In the ocean's 'twilight (i.e., mesopelagic) zone' depths of 600 to 800 meters where little light penetrates lives a peculiar creature. The barreleye fish, a member of the clade Protacanthopterygii (PROH-tuh-kan-THOP-tuh-RIJ-ee-eye), looks like nothing you've ever seen unless, of course, you've seen a barreleye fish. It has a thick body, a tiny, pouty mouth, and oddest of all, a clear, liquid-filled dome on its head that contains its tube-shaped, upward-gazing eyes. Though it was first discovered in 1939, little is known about this unusual fish because we've only recently developed the technology to bring the delicate deep sea animals to the surface for study.
The barreleye had been known for 50 years, but no one knew about its amazing adaptations until Monterey Bay Aquarium Research Institute researchers videotaped the animal alive, in its native habitat, using a robotic subersible.
In the barreleye fish, the lens of each eye is naturally oriented upwards, but the muscles can be used to rotate the eyes so that the fish can view an object in front of its rostrum (or "nose").
From what scientists have so far gathered, barreleye fish seem to use their odd eyes for spotting planktonic crustaceans and the other small animals on which they feed. Since such organisms are often trapped in the stinging tentacles of jellyfish, scientists have hypothesized
that the fish's clear dome of a forehead helps protect the eyes of the fish from stings. But why the tubular eyes? In low light levels, seeing requires the maximum amount of light coming into the eye possible. That means having a big, exposed lens (as we see in the barreleye). But basic physics dictates that larger lenses focus light a further distance away. The bigger the lens, the larger the eye needs to be to get a clear image. If the barreleye fish had normal looking, spherical eyes, they would be huge
and take up most of its head, but a tubular eye allows the eye to collect a lot of light and focus it the right distance away without devoting the entire head to eyes. In addition, the barreleye's tubular eyes work well for seeing in low light levels because they are both oriented the same direction up, where most of the light is coming from. Since each eye picks up light from the same place, the fish gets a much clearer picture of what's happening in the waters above.
The barreleye is not the only fish in the sea with tubular eyes. On this phylogeny, you can see that this trait is found in many different lineages of deep sea dwelling fish. Since these lineages are not particularly closely related to one another, it's clear that this is an example of convergent evolution. When different lineages are presented with the same challenges in their living environments, they sometimes wind up evolving similar adaptations that help cope with the difficulties.
Helfman, G. S., Collette, B. B., and Facey, D. E. 1997. The Diversity of Fishes. 535 pp. Blackwell Publishing, Malden, MA.
Wiley, E. O. and Johnson, G. D. 2010. A teleost classification based on monophyletic groups. In J. S. Nelson, H.-P. Schultze, & M. V. H. Wilson (eds) Origin and Phylogenetic Interrelationships of Teleosts. Pp. 123-182, Verlag Dr. Friedrich Pfeil, München, Germany.