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Gymnotiformes Fossil range: Late Jurassic - Recent[1]
Black ghost knifefish, Apteronotus albifrons Black ghost knifefish, Apteronotus albifrons
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Actinopterygii Order: Gymnotiformes
Families
Apteronotidae Gymnotidae Hypopomidae Rhamphichthyidae Sternopygidae

The Gymnotiformes is a lineage of ostariophysan teleost electric fishes. Common names found in the literature include the Neotropical electric fishes, South American electric fishes, or American knifefishes. They are primarily freshwater inhabitants and have organs adapted to the generation of electric fields.

The best-known species is the electric eel (Electrophorus electricus) which uses powerful electric shocks (up to 600 V) for hunting and self-defense. Other species familiar in the aquarium trade are: the black ghost knifefish (Apteronotus albifrons), the glass knifefish (Eigenmannia virescens), and the banded knifefish Gymnotus carapo.

Taxonomy

There are currently about 135 known species over 31 genera contained in 5 families, and at least 38 or so additional species are known and are yet to be formally described^[2]. The actual number of species in the wild is unknown^[3]. This group is thought to be the sister group to the Siluriformes^[4].

The families are classified over suborders and superfamilies as below.^[5]

Order Gymnotiformes

Suborder Gymnotoidei

:: Family Gymnotidae (banded knifefishes and electric eel)

Suborder Sternopygoidei

: Superfamily Rhamphichthyoidea

:: Family Rhamphichthyidae (sand knifefishes)

:: Family Hypopomidae (bluntnose knifefishes)

: Superfamily Apteronotoidea

:: Family Sternopygidae (glass knifefishes)

:: Family Apteronotidae (ghost knifefishes)

Distribution and habitat

Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid Neotropics, ranging from Guatemala to Northern Argentina. They are nocturnal fishes.

Evolution

Gymnotiformes are regarded as the among more derived members of Ostariophysi, a lineage of primary freshwater fishes. They arose in the western portion of Gondwana in what is now South America before the geological sepatation with Africa about 120 million years ago. The only known fossils are from the Miocene about 10 million years ago of Bolivia^[6].

Gymnotiformes has no extant species in Africa. This may be because they did not spread into Africa before South America and Africa split, or it may be that they were outcompeted by mormyrids, which are similar in that they also use electrolocation.^[2].

Physical characteristics

Gymnotiformes have a knife-shaped, eel-like body that is either compressed or cylindrical. They lack pelvic fins or a dorsal fin. The anal fin is extremely long; this fin is undulated to allow the fish to move both forwards and backwards. The caudal fin is absent or, in the apteronotids, greatly reduced. The gill opening is restricted. The anal opening is under the head or the pectoral fins^[7].

These fish possess electric organs that allow them to produce electricity. In most Gymnotiforms, the electric organs are derived from muscle cells. However, in adult apteronotids they are derived from nerve cells. All gymnotiform species generate weak (<1 V) electric fields, using them for navigation and communication. The electric discharge is continuous, being generated day and night throughout the entire life of the individual. Certain aspects of the electric signal are unique to each species.{Albert and Crampton, 2005}

See also

• Electric fish
• Bioelectromagnetism

References

External links

• Gymnotiform overview
• Photos of various gymnotiforms