The Pacific barreleye fish Macropinna microstoma brings new meaning to the phrase ‘clear-headed' says Dave Brian Butvill.
Its skull is transparent, as is the skin on its head and most of its face. Though these features don’t actually help the barreleye fish to think, they do enable it to see things more clearly. It’s the only animal known to have eyes below the skin inside its head. This odd design protects these vital organs, on which the fish’s life depends.
The barreleye, one of the weirdest fish in the world, lives at depths of up to 1km throughout the northern Pacific Ocean, where little light reaches.
There, this pint-sized fish – which is just a few centimetres long – specialises in stealing prey from jellyfish, tracking the ghostly predators by their shadows or faint bioluminescence.
The special tube-shaped eyes, after which it is named, make this stealthy stalking possible. Like tiny binoculars, they provide tunnel vision with a sharp focus on minute details. The fish’s green lenses also highlight any firefly-like glows in the dark.
After finding a jellyfish, the barreleye picks at the prey stuck to its stinging tentacles. This small fish is built for careful manoeuvring, but its bulgy, fragile eyes could still easily be damaged if they weren’t behind a protective shield.
Keeping a clear head is also extremely convenient. Unlike any other fish, a barreleye can shift its gaze from forward to straight up without moving its body. This allows it to scan its environment for predators with little effort and in the blink of an eye.
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Main image: Daniel Smith/MBARI
Monterey Bay Aquarium Research Institute (MBARI), is a nonprofit oceanographic research centre dedicated to advancing marine science and engineering to understand our changing ocean. Read more about their studies into the barreleye fish