given to their habitat preferences. Wild-caught fish generally are more demanding of their preferred conditions, and this can be even more important when the fish itself is very sensitive with respect to water parameters and environmental conditions.
To replicate their habitat, the aquarium should have a dark (black is ideal) sand or fine gravel substrate, and plenty of aquatic plants like Echinodorus (swords), Cabomba, Pennywort, and floating plants. Water flow from the filter should be minimal, with subdued lighting that may partly be achieved with floating plants. This fish should always be introduced to well-established aquaria, never to a new setup; they are highly sensitive to dissolved nitrogenous waste, nitrates, and fluctuating water conditions such as are common in newly-established aquaria.
Sexual dimorphism, like most other tetras, can be difficult to determine but male cardinals are often smaller and always slimmer in comparison with the females when viewed from above. Spawning is not easy to achieve. They are egg scatterers and require very soft and acidic water and darkness.
The iridescent blue/green line contains pigment cells known as iridophores (or sometimes guanophores); these reflect light using plates of crystalline chemochromes made from guanine. In light, these generate iridescent colours due to the diffraction of light within the stacked plates.
The cardinal tetra shares a very similar appearance with three other distinct species, all commonly referred to as "neons" because of the blue/green lateral line. Three of these, the neon tetra [Paracheirodon innesi], cardinal tetra [Paracheirodon axelrodi] and false or green neon tetra [Paracheirodon simulans], are available in the hobby; the fourth species was discovered by Heiko Bleher in 2006 and at the time of writing has yet to be described and named. According to DNA studies, this new species is very close to the true Neon Tetra, whereas the Cardinal Tetra is genetically closer to the Green/False Neon Tetra.
When it was first discovered in the early 1950's, specimens were examined and described by prominent ichthyologists in the United States with the results published in two journals. L.P. Schultz published in the March-April 1956 issue of Tropical Fish Hobbyist and named the fish Cheirodon axelrodi, while Dr. George S. Meyers and Stanley Weitzman published in the University of Stanford Ichthyological Bulletin and named the fish Hyphessobrycon cardinalis. It was determined that the Schultz article had been published on February 20, 1956, while the Meyers & Weitzman article was published the day following. Under the rules of the ICZN [International Code of Zoological Nomenclature] priority must go to the first published name; the fish was therefore officially named Cheirodon axelrodi. The species epithet honours Dr. Herbert R. Axelrod, although this distinguished scientist had no direct connection with the discovery of this fish.
In 1960, Dr. Jacques Gery erected the genus Paracheirodon for the type species P. innesi (Neon Tetra) and the present species was moved into the new genus. The name Paracheirodon derives from the Greek; the older genus name Cheirodon means hand [cheir] and teeth [odon], and the prefix para means "beside" to distinguish Paracheirodon from Cheirodon.
Paracheirodon was formerly considered within the Subfamily Tetragonopterinae, but this classification is now deemed incertae sedis [Latin for "of uncertain placement"]. J. Marcos Mirande (2009) proposed several revisions to the Family Characidae based upon phylogenetic diagnosis; some genera have been moved to a new Sub-Family, while others are now (temporarily) assinged to a specific clade within the Family pending further study. The following year, Javonillo et.al. (2010)determined that the Sub-Family Tetragonopterinae should only be used for species within the genus Tetragonopterus. One of the co-authors, the emminent ichthyologist Dr. Stanley Weitzman, has often suggested that the entire Characidae family is likely to be significantly restructured in the light of scientific advancements.
Bleher, Heiko (2008), "A Story of Four Neon Tetras," Part 1: The First Three Neons (August 2008) and Part Two: And Now a Fourth Neon! (October 2008), Tropical Fish Hobbyist.
Javonillo, Robdert, Luiz R. Malabarba, Stanley H. Weitzman and John R. Burns (2010), "Relationships among major lineages of characid fishes (Teleostei: Ostariophysi: Characiformes), based on molecular sequence data," Molecular Phylogenetics and Evolution, Vol. 54, No. 2 (February 2010).
Mirande, J. Marcos (2009), "Weighted parsimony phylogeny of the family Characidae (Teleostei: Characiformes)," Cladistics, Vol. 25, No. 6 (July 2009).
Cardinal Tetra Diet
Cardinal tetras are carnivorous in nature and are not fussy eaters so they will readily accept most prepared foods such as flake and frozen bloodworms and daphnia.
Grows to 2 inches, usually smaller.
Minimum Tank Suggestion
24 inches in length.
Ideal water parameters for Cardinal Tetra
Soft (hardness below 4 dGH) acidic (pH below 6.0) water, temperature 23-27C/73-81F. May tolerate harder and slightly basic water short-term, but will be more susceptible to disease and a shorter life span.