I'm trying to figure out what exactly true eels possess that make them different from other so called "eels." At first I though it was the dorsal, anal, and caudal fins being fused together, but I was looking at pictures of those fire eels to see if there was a difference, but all their fins look fused too! Can someone please help me clarify what makes a true eel a true eel so my head doesn't explode?

Being an anguilliform is the defining characteristic. The eel bauplan has sprung up many times, but the word "eel" was first applied to the European eel and its relatives, so they get to be the "true eels".

The only trait I know of that cleanly divides anguilliforms from all other "eels" is the leptocephalus larva stage (which is is also found in the non-eel-like orders Elopiformes (tarpon) and Albuliformes (bonefish)). All anguilliforms lack pelvic fins, but this is a common trait among "eels". I'm sure there are skeletal, myological, and biochemical differences as well, but probably nothing you can point to on a live fish and say, "Ah, that's a true eel characteristic".

Being an anguilliform is the defining characteristic. The eel bauplan has spring up many times, but the word "eel" was first applied to the European eel and its relatives, so they get to be the "true eels".

The only trait I know of that cleanly divides anguilliforms from all other "eels" is the leptocephalus larva stage (which is is also found in the non-eel-like orders Elopiformes (tarpon) and Albuliformes (bonefish)). All anguilliforms lack pelvic fins, but this is a common trait among "eels". I'm sure there are skeletal, myological, and biochemical differences as well, but probably nothing you can point to on a live fish and say, "Ah, that's a true eel characteristic".

Thanks! When I explain to people that I have a true eel, they want to know exactly what makes a true eel a true eel. Thanks for clearing things up!

Newt, thanks for using bauplan.

So Newt, is there no truth to the idea that true eels have no pectoral fins while non-true eels have pectoral fins?

Sunny- glad to help.

Fundulus- I couldn't remember where the umlauts go! Oh well.

Mudminnow- It is pelvic fins that true eels lack. At least some, including Anguilla, have pectoral fins. On the other hand, some "false" eels, such as swamp eels, lack paired fins altogether, while others, such as electric eels, have the same pattern as Anguilla- pectoral fins present, pelvic fins absent.

Mudminnow- It is pelvic fins that true eels lack. At least some, including Anguilla, have pectoral fins. On the other hand, some "false" eels, such as swamp eels, lack paired fins altogether, while others, such as electric eels, have the same pattern as Anguilla- pectoral fins present, pelvic fins absent.
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Ah, thanks Newt. I think my confusion comes from where I was raised, for I was always told that the monkey-faced eels and wolf eels that lived close to my house were not eels but the moray eels were. So naturaly, when I moved to the east coast and saw american eels that looked more like monkey-faced eels than moray eels, I assumed the american eels (Anguilla) weren't really eels either.

Fundulus- I couldn't remember where the umlauts go! Oh well.

You only need an umlaut on the plural form – baupläne.

Thanks! That's good to know.

Mudminnow- that makes sense, but Anguilla are definitely "true eels". They are, in a sense, the truest of them all. If some taxonomist comes in and chops Anguilliformes all up, whichever bit keeps Anguilla will remain the true eels.

A correction to my earlier statement: two families of "eels" related to Anguilliformes also have leptocephalus larvae. These are the Notacanthidae (deep sea spiny eels) and Halosauridae (halosaurs). These are both exclusively deep-sea groups which together form the order Notacanthiformes, sometimes considered suborder Notacanthoidei of Albuliformes.

However, neither adults nor leptocephali of these fishes look much like true eels. The leptocephali are extremely long and attenuate, not leaf-shaped like anguilliform leptocephali. Adult halosaurs have long, lizard-like snouts, and distinctly separate body and elongate tail. They look a bit like stretched-out chimeras. Deep sea spiny eels are deepest near the front and taper towards the rear, somewhat like knifefishes. Both groups retain pelvic fins, have reduced or absent caudal fins, and the dorsal fin (if present) is separate from the other median fins.

Also, four abberant deep-sea anguilliform families (Cyematidae, Eurypharyngidae, Monognathidae, Saccopharyngidae) are sometimes placed in a separate order, Saccopharyngiformes. This group includes the gulper, pelican, and arrow (snipe) eels. They also have leptocephalus larvae but have V-shaped myomeres (all other fishes have W-shaped myomeres). All have reduced skeletal elements and bizarre mouth morphologies.

