Characiformes

(Characins)

Class: Actinopterygii

Order: Characiformes

Number of families: 11

Evolution and systematics

Characiformes are members of the superorder Ostariophysi, which contains nearly three-quarters of all freshwater fishes in the world. Characiformes possess several specialized adaptations, common to most Ostariophysi, which enable them to thrive in freshwater environments. One specialized anatomical feature is the presence of the Weberian apparatus, a linkage of bones called ossicles, derived from the vertebrae immediately following the skull, that connect the inner ear and the swim bladder; this structure improves the hearing ability of the fishes. Another evolved characteristic is the production of Schreckstoff, an alarm pheromone released into the water by an injured fish that triggers an escape response in other members of the species. Certain Characiformes, such as blind cavefish, do not produce these pheromones, as such a substance would not enhance a blind fish's ability to escape predators. Other species do not respond to the presence of the pheromones; for example, many characins preyed on by piranhas produce Schreckstoff, but piranhas do not flee when feeding on these species. Finally, 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.

Lineages of Characiformes date back more than 100 million years. Morphological and genetic evidence suggests that much of the species diversity of this group likely developed prior to the time when Africa and South America split into separate continents. The classification of the Characiformes into families has proven complex and controversial. The taxonomic relationships have been revised considerably in the last 30 years, and the classifications continue to change as new evidence becomes available. Although some taxonomists consider there to be up to 16 distinct families, most recognize 11 families: Alestiidae, Anostomidae, Characidae, Citharinidae, Ctenoluciidae, Curimatidae, Erythrinidae, Gasteropelecidae, Hemiodontidae, Hepsetidae, and Lebiasinidae. These families encompass species that are diverse in life form and behavior, with examples including the blind cavefish and predatory piranhas, as well as popular aquarium species such as tetras.

Physical characteristics

Most species of Characiformes are small fishes, although size and shape vary widely throughout the order. The smallest member of the order is the Bolivian pygmy blue characin (Xenurobrycon polyancistrus), which attains a maximum size of around 0.5 in (1.4 cm); the largest species, the giant tigerfish (Hydrocynus goliath) grows to 4.3 ft (1.33 m).

Most Characiformes are brightly colored and often silvery, but some are also brilliant shades of red or blue. Characiformes vary widely in body form. Some species have long and slender bodies, while others, such as hatchetfishes, are deep-bodied and laterally compressed. The body is covered in scales, and the lateral line is often decurved or may be incomplete in some species. Most species have an adipose fin, a short fin between the dorsal and caudal fins. Pelvic fins generally have five to twelve rays, and the anal fin may be short to moderately long, with 45 rays or fewer. Characiformes lack sensory barbels, but they typically have large eyes to heighten visual acuity.

Distribution

Characiformes are found in fresh waters of Texas and Mexico in North America and are widely distributed in Central America, South America, and Africa. Over 1,300 species, 252 genera, and 10 families inhabit South America. At least 176 species in 23 genera and 4 families occur throughout Africa.

Habitat

As a dominant group of freshwater fishes, Characiformes inhabit all types of fresh waters, including weedy river edges, stagnant ponds, rushing streams, and even underground caves. Certain species also possess morphological adaptations, including the ability to breath air or swollen lips that facilitate gas exchange in the upper portion of the water column, that enable them to move into shallow waters of floodplains and flooded forests.

Behavior

The diversity of species represented by the Characiformes necessitates a diversity of behaviors. Many species travel in schools under certain conditions. Some species, such as members of the Ctenoluciidae, form schools as juveniles but become solitary as adults. Other species, including members of the Curimatidae, piranhas, and tetras, travel in large aggregations during all life stages. However, certain species, such as the blind cavefish, rarely gather in groups, and do not form organized aggregations. Although many of the smaller species of Characiformes live their whole lives in a limited geographic area, some species undertake extensive seasonal or spawning migrations.

Courtship behaviors and migrations are associated with spawning in some members of the Characiformes. These unique courtship behaviors often involve fin displays to attract mates as well as elaborate swimming patterns to lure the mate to the spawning site. In addition to complex courtship rituals, some species possess morphological adaptations to attract mates. Males of certain species may utilize sensory cues, including pheromones and visual lures, to gain the attention of females.

