A newly discovered fish is just the second vertebrate known to use both lenses and mirrors to focus light in the eye. The eyes of the glasshead barreleye, Rhynchohyalus natalensis, are described by an international team of researchers, including some from the University of Western Australia and the University of Queensland, in a paper published in the Proceedings of the Royal Society.

The glasshead barreleye is a denizen of the mesopelagic zone, the twilight ocean, where downwelling sunlight is too dim for photosynthesis. Barreleye fish hang in the water with large, cylindrical eyes pointed at the surface, waiting for the silhouette of their prey to pass above them. For this, the glasshead use the lenses of their eyes to focus light onto their retinas. But they also have mirrors within their eyes that are able to focus the bioluminescent flashes of light produced by other mesopelagic inhabitants that arrive from the sides and below.

It’s likely to be important for the glasshead barreleye to detect these flashes because they may signal the approach of a predator or the presence of feeding prey. The authors use modelling to show that the mirrors are capable of producing a well-focused image on the retina, allowing the glasshead to judge the distance and direction of the flashes.

The only other species of vertebrate with similar eyes is the brownsnout spookfish, Dolichopteryx longipes, which is another barreleye fish. Differences in the way the mirrors are formed indicate that the two species evolved their eyes independently. The brownsnout spookfish forms its mirrors out of different tissues and light is focused by changing the angle of the reflective crystals within the mirror depending on their location. In the glasshead barreleye, the reflective crystals are arranged parallel to the surface of the mirror and it’s the shape of the mirror that focuses the light.

The evolution of two remarkably similar, yet unique eyes that are capable of producing near 360-degree vision is clearly an advantage in the inky blackness of the mesopelagic. Moreover, using mirrors to focus light in the eye offers advantages over thick lenses because they’re less prone to aberrations. It is interesting, then, that mirrors are not found more widely in vertebrates, particularly in the deep sea.

Communicated by David Semmens

A live specimen of the Pacific barreleye, Macropinna microstoma, filmed by the Monterey Bay Aquarium Research Institute.