Hank Green can tell you the full mystery:

youtu.be/acEIGorImGs?si=_xi2IF-GEssAuyZ-

tl:dw: We knew that’s where baby eels came from but we didn’t know how the adults got there or what the larvae looked like. Baby eel larvae was misidentified as another species and adult eel can take up to 18 months traveling at the bottom of the ocean to get there, during which time they grow their gonads which was another mystery.