This is truly an unusual-looking fish. But what makes it more peculiar is its very unique feature – its head. Believe it or not, its head is totally transparent. It is not empty, it’s just clear.
Otherwise known as spook fish, a barreleye fish is a small deep-sea fish found in tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans. Its eyes are barrel-shaped and tubular, from which it derived its name. These eyes are generally directed upwards to detect shadows of available prey; yet, they can be directed forwards as well.
The eyes of the barreleye fish are extremely sensitive to light. They are capped by bright green lenses. The green pigments help filter out sunlight coming directly from the sea surface, enabling the fish to detect the bioluminescent glow of animals overhead. They point upward when the fish is looking for food overhead, but forward when it is feeding. Hence, the fish is very much adapted to the pitch-black environment of the deep-sea.
The fish’s eyes are located inside a fluid-filled shield on its head. This structure allows the fish’s sensitive eyes to collect even more light, as well as protect them from the nematoctyes or the stinging cells of the siphonophores, a type of jellyfish, from which the fish feeds. Most existing pictures of the fish do not show the transparent shield, probably because this part is so fragile that it is destroyed when the fish is brought up to surface by nets.
The two spots above the animal’s mouth are called nares – its olfactory organs which are similar to human nostrils. Further, the fish have large, flat fins which allow them to maneuver precisely, as well as to remain almost motionless in water. Its small mouth suggests its being selective in feeding. However, its digestive system is large, indicating that the fish is a voracious eater of small animals like jellyfish.
The fish was first described by marine biologists in 1939, but it was only in the recent years that images of the fish with its head intact were captured.