Fishy character: The barreleye has extremely light-sensitive eyes, capped by bright green lenses, that rotate within its transparent head. The eyes point up (shown here) when the fish is looking for food overhead, and point forward when it feeds. The two spots above the fish's mouth are olfactory organs called nares, which are analogous to human nostrils.
Credit: MBARI
SYDNEY: A new study has shown that a bizarre deep-fish can rotate its eyes to look up through the totally transparent roof of its head in search of prey.
Though the barreleye fish, Macropinna microstoma, was discovered almost 70 years ago, most specimens were damaged when brought to the surface in nets.
The research from the Monterey Bay Aquarium Research Institute (MBARI), in California, is the first to show that the animal has a transparent fluid-filled sack in place of the roof its skull, and that it can rotate its tubular eyes to either look up or forwards through its head to focus on prey.
Lucky break
The scientists, who report the find in a recent edition of the journal Copeia, were lucky enough to capture a specimen and bring it up to the surface unharmed, where it swam in an aquarium aboard their research ship for several hours. The team also used a specialised underwater ROV (remotely operated vehicle) to monitor the fish at depths of up to 800 m below the surface
"We realised that we were observing a deep-sea animal that no one had ever seen alive before," said expedition leader Bruce Robison. "It was quite different from what everyone had imagined. That made us curious about its extraordinary eyes and how they work."
The fish, typically found in deep waters off the coast of central California, was originally thought to have its eyes fixed upward, meaning it could only see what was directly above it. The tubular eyes are an excellent source for collecting light, but have a very narrow field of vision.
Deep-sea loitering
The barreleye lives in conditions of almost complete darkness more than 600 m below the surface, where it remains still, floating in the deep waters, and uses its excellent eyesight to search for silhouettes of prey from overhead.
"There is a yellow pigment in the lenses … that helps the fish to distinguish between down-welling sunlight and the bioluminescence that other animals use to conceal their presence with the sunlight as a background," Robison said.
It is now thought the fish hangs in a horizontal position, pointing its eyes up through its head to locate faint silhouettes of its prey. When it spots one, it rotates its eyes back into the forward-facing position and swims upward to capture it.
The researchers also speculate that the barreleyes follow jellyfish, and use their eyesight to manoeuvre carefully through their stinging tentacles and grab small trapped organisms.
