OK you may think, a deep-sea fish with a transparent head is unusual, but then many, if not most, deep-sea fish are unusual.
But look closely at the barreleye's eyes and there is a good chance you are looking at its nose. What appear to be eyes, those small round objects just above his mouth, are actually olfactory organs. They are basically fish nostrils.
The eyes are actually those green demi-spheres inside the top of its head. Its eyes are shaped like barrels and thus its name. Spherical eyes are not as well adapted for the dark deeps in which it the barrel fish lives and its eye can swivel inside its head to look up, forward, or and even to backwards through the top rear of its cranium to see if there is anything coming from behind and above.
The name 'barreleye' drives from the fact that because their eyes are tubular in shape. Barreleyes typically live near the depth where sunlight from the surface fades to complete blackness. They use their ultra-sensitive tubular eyes to search for the faint silhouettes of prey overhead.
Although such tubular eyes are very good at collecting light, they have a very narrow field of view. Furthermore, until now, most marine biologists believed that the barreleye's eyes were fixed in their heads, which would allow them to only look upward. This would make it impossible for the fishes to see what was directly in front of them, and very difficult for them to capture prey with their small, pointed mouths.
The barreleye (Macropinna microstoma) is 15cms long (6"), lives at depths of more than 600 meters (2000') and, although it was first discovered in 1939, the transparent nature of its head wasn’t known as the only specimins examined were caught in nets and dead, and when dead the transparent nature of its skull disappears so it was thought that the fish could not see forwards.
It was not until recently that a team from the Monterey Bay Aquarium Research Insitute sent remotely operated vehicles to the ocean floor to study the Barreleyeand make the remarkable discovery that the fish had a transparent skull and could rotate its eyes through many angles. They also managed to capture a live specimen, and over several hours observed the fish rotating its eyes.
There are more things in heaven and earth (and underwater), Horatio, . . . .
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