The Rhynchohyalus natalensis in a recent paper was caught about 1000 meters under the Tasman Sea and it has two pairs of eyes, allowing it to spot danger from every angle. One pair is upward-facing tubular eyes, to spot danger from above, while another set is on the side of its head, to detect bioluminescence from deep sea creatures.
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
