The second type of eye is typically associated with invertebrates. The authors write that this is only the second instance in a vertebrate, after Dolichopteryx longipes, with both reflective and refractive optics.
I particularly liked the description of how they modeled the optics and image focusing. The authors have the same question I have; this is cool, so why isn't it more common?
Gross morphology of the eyes of R. natalensis. (a) Lateral view of specimen shortly after capture; (b) dorsal view of head showing the spherical lenses of the dorsally directed tubular eyes; (c) ventral view of the head showing the silvery lateral walls and the dark cornea of the diverticulum—the red arrows indicate a medial notch in the diverticular cornea, enlarging the visual field caudo-medially; (d) lateral view of the right eye—note the reflection of the flashlight (blue arrow) from the diverticular mirror located inside the eye and observed at the time of collection; (e) MRI section of the right half of the head showing the tubular eye including the lens and the lensless diverticulum; (f) 25 μm thick resin-embedded histological section of the eye with the lens removed. In (d,e) the margins of the ventro-laterally facing diverticular cornea are indicated by arrows. Credit: DOI:10.1098/rspb.2013.3223
Citation: J. C. Partridge, R. H. Douglas, N. J. Marshall, W.-S. Chung, T. M. Jordan, H.-J. Wagner, 'Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis)', Proc. R. Soc. B 7 May 2014 vol. 281 no. 1782 20133223 DOI:10.1098/rspb.2013.3223