Body eel-like (compressed or cylindrical); pelvic girdle and fins absent; dorsal fin absent (but see family Apteronotidae); anal fin extremely long (more than 100 rays and extending from near pectoral-fin origin to near posterior tip of body) and employed in forward and backward movements; caudal fin absent or greatly reduced (present only in the apteronotids); restricted gill openings; anal opening under head or pectorals; basal pterygiophores to anal fin with only one section (radial) and a hemispherical cartilaginous head that articulates the fin rays (allowing them to move in a circular motion); electric organs present; suboperculum absent; palatine not ossified; maxilla rudimentary (except Electrophorus). The electric organs are derived from muscle cells in most groups (myogenic), or from nerve cells in adult apteronotids (neurogenic). Like catfishes, gymnotiforms are nocturnal. They probably arose in the Neotropical region. They are thought, on the basis of a cladistic study by Fink and Fink (1981, 1996), to be the sister group to the siluriforms (see above under Otophysi).
Five families, 30 genera, and about 134 species (at least 38 additional species are being described giving at least 173 species known, and many additional species no doubt remain to be discovered) (Albert and Crampton, 2005). The classification follows Albert and Campos-da-Paz (1998), Albert (2001), and Albert and Crampton (2005). See family Sternopygidae below for the only known fossil species.
Suborder Gymnotoidei. One family, two genera, Gymnotus (32) and the mono-typic Electrophorus, with 33 species (Albert and Crampton, 2005).
Family GYMNOTIDAE (161)—nakedback knifefishes. Freshwater; North (southern Mexico only), Central, and South America.
Body rounded or partially so (adult body depth greater than half the body width at the anal-fin origin); body cavity very long with 31-51 (more than 100 in Electrophorus) precaudal vertebrae. Maximum length about 2.2 m TL, attained in Electrophorus electricus; species of Gymnotus reach up to 100 cm. The common name in English for the family is appropriate for the order but is retained here.
Gymnotus (banded knifefishes).
Small scales present; mouth superior; anal fin terminating at a point near the tip of the tail; body subcylindrical; weak electrical discharge. The genus Gymnotus, currently with 32 species (Campos-da-Paz, 2003; Albert and Crampton, 2003a, 2005) is substantially more diverse than previously recognized (e.g., Nelson, 1994, recognized only three species). This is the most widespread genus of the order, extending from southern Mexico ( G maculo-sus) to Argentina ( G inaequilabeatus); also in Trinidad ( G. carapo). Electrophorus electricus (electric eel).
This species was formerly recognized in the family Electrophoridae (electric knifefish), but was placed with the gymnotids in its own suborder; placement here in the same family with Gymnotus is based on studies demonstrating their close relationship (Albert and Campos-da-Paz, 1998; Albert, 2001). Electrophorus is unique among gymnotiforms in having large electric organs producing lethal discharges (up to 600 volts) for stunning prey (high voltage, low amperage), vas-cularized oral respiratory organ (they can breathe air), and continuous addition of vertebrae throughout life. In addition: scales absent; mouth terminal; anal fin continuing to the tip of the tail; body rounded. Northern South America (primarily Orinoco and Amazon River basins).
Suborder Sternopygoidei. Body compressed (rarely cylindrical); precaudal vertebrae 12-26 (except Sternopygus, which can have as many as 30). Four families, 28 genera, and 101 species.
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