A fish with a cylindrical eye that points upward to see its prey has been discovered by German scientists.
Named the glasshead barreleye, the fish has an eye that points up to see potential mates or prey. The eye also has a mirror-like retina capable of detecting bioluminescent flashes giving the fish a larger field of vision. The findings, published in the latest Proceedings of the Royal Society, describe the previously unknown type of eye.
The 7-inch long glasshead barreleye, officially known as rhynchohyalus natalensis, was caught in the Tasman Sea between Australia and New Zealand. The fish’s reflector eye came as a surprise since they are usually found in invertebrates like mollusks and crustaceans.
The eye has a conventional round lens on top. Light from above hits the main retina. Bioluminescence enters through a cornea on the left and hits the secondary, silvery lens, to be focused on the second retina made of guanine crystals.
"The mirror here is formed from the silvery skin of the eye, and the crystals are arrayed almost parallel to the surface of the mirror," Professor Hans-Joachim Wagner from the University of Tübingen's Institute of Anatomy said. "Obviously, a broad field of vision is an advantage even at great depths if similar structures develop independently to ensure it."
The glasshead barreleye and the brownsnout spookfish are the two known vertebrates with reflector eyes. While they belong to the same family, their eyes have different tissue and structures. The spookfish’s cylindrical eye grows out of a layer of pigment on the retina and the angle of the reflective crystals varies depending on their position within the mirror. In the glasshead barreleye, the crystals are flatter and the images formed rely on the roundness of the reflective surface. Both fish appear to have four eyes, but they are in fact two eyes that are split into two connected parts.
"Very little light penetrates beneath about 1,000 meters of water, and like many other deep-sea fish, the spookfish is adapted to make the most of what little light there is,” Professor Julian Partridge of Bristol University in England told the BBC at the time the first spookfish specimen was captured in 2009. "At these depths it is flashes of bioluminescent light from other animals that the spookfish are largely looking for.The diverticular eyes image these flashes, warning the spookfish of other animals that are active, and otherwise unseen, below its vulnerable belly."