The electric eel Electrophorus electricus is one of those animals that seem to border on the mythical. Most people will have come across some sort of reference to their existence, and may even have seen some sort of intended depiction of one in cartoon form. However, said depiction will probably bear little if any resemblance to a real-life electric eel. Most commonly, it will look more like a standard Anguilla eel, to which true electric eels are not close relatives. Instead, electric eels belong to a uniquely South and Central American group of fish, the Gymnotiformes.
The Gymnotiformes, commonly known as the Neotropical knife-fishes, are more closely related to catfish than they are to anguillid eels. They are characterised by an elongate body form, lacking the dorsal fin of other fish. The anus has been moved forward relative to other fish: in some gymnotiforms, the anus is actually in front of the pectoral fins, just behind the head. The anal fin that runs behind the anus has become greatly elongated, and instead of swimming by undulating the body from side to side like other fish, gymnotiforms swim by undulating the anal fin alone while the main body remains more or less rigid. This unusual swimming style is directly related to another distinctive feature of the gymnotiforms: their production of an electrical field. Many fish are able to passively sense electrical fields in the water: gymnotiforms take the next step and generate their own electrical field, which they use to sense their surrounding environment (Albert & Crampton 2005). As a result, they can live and hunt effectively at night and in turbid waters with poor visibility. They can also use their electrical fields for communication, signalling their moods and identities to other fish. The connection between electricity generation and swimming style is that, if gymnotiforms swam in the manner of other fish, their changes in body aspect would create changes in the shape of their electrical field. Holding the body more or less rigid means that the electrical field also remains constant, and any distortions must be caused by something external. Another group of fishes found in Africa and Asia that also navigates by electricity, the Notopteridae, has evolved a very similar appearance and swimming style to the gymnotiforms (and are also known as knife-fishes), but are entirely unrelated phylogenetically.
The electric eel is something of an outlier among gymnotiforms. For a start, it's a monster: electric eels can be over two metres in length, while other gymnotiforms are all much smaller. The electric eel has also had a Susan Storm-style upgrade, and weaponised its electrosensory system. Electric eels can produce up to 600 volts of electricity, allowing them to stun reasonably large prey. The closest relatives of the electric eel are the banded knife-fishes of the genus Gymnotus; both are predators of fish and other aquatic animals. Males of at least some Gymnotus species and the electric eel build nests that the females lay their eggs into; males of Gymnotus carapo have been recorded to mouth-brood larvae.
The remaining gymnotiforms were placed by Albert (2001) in a clade called the Sternopygoidei; these taxa have a smaller gape and feed on correspondingly smaller prey (some are planktivores). Two families, the Hypopomidae and Rhamphichthyidae, are united by the lack of teeth in the oral jaws; rhamphichthyids also have a very long and tubular snout. The other sternopygoids are placed in the families Sternopygidae and Apteronotidae; a distinctive feature uniting these two families is that they produce a wave- or tone-type electrical field instead of the pulse-type electrical field of other gymnotiforms. Pulse-type species produce discrete pulses of electricity at a lower frequency, while wave-type species produce a continuous series of electrical discharges at a much higher frequency (Albert 2001). While Albert (2001) regarded the pulse-type electrical field as ancestral for the gymnotiforms and the wave-type field as derived, other authors have preferred the opposite scenario. Sternopygids retain well developed eyes, in contrast to the reduced eyes of other gymnotiforms, while apteronotids are the only gymnotiforms to retain a caudal (tail) fin. If the wave-type families form a derived clade, then either these features were lost independently in the other families, or they represent reversals to an ancestral type.
One final thing to note is that the gymnotiforms have been going through something of a taxonomic boom, with many new species described in recent years. Albert & Crampton (2005) estimated that the total number of species out there could be nearly twice the 135 that had been named so far. In South America, it turns out, the streams are alive with the buzz of electricity.
Albert, J. S. 2001. Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei). Miscellaneous Publications, Museum of Zoology, University of Michigan 190: 1-129.
Albert, J. S., & W. G. R. Crampton. 2005. Diversity and phylogeny of Neotropical electric fishes (Gymnotiformes). In: Bullock, T. H., C. D. Hopkins, A. N. Popper & R. R. Fay (eds) Electroreception, pp. 360-409. Springer: New York.