Feeding ecology and diet

As reflected by their diversity of body shapes, Characiformes exhibit remarkable feeding specializations and exploit all available trophic modes. Many predatory species, particularly those of the Characidae, have well-developed teeth that enable them to feed on other fish. Some species, such as tiger-fishes, solitarily stalk their prey: others, such as piranhas, engage in voracious group predation. Other carnivorous species pick or suck invertebrates from the substrate. In contrast, some Characiformes are strictly herbivorous, feeding on plants, fruits, or seeds. Other species filter plankton or are detritivores that feed on mud, algae, and ooze. Among the most remarkable feeding adaptations observed in freshwater fishes, certain Characiformes survive by eating the scales or pieces of fins from other fishes.

Many Characiformes are small-bodied and, thus, fall prey to larger fish species. In fact, predatory piranhas may indiscriminately attack a variety of food items, including smaller characins. Humans are the major predator of some characins. Small species are harvested as bait for fisheries and as trade items for the aquarium industry. A variety of the larger species, particularly tigerfishes, pacus, and piranhas, sustain important local fisheries in South America and Africa.

Reproductive biology

Most Characiformes broadcast their eggs and devote little parental care to their young. The eggs are often scattered among aquatic plants, and the vegetative cover confers some protection to young by providing shelter from predators. In a few species, including members of the Characidae, the male inseminates the female, and she may retain the sperm cells in her ovary for a period of days to months. However, fertilization does not take place until the eggs and sperm are shed into the water column at the same time.

Certain Characiformes exhibit an assortment of more specialized breeding behaviors. Members of the Erythrinidae construct nests for their eggs, and the African pike characins of the family Hepsetidae deposit eggs in a bubble-like nest of floating foam. To avoid egg predation, female splash tetras (Copella arnoldi) jump out of the water and lay their eggs on the underside of overhanging vegetation or rocks. The males follow, fertilize the eggs, and remain in the area to splash water onto the eggs until they hatch. Members of another species, Brycon petrosus, crawl onto the banks of rivers to lay their eggs.

Conservation status

The IUCN Red List includes 7 Characiformes. Six are listed as Data Deficient, and one, the naked characin (Gymnocharacinus bergii), is listed as Endangered. Although international conservation concerns have not been recognized for other species, many Characiformes are harvested for the aquarium trade, an industry that is monitored and regulated only in certain countries or localities.

Significance to humans

Many Characiformes, including tetras, hatchetfishes, and pencilfishes, are popular aquarium fishes. Others are important as a food resource for humans. Although some species are traded commercially, many are relied upon to meet the subsistence food needs of communities living along tropical rivers. Examples include members of the Citharinidae (dorados) and Serrasalminae (tambaqui and pacus). In addition, some Characiformes, such as tigerfishes, are popular recreational targets that attract anglers from around the world.

Species accounts

Resources

Berra, T. M. Freshwater Fish Distribution. San Diego: Academic Press, 2001.

Gery, J. Characoids of the World. Neptune City, NJ: Tropical Fish Hobbyist Publications, Inc., 1977.

Araujo-Lima, C. A. R. M., and E. C. Oliveira. "Transport of Larval Fish in the Amazon." Journal of Fish Biology 53, Supplement A (1998): 297–306.

Loubens, G. and J. Panfili. "Biologie de Piaractus brachypomus (Teleostei: Serrasalmidae) dans le bassin du Mamoré (Amazonie bolivienne)." Ichthyological Explorations of Freshwaters I 12, no. 1 (2001): 51–64.

Orti, Guillermo, and Axel Meyer. "The Radiation of Characiform Fishes and the Limits of Resolution of Mitochondrial Ribosomal DNA Sequences." Systematic Biology 46, no. 1 (March 1997): 75–100.

Ribeiro, M. C. L. B., and M. Petrere, Jr. "Fisheries Ecology and Management of the Jaraqui (Semaprochilodus taeniurus, S. insignis) in Central Amazonia." Regulated Rivers Research and Management 5, no. 3 (1990): 195–215.

Roberts, T. R. "Osteology and Relationships of the Prochilodontidae, a South American Family of Characoid Fishes." Bulletin of the Museum of Comparative Zoology 145, no. 4 (1973): 213–235.

Ruffino, M. L., and V. J. Isaac. "Life Cycle and Biological Parameters of Several Brazilian Amazon Fish Species." NAGA, The ICLARM Quarterly 18, no. 4 (October 1995): 41–45.

Schrieber, R. "The African Darter Tetra, Nannocharax fasciatus." Tropical Fish Hobbyist 41, no. 4 (1992): 132–135.

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[Article by: Katherine E. Mills, MS; Elizabeth Mills, MS]

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