This unusual fish might be the most bizarre creature ever found lurking in the deep ocean. Sometimes called a "spook fish," no doubt because of its strange appearance, the barreleye has a completely transparent head.
The purpose of the clear head is that its eyes, which are located inside the head, can look straight up as it swims, presumably so that it can detect the silhouettes of available prey. Its eyes can rotate within the socket so that the fish can look in multiple directions, which would be impossible if not for the transparent cranium.
Check out this incredible video of a live barreleye as it swims.