"At an age of 6-30 years, eels begin to undergo a remarkable series of changes, eyes are enlarged, head becomes pointed, skin on the back darker, while that on the belly becomes shiny and silvery."
http://filaman.uni-kiel.de/summary/SpeciesSummary.php?genusname=Anguilla&speciesname=anguilla
http://filaman.uni-kiel.de/summary/SpeciesSummary.php?genusname=Anguilla&speciesname=anguilla