A small tropical fish has been shown to use mirror-eyes on the side of its head to help see its prey in its dark deep-sea habitat - and is the subject of a paper published in the prestigious journal Proceedings of the Royal Society B.
It is only the second vertebrate known to have mirror vision and to be able to see a focused image by reflection rather than using a lens as most vertebrates (including humans) do. The fish is closely related to the first animal found to have such ‘side-mirrors' although the mirrors in each fish are very different. Both fish also have conventional eyes, although they are on the top of their head and look upwards into the minimal sunlight filtering down, rather than forwards as in most animals. The fish use their mirror-eyes to see underneath and to either side.
The discovery of the mirror eyes of both fish was made by a team that included Adjunct Professor Julian Partridge of The University of Western Australia's School of Animal Biology. Professor Partridge, who is also in the University of Bristol's Vision Ecology Research Lab, is the lead author of today's paper.
The first, the brownsnout spookfish (Dolichopteryx longpipes) was described four years ago and the second, the glasshead barreleye fish (Rhynchohyalus natalensis), was collected last year. Their habitats can be as much as half a kilometre under the sea surface where many animals are bioluminescent and much of vision is to do with seeing dim flashes of blue light in an otherwise dark world.
Scientists are excited about the discovery because not only is it unexpected to find a vertebrate with mirror vision, it is even more remarkable to find two, with mirrors that are so different despite the fish belonging to the same family.
The barreleye fish's mirrors are made of layers of two transparent substances stacked in such a way that they become reflective. The substances are guanine and cytoplasm and together create a smooth silvery surface from which light bounces onto the retina. Focus depends on the shape of this surface.
The brownsnout spookfish's mirrors are made of the same substances but instead of being smooth, the surface is composed of angled reflective crystals that shine light onto the retina, and the arrangement of the plates allows the light to be focused.
In both fish, the mirrors extend the restricted visual field of their tubular eyes - and have ensured the animals' survival for more than 35 million years. Remarkably, the mirrors have evolved from different layers of retinal tissue in the two species; providing a similar solution by different mechanisms.
"This is a finding that not only shows the marvellous diversity of eye design, but also what an amazing and little-known place the deep-sea ocean is," Professor Partridge said.