This is a video hitting us with some pop-up facts about leptocephalus ("slim head"), the almost completely transparent larval stage of eels (among other members of the superorder Elopomorpha). Prior to their metamorphosis into the juvenile glass eel stage of their lives, the eels contain only tiny organs, a tube for a gut, and no red blood cells, resulting in their invisibility cloaks. Speaking of -- I actually bought an invisibility cloak from an online wizard supply company and I plan on giving it a test just as soon as this business meeting gets too boring for me to bear anymore, which is right about now. *tosses cloak over heads, bee-lines it for the conference room door* "We can all still see your legs you know." *removes cloak, sulks back to seat at table* Man, I accidentally bought a kids size and their return policy is bullshit.
Keep going for the video while I speculate if eels evolved this trait to look like clear pieces of plastic ocean garbage.
Thanks to Joshua S, who doesn't need to be transparent to be invisible because he's a ninja.