A Bizarre and Unique Type of Eye Discovered

D. A. Flynn (CSIRO)

Scientists have described details of a fish, called the glasshead barreleye, which has evolved quite bizarre but highly specialized optics. The results were published in the journal Proceedings of the Royal Society B yesterday, and document a unique type of eye which demonstrates a clever solution to living in a tricky environment. 

A team led by Professor Hans-Joachim Wagner analysed a single mesopelagic dwelling fish caught in the Southern Tasman Sea between 800 and 1000 meters deep. Life in the mesopelagic zone can be problematic in terms of vision. This is because the sunlight that travels through this zone is highly directional, which has driven the evolution of upward-facing tubular eyes. This maximizes the sensitivity of the eyes to the daylight that creeps through the water. At night time and in deeper waters, however, the majority of light is produced by bioluminescent organisms which could come from any direction. Animals dwelling in this area therefore need a different type of eye, since the restricted field of vision tubular eyes give would be useless at observing objects from the side or below. 

So what happens when a fish wants to be able to see both? Some fish have overcome the restricted field of vision that tubular eyes present by using extensive eye movements. Another way to extend the visual field is by developing reflector eyes. This type of eye is lined with "mirrors" which reflect the image to a focus at a central point. This is what the glasshead barreleye, or R. natalensis has done, allowing focus of light from ventro-lateral sources; the front and sides. This allows this fish to both see objects such as predators or potential mates silhouetted from above, and bioluminescence which may occur from other directions, giving the fish a wide field of view. 

Reflector eyes are rarely found in vertebrates; they've been documented in invertebrates such as scallops, but this represents only the second case of a vertebrate possessing such an optical system. The other is a relative fish called the brownsnout spookfish, or D. longipes. This fish focuses light from above using a lens, and light from below using a curved mirror composed of small reflective plates made of guanine crystals. So what's so unique about this research then? Well, the reflective parts of the eye differ structurally between the fish, and perhaps more intriguingly, they arose from different tissues. This means that the fish adopted different ways to find a solution to the same problem, which is exciting. The fish needed better fields of vision than what one type of eye provided, and thus changes were driven that independently resulted in the evolution of two different fish with similar yet unique specialist adaptations. 

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