Interesting note: some notacanthids have leptocephalus larvae larger than the adults, as in paradox frogs (and many insects).

A correction to my earlier statement: two families of "eels" related to Anguilliformes also have leptocephalus larvae. These are the Notacanthidae (deep sea spiny eels) and Halosauridae (halosaurs). These are both exclusively deep-sea groups which together form the order Notacanthiformes, sometimes considered suborder Notacanthoidei of Albuliformes.

However, neither adults nor leptocephali of these fishes look much like true eels. The leptocephali are extremely long and attenuate, not leaf-shaped like anguilliform leptocephali. Adult halosaurs have long, lizard-like snouts, and distinctly separate body and elongate tail. They look a bit like stretched-out chimeras. Deep sea spiny eels are deepest near the front and taper towards the rear, somewhat like knifefishes. Both groups retain pelvic fins, have reduced or absent caudal fins, and the dorsal fin (if present) is separate from the other median fins.

Also, four abberant deep-sea anguilliform families (Cyematidae, Eurypharyngidae, Monognathidae, Saccopharyngidae) are sometimes placed in a separate order, Saccopharyngiformes. This group includes the gulper, pelican, and arrow (snipe) eels. They also have leptocephalus larvae but have V-shaped myomeres (all other fishes have W-shaped myomeres). All have reduced skeletal elements and bizarre mouth morphologies.

Interesting note: some notacanthids have leptocephalus larvae larger than the adults, as in paradox frogs (and many insects).

Cool stuff, Newt. I find it humorous that you say "bizarre mouth morphologies" as one of the traits. I can imagine a key for eels saying "Bizarre looking mouth, Go to 4".

The common eel Anguilla rostrata in American waters is a catadromous fish, it spawns in salt water but spends a great part of it's life in fresh water streams. It goes out to sea to spawn unlike the salmon which goes upstream into fresh water and spawns and then spends most of it's life at sea. The European eel behaves in much the same way and although it's breeding grounds overlap the American species breeding ground they do not cross nor do they visit one another's shores. Paraphrased from The Book of Fishes 1952 Edition published by NGS.

So I would imagine that this peculiar spawning behavior could possibly be a major defining characteristic when it comes to eels however I am not certain.

Edited by JCarpenter, 25 January 2010 - 12:48 AM.

The common eel Anguilla rostrata in American waters is a catadromous fish, it spawns in salt water but spends a great part of it's life in fresh water streams. It goes out to sea to spawn unlike the salmon which goes upstream into fresh water and spawns and then spends most of it's life at sea. The European eel behaves in much the same way and although it's breeding grounds overlap the American species breeding ground they do not cross nor do they visit one another's shores. Paraphrased from The Book of Fishes 1952 Edition published by NGS.

So I would imagine that this peculiar spawning behavior could possibly be a major defining characteristic when it comes to eels however I am not certain.

That's probably a characteristic trait of Anguillidae, but not the whole order Anguilliformes. Aren't morays in the order? They don't really live in freshwater, with very few exceptions.

Yeah, morays, congers, and various other non-catadromous groups are included in the order.

Here's the description of the order from FishBase:

Pelvic fins and skeleton absent; pectoral fins and girdle absent in some; pectoral fins, when present, at least midlateral in position or higher and skeleton lacking bony connection to skull (posttemporal absent); dorsal and anal fins confluent with caudal fin (caudal fin rayless or lost in some); scales usually absent or, if present, cycloid and embedded; body very elongate (eel-like); gill openings usually narrow; gill region elongate and gills displaced posteriorly; gill rakers absent; pyloric caeca absent; maxilla toothed, bordering mouth; the two pre-maxillae (rarely absent), the vomer (usually), and the ethmoid united into a single bone; branchiostegal rays 6-49; swim bladder present, duct usually present; oviducts absent; opisthotic, orbitosphenoid, mesocoracoid, gular plate, posttemporal, postcleithra, supramaxilla, and extrascapular bones absent; ossified symplectic absent (cartilaginous one present in Synaphobranchidae); hyomandibular united with quadrate; ribs present or absent.

Unfortunately it does not indicate which traits or combinations of traits are unique to the order.