Hollywood has created some strange creatures, but I don’t think even they can match some of the strangeness that can be found in the depths of the ocean. Let's explore the lives of one of these creatures now. These are the Barreleye fish. The Barreleyes form a family known as Opisthoproctidae. What makes these creatures so unique is the morphology of their heads. Their eyes are based in the middle of their skulls, and they look dorsally (or up) through their skulls to see the silhoette of their prey against the light at the surface of the water. OK, this may blow your mind, so let's dive into this properly.
These fish live at depths of between 400 and 2500 m (1300–8000 ft) where there is very little light. However, what little light does reach those depths, the majority comes from up above (a very little amount is produced by bioluminesent creatures as well). So these fish are adapted to catch the small amount of light raining down from the surface. They hunt by swimming below their prey and looking for the silhoette. They have been known to steal food from jellyfish as well.
Now for the interesting part: since the fish is constantly looking up, its eyes face up, making it easier for the fish to continue swimming around. However, having eyes constantly looking up complicates things because, generally, the skull is there to protect the head of the animal. Instead, the skull sits behind or below the eyes, depending on the species. Because it is so dark in their environments, their eyes are enormous and sit beneath the skin in the middle of the head, where they are protected.
The structure of the eye is also different to other vertebrates. To help them gather enough light, they are currently the only known vertebrate to use mirrors in their eyes to help them gather as much light as possible.
These fish have small mouths and no teeth. Instead, they have an organ just behind their fourth gill arch that is used to grind and crush their food. The organ performs the same function as the gizzard in birds.
In all species, the fins are very small. There is one genus of barreleyes where the pectoral fins are greatly elongated. It is believed that this helps with remaining stationary but could use more study.