I am tempted to say I'm not believing it just 'cause the source is Wikipedia, where they had Liz Smith-the-gossip-columnist married to someone she never married & she says it took her months to get them to take it out. And the eels don't have Liz Smith-level clout to correct any wrongos about them.Eels have fins but not scales:
Wikipedia wrote:
True eels are elongated fishes, ranging in length from 5 centimetres (2.0 in) in the one-jawed eel (Monognathus ahlstromi) to 3.75 metres (12.3 ft) in the giant moray.[2] They have no pelvic fins, and many species also lack pectoral fins. The dorsal and anal fins are fused with the caudal or tail fin, to form a single ribbon running along much of the length of the animal.
But let's grant that the Eel-Wiki entry was lifted directly from some legit bio book (as it most likely was, lifting with or without attribution being another unsavory characteristic of Wikipedia):
Back in biblical days, that fin fused into a ribbon would not cut it as a fin. Modern biologists/anatomists may say "fused fin!", but my ancestors, the ones whose descendants today are debating whether adult swordfish are kosher because they've lost their baby scales, would have taken one look at those eels & said "not a fish!!!!!". Their descendants certainly do.
I admit I am tempted to check a site on kashrut & see if those eels that do have a pectoral fin make it onto the list. My stomach is not tempted, though. My stomach is very untempted.