A Fish With Eyes Inside Its Head, by Aaron Betancourt

The Pacific Barreleye Macropinna microstoma is a truly extraordinary fish first discovered in 1939 with an incredibly bizarre morphology unlike any other fish in the world (Robison and Reisenbichler 2008).  The Barreleye has a relatively narrow distribution for a marine fish species ranging from Western North America, to Eastern Asia (Eschmeyer et al. 1983).  This species of fish is relatively small, as most individuals are only half an inch long while larger specimens may grow to a foot and a half (Baidya 2017).  The Pacific Barreleye is an incredibly unique fish species that is best defined by its large telescopic eyes located inside its head, its bioluminescent organs, and its feeding and behavioral patterns.

The Barreleye’s claim to fame are its eyes which are located inside its head underneath a transparent dome-like hood.  These eyes resemble barrels in shape thus how the Barreleye got its name (Poulsen et al. 2016).   For quite a while after discovery, the Barreleye’s eyes were thought to be fixed in place thus leaving the fish only able to look in an upwards direction without moving its head or body; however, it has been recently discovered that these eyes are able to rotate in place allowing the Barreleye to either look up to view its surroundings, or look forwards to see where it is swimming or look at the food it is about to consume (Robison and Reisenbichler 2008).  The Barreleye lives in the deep sea and so advanced sight is a tremendous advantage that few marine fish at that depth have.  The tubular eyes of the Barreleye are filled with a fluid that makes the eyes more sensitive to light than other fish.  They can detect even the smallest bits of light enabling it to see prey in its near pitch black environment (Baidya 2017).   

 One major question about the Barreleye was why its eyes where located inside its head and not outside like most other fish.  It is now believed that the translucent covering’s purpose is to protect the eyes of the Barreleye from damage, especially from the stingers of jellyfish (Eschmeyer et al. 1983).  At first glance it may seem like the Pacific Barreleye actually has eyes on the outside of its head, but that would be incorrect to think that.  Those eyelike spots on the outside of the head are actually olfactory sacs used for smell (Baidya 2017).  Along with its sight the Barreleye uses these tools to hunt prey.  Click here to watch video of the Pacific Barreleye.

Barreleye hunting technique.    Illustration by Ocean Syrup.    CC by 2.0

Another unique feature the Pacific Barreleye possesses is bioluminescence as is common for deep sea creatures.  They have soles on their undersides which are bioluminescent organs that the Barreleye can change the amount of light emitted from (Poulsen et al. 2016).  Because there is some form of light present where the Pacific Barreleye lives—although very minimal—it has been hypothesized that due to the location of the bioluminescent organ on the underside of the fish, the soles primary purpose is to provide counter shading to the Pacific Barreleye (Poulsen et al. 2016).  This counter shading is used by the Barreleye to help hide itself from predators by making its underside match as closely in color to what little light is making it down from above so that it blends in and predators below struggle to see it (Kelley et al. 2017).  One other potential use of this bioluminescent organ is communication between individual Pacific Barreleyes, but this is much less studied due largely to the fact that the Barreleye lives in such deep water where species are much harder to study (Poulsen et al. 2016).

The morphological adaptations the Barreleye possess are key in understanding the behavior of the fish and how that behavior aids it in everyday survival.  The largest adaption the Pacific Barreleye has other than its name giving eyes, are its incredibly large pectoral fins.  The fins are shaped in a way that the Pacific Barreleye can extend them in such a way that they hit a full stop in the water and remain motionless (Robison and Reisenbichler 2008).  It is hypothesized that the Pacific Barreleye sits completely still in the water column using its incredible vision to detect the faintest bioluminescent glow of prey while using its own bioluminescence to remain undetected (Robison and Reisenbichler 2008).  Upon finding prey it then sneaks up on it and ambushes it catching the prey in its small mouth.  Prey for the Pacific Barreleye ranges from small planktonic organisms to crustaceans and even some jellyfish (Baidya 2017).  The Barreleye’s inclusion of jellyfish in its diet explain the presence of the shield over its eyes as getting up close and personal with a jellyfish could leave the Barreleye stung and its eyes damaged if not for the shield.  It is also believed that the Pacific Barreleye interacts with jellyfish in another unique way.  Robison and Reisenbichler (2008) believe that the Pacific Barreleye uses its large fins to maneuver around the stingers of jellyfish and steal prey from them by quickly plucking them away from the stingers. 

The Pacific Barreleye is an incredibly strange creature due in large part to its remarkably sensitive eyes located inside of its own head, its bioluminescent soles which hide it from predators, as well as its feeding habits of motionless hunting and stealing food from jellyfish.  Little research has been done on these remarkable creatures because of their location in the deepest waters of the ocean.  What work that has been done has been to characterize its most striking features as well as determine the true number of species of Barreleye present.  More research on these fish is needed to understand in what ways the Barreleye uses its bioluminescence to communicate, as well as to understand if there are more reasons why it has evolved its eyes inside of its own head.

References

Baidya, S. 2017, October 17. Barreleye Fish Facts: 30 Facts You Will Need For Your Homework. https://factslegend.org/30-barreleye-fish-facts-truly-bizarre/.

Eschmeyer, W. N., E. S. Herald, and H. Hammann. 1983. A Field Guide to Pacific Coast Fishes of North America. Houghton Mifflin Company.

Kelley, J. L., I. Taylor, N. S. Hart, and J. C. Partridge. 2017. Aquatic prey use countershading camouflage to match the visual background. Behavioral Ecology 28(5).

Poulsen, J. Y., T. Sado, C. Hahn, I. Byrkjedal, M. Moku, and M. Miya. 2016. Preservation Obscures Pelagic Deep-Sea Fish Diversity: Doubling the Number of Sole-Bearing Opisthoproctids and Resurrection of the Genus Monacoa (Opisthoproctidae, Argentiniformes). Plos One 11(8).

Robison, B. H., and K. R. Reisenbichler. 2008. Macropinna microstoma and the Paradox of Its Tubular Eyes. Copeia 2008(4):780–784.