The barreleye (Macropinna microstoma) has extremely light-sensitive eyes that can rotate within a transparent, fluid-filled shield on its head. The fish’s tubular eyes are capped by bright green lenses. The eyes point upward (as shown here) when the fish is looking for food overhead. They point forward when the fish is feeding. The two spots above the fish’s mouth are are olfactory organs called nares, which are analogous to human nostrils. In the second image, you can see that, although the barreleye is facing downward, its eyes are still looking straight up. This close-up “frame grab” from video shows a barreleye that is about 140 mm (six inches) long. Image: © 2004 MBARI
Crazy doesn’t even come close to how freakin’ wierd this fish is. At my old stomping grounds, MBARI, Bruce Robison and Kim Reisenbichler now know why. Desribed in 1939, Macropinna microstoma, the barreleye fish, isn’t exactly new to science. All the species in the Opisthoproctidae family are known for having ultra-sensitive tubular eyes that face upward, well adapted for collecting light. Slight problem. If the eyes constantly face upward, looking forward to capture prey with their wee little mouths is impossible.
Using MBARI’s remote operate vehicles Robison and Reisenbichler were able to view the barreleyes in the ocean between 600 and 800m. The found that the eyes of Macropinna can rotate within transparent shield that covers the fish’s head, allowing it to look at whatever it wants. This transparent, fluid-filled shield that covers the top of the fish’s head was unknown to science, i.e. existing descriptions and illustrations do no show it. Likely, when previous specimens were caught in deep-sea trawls there were damaged or lost.
But the craziness of this fish doesn’t stop with the clear skull or pivoting eyeballs…
In addition to their amazing “headgear,” barreleyes have a variety of other interesting adaptations to deep-sea life. Their large, flat fins allow them to remain nearly motionless in the water, and to maneuver very precisely (much like MBARI’s ROVs). Their small mouths suggest that they can be very precise and selective in capturing small prey. On the other hand, their digestive systems are very large, which suggests that they can eat a variety of small drifting animals as well as jellies. In fact, the stomachs of the two net-caught fish contained fragments of jellies.
And now for the video…
Bruce H. Robison, Kim R. Reisenbichler (2008). Macropinna microstoma and the Paradox of Its Tubular Eyes Copeia, 2008 (4), 780-784 DOI: 10.1643/CG-07-082