One of the more difficult hurdles of becoming an aquaristic aficionado (or is that a-fish-ee-oh-nah-doh!) is the mastery of so much novel and specialized terminology. Adding to this considerable confusion is the field of "common" names of livestock. Think of the disparate fishes called "basses", "snappers", "sharks"...

    Here we'll be tacking freshwater "eels". Just what are these? Rather than a taxonomic grouping, these eel-like fishes share a long sinuous profile, oh-so-many head-lengths divisible into their body length. Other than physical semblance and a propensity for jumping out of their aquarium containers, these animals have little else in common. 

    For fun and your personal knowledge, let's detail these fishes en toto, and (next) delve into the one principal family of freshwater eels used in the pet-fish interest (through they're not true eels either), the Mastacembelidae. 

Classification:

True Eels: 

    In the grand scheme of ichthyological classification the true eels are places as some of the least advanced true bony fishes (Class Osteichthyes, after the Jawless Fishes called Lampreys and Hagfishes, Class Agnatha, and the Cartilaginous Fishes called Sharks, Rays, Skates and Chimaeras, Class Chondrichthyes). These are the three Classes of the Subphylum Vertebrata... the other four: the amphibians, reptiles, birds and mammals. An example of a Green California Moray Eels systematics:

Class Osteichthyes (Bony Fishes)

    Order Anguilliformes (True Eels)

        Family Muraenidae (Moray Eels)

            Gymnothorax mordax ("Naked chest, biter")

       This, the true eel group, Order Anguilliformes is typified more by what it's members lack than what they possess. An older, alternative name for the Order is Apodes, literally "without feet"; in reference to the absence of pelvic fins and skeletal support for same in true eels. Some, like Moray Eels (Family Muraenidae) even lack pectoral fins and girdle. Their body skeleton lacks a bony attachment to the skull. Most true eels have no scales; or if they do, these are small, of a cycloid variety, embedded in their skin. 

    Anguilliforms have small gill openings, lack gill rakers and the out-pocketings of the gut called pyloric caecae. Gone also are the bones of the head that are prominent in advanced bony fishes. A clincher for uniting the three Suborders, fifteen Families, 141 genera and approximately 738 true eel species is their shared larval stage, the leptocephalus. These strange, transparent marvels swim about as pelagic young, metamorphosing and settling down as currents and swimming dictate. 

Freshwater Eels, Real & Not:

    Of the true eels that occupy freshwater full time, part or as visitors, only a few species grace our aquariums.

    The Family Anguillidae are termed "freshwater eels", but are usually catadromous, sort of the opposite of anadromous fishes like the sea-run Salmon, these fishes return to the sea to spawn. Anguillids live in freshwater or estuaries as young to adults, migrating to the ocean to spawn. Young return as leptocephali and enter freshwater in a stage of development known as elvers. Occasionally the stars of long distance spawning, the European Anguilla anguilla and A. rostrata of North America are offered to hobbyists. These somber, uniform-colored animals don't do much behaviorally, get too large for most home systems, and rank high as escape artists. 

    Lamprey Eels and Hagfishes, sometimes called Slime Eels, have already been mentioned as the least advanced fishes they are. These parasites are only put on display in public aquariums and then only in areas where they cause fisheries troubles (e.g., the Great Lakes). Some freshwater species, all ugly.

    Lungfishes; the Dipneustian fishes of Australia, Africa and South America have at times been called eels, and they are freshwater... only distantly related to true eels.     

    Knifefishes as eels. The Order Gymnotiformes contains many fishes with eel-like bodies (rounded or compressed, missing pelvic fins and girdle, no dorsal, long-continuous anal fins; restricted gill openings...). Of the six families, the Electric Knife/Eel (Electrophoridae) and Central and South American Gymnotidae are most often labeled as "eel" knifes by writers and retailers. Electric Eels (Electrophorus electricus) are only for folks with the resources to dedicate to a large (these fish grow to more than two meters in length) specimen tank. The Carapo (Gymnotus carapo) is frequently seen, and many species of Gymnotiformes make interesting, intelligent aquarium specimens; though they are decidedly not "true eels".

