If every fish had four eyes and a 360-degreee field of vision, we’d only be eating chips. Fortunately, only the Glasshead barreleye can claim this distinction.
The journal Proceedings of the Royal Society B reports that a research team led by Professor Hans-Joachim Wagner of the University of Tubingen's Institute of Anatomy in Baden-Wurttemberg, Germany, discovered the Glasshead barreleye, scientifically known as the rhynchohyalus natalensis, at depths between 800 and 1000 meters in the Tasman Sea between Australia and New Zealand. They were surprised the fish had four eyes but, with the murky water and lack of light at that depth, it makes sense.
The Glasshead barreleye’s main two eyes are cylindrical and point up to spot the dim shadowy images of food and other fish looking to make it food. The second set of eyes contain layers of silver guanine crystals which act as a curved mirror to focus light onto the retina. These eyes see bioluminescence created by deep-sea creatures below and to the sides of the Glasshead barreleye, giving it an extremely wide field of vision.
While extremely unusual, the Glasshead barreleye’s four eyes are not unique. It’s often seen in mollusks, crustaceans and other invertebrates but only in one other vertebrate – the brownsnout spookfish, a close relative of the Glasshead barreleye found in the Atlantic, the Gulf of Mexico, the Pacific and the South China Sea.
Before any Glasshead barreleyes or their fans write angry letters, the term “four-eyed” is really a misnomer. Both it and the spookfish actually have two eyes that are split into two connected parts. However, that doesn’t take anything away from the fact that the 7-inch barreleye is one scary-looking fish. It’s a good thing those four eyes can’t see themselves.