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The vast number of small-sized fish species makes the group of fishes we commonly term characins one of the most popular for home aquaria. Characins—or more correctly characids—belong to the scientific order Characiformes. The name comes from the Greek charax [a marine fish] plus the Latin forma [shape]. This is a fairly recent taxonomic revision; previously these fish were all considered to be within a single Family, Characidae, included within the order Cypriniformes.

The oldest characid fossil record comes from the late Jurassic period, more than 145 million years ago. Several ichthyologists now view the characins as representing the oldest and most ancient of freshwater fishes, and Boulenger considered them the ancestors of the naked catfish, barbs, eels and Knifefish.

The ancestor of all modern Characiformes appeared when the two southern continents of Africa and South America were part of the supercontinent called Gondwana [Lundberg 1993; Ortí and Meyer 1997]. The breakup of the two continents allowed the characins in what became South America to take full advantage of the potential [=empty or not fully-exploited] ecological niches, and the characids underwent explosive radiation unlike any other in all the vertebrates except for the marsupials in Australia. The diversification of the cichlid fishes in the African rift lakes during the past 15,000 years is a more recent though much smaller-scaled example.

The divergence in ecological specialization among the characids is truly remarkable. With respect to feeding, many are predatory, several are herbivore, some survive by eating fish fins and scales, some are mud-eaters, and some sift plankton from the water; in other respects, some “hop” on the substrate, some leap from the water and glide, some jump out of the water and spawn on terrestrial vegetation, some swim with their head vertically downward, some are capable of air-breathing, and some exhibit nesting behaviours and parental care. No other order of fish possesses such diversity.

There are currently more than 1674 described species in approximately 270 genera within 19 Families. Roughly 20% of the characin families with approximately 200 species occur in Africa; the remainder are in South America with a few having made their way into Central America, Mexico and southern Texas no later than 5 million years ago. The taxonomy of the characids is still changing and will continue to do so for many years due to increasing study of fish populations. For instance, there are currently some 304 species of aquarium fish in the Hemigrammus clade which includes all Hemigrammus and Hyphessobrycon species, and nearly all of these are deemed incertes sedes, or of uncertain placement.

For the aquarist, what makes a fish a characid?

Ø Externally, many but not all species possess an adipose fin, a small fleshy fin positioned on the dorsal ridge between the dorsal and caudal fins; in some of the pencilfishes this fin may be present or absent within the same species. Other fish groups sometimes have this fin, such as most of the catfishes and the salmonids, neither of which are related to the characins.

Ø When a characin is injured, it releases an alarm pheromone called Schreckstoff into the water that triggers an escape response in other members of the species. Other species do not respond to the presence of the pheromones. This chemical warning system may explain their heightened sensitivity to medications and fluctuating water conditions in the aquarium.

Ø The presence of a moveable upper jaw confers a feeding advantage to Characiformes. In some species, the jaw is protrusible, enabling the fishes to use suction pressure to capture prey.

Ø All characins are egg-layers and except for the species within the Mimagoniates which have internal fertilization, all use external fertilization.

Ø But perhaps most importantly, all possess the Weberian apparatus, a modification and fusing of the anterior-most 4-5 vertebrae containing a series of ossicles (movable bony parts) and ligaments that connects the swim bladder to the inner ear. Some other fish groups contain rudimentary signs of this, but in the characids it is fully developed. This greatly heightens their sense of hearing and may account for the characids’ instinctive ability to sense danger before it occurs.

All aquarium species of characins are shoaling fish, meaning they live together in groups; this may be for defence against predators, but many species have an interactive social structure within the group. Depending upon the size of the aquarium, most species should be in a group of at least five or six and preferably eight or more. Keeping these fish individually or in pairs frequently causes significant stress which leads to health problems and a shortened lifespan.

All characins are highly sensitive to water parameters and conditions; the water quality should be stable, and use of medications should be avoided unless absolutely essential. Salt should never be used in an aquarium with characids; with one or two exceptions, all species suffer stress with levels above 100 ppm [= .38 of one gram, or 1/6 of a level teaspoon, per gallon] and many cannot tolerate more than 60 ppm [source: Dr. Stanley Weitzman].

Several families within the characids provide suitable aquarium fishes, but there are three that have become very popular with home aquarists. The largest of these are the tetras; the common name "tetra" comes from one of the families, Tetragonopterinae, which name is derived from the Greek tetra (=four), gonia (=angle) and pteron (=wing). Until quite recently, this family was assumed to include most of the common tetra, but DNA and cladistic studies are now changing this, and none of the popular tetra are actually in this family.

The second largest group is the pencilfishes, so called because of their linear narrow profile; scientifically these are within the Nannostomini tribe of the subfamily Pyrrhulininae in the family Lebiasinidae.

The third popular group are the hatchetfishes, so named from their distinctive hatchet-shape keel, and scientifically in the Gasteropelicidae family.

Specific traits of these fishes are included in their respective profile.

Byron Hosking
April 24, 2010
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