Fish eyes do the coolest things

Ed Yong over at Not Exactly Rocket Science beat me to the punch on this one.  You should check out his summary of a new paper by a group of excellent fish eye people on the spookfish, Dolichopteryx longipes.  Like many mesopelagic fishes that live in these low light conditions, the spookfish has tubular shaped eyes that look straight up to try and spot the shadows cast by soon to be prey items.  This oddly shaped eye allows the fish to collect as much light as possible from above, but it does not allow the fish to see around or down.  To do this some mesopelagic fishes have a secondary retina that looks laterally and ventrally, but this part of the eye does not use a lens to focus light.  It was thought that the images produced by this secondary retina would be crude, but the new paper by Wagner et al. on the spookfish shows that instead of a lens, this species uses a reflective surface, yes a mirror, to focus light on its secondary retina.  This is the first described example of a vertebrate eye that uses reflective optics to focus light, but may not be the last.