Fish with a transparent head
Opisthoproctidae are a weird family of fish commonly known as Barreleye fish or Spook fish.
This is probably due to the bizarre adaptations to their eyes which they need for the dark areas on the depths of the ocean in which they live. They have enormous eyes with massive lenses which usually face upward but can be rotated to face forward if they want to change their field of view. Their dome shaped head is transparent in order to refract light into their massive eyes.
Opisthoproctidae can be found in all the world's oceans at depths of 400-2,500m