Anal, caudal, and pelvic fins present; caudal peduncle narrow; caudal fin deeply forked; teeth present on parasphenoid and tongue; 6-8 branchiostegal rays; dorsal fin rays 12-91; anal fin rays 20-70; dorsal and anal fins usually opposite and placed back on body; vertebrae 37-64.
The mouth is extremely variable in mormyrids. In some there is a very elongate proboscislike snout with a terminal mouth (e.g., Gnathonemus curvirostris); in a few there is an elongate lower jaw (e.g., Gnathonemus petersix), whereas in others there is a rounded snout with an undershot mouth (e.g., Marcusenius). The fish shown above has a moderately developed proboscislike snout. Some bottom-feeding mormyrids have a chin barbel that is absent in the midwater species. Length reported up to 1.5 m; the maximum length in most species is 9-50 cm.
Some mormyrids and the one gymnarchid are known to transmit weak electric currents and to be capable of detecting extremely weak charges. They are primarily nocturnal fishes and may use these currents to locate objects. Mormyrids, at least, appear to have considerable learning ability. Their brain size (largely cerebellum), relative to body weight, is comparable to that of humans. There is evidence that the family Mormyridae is paraphyletic without the inclusion of Gymnarchus; both groups share the following: maxilla toothless; enormous cerebellum; eyes usually small; electric organs derived from caudal muscles; intracranial penetration of swim bladder; flagellum lost in spermatozoa (Jamieson, 1991).
About 18 genera (e.g., Brienomyrus, Campylomormyrus, Gnathonemus, Hippopotamyrus, Hyperopisus, Marcusenius, Mormyrops, Mormyrus, Petrocephalus, Pollimyrus, and Stomatorhinus) and about 201 species (Kramer and van der Bank, 2000; Kramer et al., 2004).
Family GYMNARCHIDAE (68)—aba. Freshwater; tropical Africa and Nile.
Anal, caudal, and pelvic fins absent; teeth absent from parasphenoid and tongue; four branchiostegal rays; elongate body; long dorsal fin (183-230 rays), which can be used for locomotion. They can move forward or backward equally well by passing reversible wavelike movements along the fin while keeping the body rigid. Vertebrae 114-120. Length reported up to 1.5 m but usually less than 0.9 m.
One species, Gymnarchus niloticus (Aba).
ELOPOCEPHALANS. The remaining three subdivisions, the Elopomorpha, Ostarioclupeomorpha (= Otocephala), and Euteleostei, are placed in the unranked taxon Elopocephala. However, as noted above, there are strong arguments by Arratia (1997, 1999, 2004) that the Elopomorpha are more primitive than the Osteoglossomorpha, but counterarguments exist by others that the Osteoglossomorpha may be the most primitive, an arrangement shown in Nelson (1994), based on Patterson and Rosen (1997). I have thus opted to maintain the classification previously given until we have clearer resolution of this problem.
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