The Barrel-eye Fish (from the family Opisthoproctidae) also know as the Spook Fish lives in the Pacific, Atlantic and Indian Oceans. It is a small deep sea fish but its most interesting features are... its transparent head and tubular eyes.
Now looking at this picture you might think this it looks like a normal dolphin-like fish, just with a transparent head, but it gets weirder! You also might assume those two small black spots above its mouth are its eyes, but really, those are its nostrils. Its eyes are those two green bulges inside its head!
The barrel-eye fish's odd features can be really explained by its lifestyle, habitat, diet and competition for prey.
Lifestyle and Habitat:
The Barrel-eye fish is a small deep sea fish. Its full length is only about 15 cm long and it lives about 600 to 800 meters below the surface which puts it at the twilight zone. Scientists think that the tubular eyes help the fish hunt prey because the barrel-eye fish lives right under its prey. Since its eyes are always looking up, this makes it easier to find food. The size of the eyes also help it see in the low light condition, since a bigger lens can take in more light.
The barrel-eye fish's diet consists of a variety of prey, from small drifting fish and animals to many types of small jellyfish. Scientists believe that the fish's eyes are green in order to eliminate sunlight and be able to see prey better. Since the barrel-eye fish lives underneath the jellies, the fish hunts them through their glow. But the barrel-eye's sensitive eyes can still pick up small traces of sunlight that manage to filter in to the ocean. The green eyes are thought to be the solution to eliminating the sunlight and focusing on the glow of the jellies.
Now you might be wondering, if the barrel-eye fish can only see up, the how will it catch its prey? Or for that matter, how will it eat? Scientists for a long time thought that the barrel-eye fish could only see upward, but a study conducted by researchers at the Monterey Bay Aquarium Research Institute, where they were able to catch one alive and study it in an aquarium showed that the fish is actually able to turn its eyes to face forward when needed. The fish also have large flat fins that allow it to stay floating motionless underwater waiting for its prey, and when it spots one, the fish swim upward, their eyes turning to face forward and they are able to catch their prey.
Competition for Prey:
The barrel-eye's main competition for prey is the siphonophores which are a type of large stringing jellyfish. The barrel-eye's strategy for hunting is to swim up to the siphonophores and steal the prey that the large jellies captured in its tentacles. The barrel-eye's small size and small mouth helps it easily maneuver around the dangerous stringing tentacles and carefully pick off food, while its transparent head, which is filled with liquid, helps protect the tube eyes from getting strung.
The barrel-eye face no threat from humans simply because it is too secluded from us. Even scientists haven't been able to observe these fish in their natural habitat until recently because they live too far underneath the surface.
The barrel-eye fish's features may seem odd or even well spooky, but the fish is perfectly suited for its environment and its survival, which its doing pretty good at.