Interesting....

Barreleye fish

By Gillian Shaw

The fish with eyes that rotate in a transparent head.

The Barreleye fish (Macropinna microstoma). Credit: MBARI

IF YOU’VE EVER WISHED for eyes in the back of your head, then you’ll be impressed by the barreleye fish (Macropinna microstoma). Also known as a kind of spookfish, this bizarre, deep-sea species features a completely transparent roof to its head. It’s something akin to the sunroof in your car – and, in fact, it’s meant to capture light in a similar way.
Found 600m or more down off the coast of California, the species lives in nearly total darkness. A transparent fluid-filled sack replaces the top of the head and two spots above the mouth, called nares, act somewhat like human nostrils, chemically sampling the water.
Experts think the fish hangs motionless in the current with its eyes swivelled upwards to spot the faint silhouettes of its prey. If it spots something it likes the look of, it rolls its eyes back to the forwards position and swims up to intercept it with its small, toothless mouth.
There are 14 species in the barreleye family of fish (Opisthoproctidae), all of which have bulging telescopic eyes protruding from the skull. M. microstoma was first discovered 70-odd years ago, but only recently did scientists bring a live specimen to the surface. It was then that they realised that its eyes were not permanently positioned facing upwards.
“We realised that we were observing a deep-sea animal that no one had ever seen alive before,” said Bruce Robison who led an expedition from California’s Monterey Bay Aquarium Research Institute to observe the fish underwater, and also brought a live specimen up to an aquarium on their boat. “It was quite different from what everyone had imagined. That made us curious about its extraordinary eyes and how they work.”
As the researchers reported in the journal Copeia in February 2009, the tubular eyes are an excellent source for collecting light, but have a very narrow field of vision. “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. He believes that barreleyes follow jellyfish, using their eyesight to carefully navigate a path through the stinging tentacles to snatch small, trapped organisms.