Optics

Double-duty deep-sea fish eye has one mirrored and one refractive lens system

March 20, 2014

Researchers at the University of Tübingen’s Institute of Anatomy (Tübingen, Germany) has discovered a fish with a previously unknown type of eye that contains two optical systems: one catoptric (mirror-based) and one dioptric (refractive).

John Wallace

The eye of a glasshead barreleye fish is cylindrical with a conventional refractive lens on top; light from above hits the main retina (green). Bioluminescence from deep-sea prey enters through a cornea on the left and hits a separate silvery surface, to be focused on the second retina (yellow-green). The fish can therefore see objects above it with its conventional cylindrical eye, and detect other deep-sea creatures to the side and below via the reflector eye looking sideways.

Researchers at the University of Tübingen’s Institute of Anatomy (Tübingen, Germany) have discovered a fish with a previously unknown type of eye that contains two optical systems: one catoptric (mirror-based) and one dioptric (refractive).¹

The aptly-named glasshead barreleye lives at depths of 800 to 1000 m. It has a cylindrical eye pointing upwards to see prey, predators or potential mates silhouetted against the gloomy light above. But the eye also has a mirror-like second lens and retina that can detect bioluminescent flashes created by deep-sea denizens to the sides and below, says researcher Hans-Joachim Wagner.

Wagner examined an 18-cm-long glasshead barreleye (rhynchohyalus natalensis) caught in the Tasman Sea between Australia and New Zealand, as part of an international research project. The results were unexpected -- reflector-based eyes are usually only found in invertebrates, such as mollusks and crustaceans, although one other vertebrate, the deep-sea brownsnout spookfish (dolichopteryx longipes) also uses a combination of reflective and refractive lenses in its eyes. The light coming from below is focused onto a second retina by a curved mirror composed of many layers of small reflective plates made of guanine crystals, giving the fish a much bigger field of vision.

Smooth mirror vs. reflective Fresnel lens

The glasshead barreleye is therefore one of only two vertebrates known to have reflector eyes; but significantly, although rhynchohyalus natalensis and dolichopteryx longipes belong to the same family, their reflective lenses have a different structure and appear to have developed from different kinds of tissue.

The glasshead barreleye has a smooth mirrored optical surface, while the brownsnout spookfish has a reflective Fresnel lens.

REFERENCE:

1. J. C. Partridge et al., Proceedings of the Royal Society B, published 19 March 2014; doi: 10.1098/rspb.2013.3223

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.