    The Eel  Catfish, Heteropneustes fossilis, is a dangerously venomous (potentially fatal) fish that this author wishes was more limited for sale. Offered as "Black African Cats" or "Fossil Catfishes" this species is more dangerous than a Lionfish (Pterois) in my opinion... in terms of the pain of envenomation and more importantly, the too-typical ignorance of its handling. 

    Swamp Eels, Family Synbranchidae, are often looked on as true eels by the trade. They are not. Sure, they have a lot of eel-like traits, being long of body and tapered, lacking pectoral and pelvic fins, possessing only vestigial dorsal and anal fins, reduced or absent scales and small gill openings...

    These widespread (Africa, Asia, Mexico, Central and South America) fishes are interesting aquarium specimens in their own right. Not to add to the common name confusion, but one Swamp Eel (Monopterus alba) is often referred to as a/the Freshwater Moray Eel.

    Bear in mind the Synbranchids can get to more than a meter long, jump out like no tomorrow, and eat tankmates they can get their capacious jaws around if you intend to try them. 

    Amphibians as eels. The Caecilians ("See-keel-ee-uns") are too-often sold as "Rubber, Worm, Leech or Slime Eels to under-informed consumers. Most of these perish in short order from infection and lack of food. Interested aquarists should read extensively before committing to trying these animals. 

    The Spiny Eels: Follow this link to the coverage of the Family Mastacembelidae.

Close:

    So, how many of these freshwater non-eels did you know already? Just think of all the marine fishes we could have included; the Sand Eels, family Hypoptychidae, that are closely related to Sticklebacks; the Wolf Eels, Anarrhichicadidae, that are actually Blennioids... please stop me.

Bibliography/Further Reading:

http://aquariacentral.com/fishinfo/brackish/eel.shtml

Anon. 1987. Mail Call column: Caecilian (re Typhlonectes natans). TFH 12/87.

Axelrod, Herbert R, Warren E. Burgess, Neal Pronek & Jerry G. Walls. 1990. Atlas of Aquarium Fishes. Reference Book v. 2 Freshwater Fishes. T.F.H. Publications, Inc. NJ. 1055 pp.

Baensch, Hans A. & Rudiger Riehl. 1993. Aquarium Atlas v.2. MERGUS, Germany 1212pp.

Burdick, J. Alan. 1970. A look at the South American knifefishes. The Aquarium 2/70.

Fenner, Robert. 1988. Electric fish. Electric fields help some aquarium fishes to find food and "see" their surroundings. AFM 12/88.

Fenner, Bob 1998. Eels, eels, eels. TFH 7/98.

Hunziker, Raymond E. III. 1986. The flat-tailed caecilian, Typhlonectes compressicauda. TFH 9/86.

Laurie, James R. 1991. Heteropneustes fossilis. FAMA 10/91.

Nelson, Joseph S. 1994. Fishes of the World, 3d ed. John Wiley & Sons, NY. 600pp.

Nico, Leo G. 1991. Fishes of the night; part 1, The Electric Eel, part 2, The weakly electric gymnotids. TFH 2/91.

Quinn, John R. 1987. Ode to an eel (Synbranchus). TFH 5/87.

Rychlinski, Robert A. 1983. Typhlonectes compressicaudus; an aquatic caecilian from the Amazon Basin. FAMA 12/83.

Riehl, Rudiger & Hans A. Baensch. 1996, 5th ed. Aquarium Atlas, v.1. MERGUS, Germany 992pp.

Riehl, Rudiger & Hans A. Baensch. 1996, Aquarium Atlas, v.3. MERGUS, Germany 992pp.

Sprackland, Robert George. 1991. The eels that aren't. Caecilians. TFH 2/91.

Sterba, Gunther. 1966. Freshwater Fishes of the World. The Pet Library, Ltd. NY 879pp.

Thomas, Scott. 1990. The knifefishes; an underwater ballet. FAMA 8/90.