Deep-Sea Fish Uses Unusual Method to See

The brownsnout spookfish Dolichopteryz longipes uses mirrors to focus its eyes. (Photo Credit: David Shale/NPL)

The brownsnout spookfish is found in tropical to temperate waters of the Atlantic, Pacific, and Indian oceans. The spookfish lives in the deep-sea, about 1000 meters beneath the ocean’s surface, where light does not penetrate. Although the existence of this deep-sea fish was discovered 120 years ago, it was not until recently that scientists were able to observe the fish up-close. Prof. Hans-Joachim Wagner of Tbingen University in Germany caught a live specimen off the island of Tonga in the Pacific Ocean. While examining the fish, scientists made an exciting discovery–the spookfish uses mirrors, rather than lenses, to focus its eyes. Flash photography was used to verify that the fish focuses its eyes with the use of mirrors. Dissection studies helped to confirm the scientists’ discovery.

Organisms that live in the deep-sea must have adaptations that let them find food and avoid predators in their low-light habitat. Deep in the ocean, the only light that exists comes from flashes of light given off by bioluminescent organisms. Bioluminescence refers to a light made by a chemical reaction within an organism. Most deep-sea bioluminescent creatures give off a blue shade of light, since that color is most easily transmitted in a marine environment. One example of a bioluminescent animal is the anglerfish. This deep-sea fish has a lighted “lure” attached to its head to attract prey.

Although the spookfish looks like it has four eyes, it actually only has two. Each eye is split into two connected halves. One half of the eye points upward, which gives the fish a view of the ocean above. The bottom half of the eye points downward, into the darkness of the abyss below.

As shown in this photo, the spookfish’s eye is made of two-connected parts. (Photo Credit: Dr. Tamara Frank)

The mirrors in the spookfish’s eyes are made up of tiny plates of guanine crystals, arranged in a stack made up of many layers. The arrangement and orientation of the crystals direct any light that enters the spookfish’s eye into a focus. The mirrors let the spookfish quickly produce bright, high-contrast images, giving it an immediate picture of what is around it. In contrast to mirrors, lenses are less efficient because they do not reflect all the light that hits them, and instead absorb some of the light.

Additional work to corroborate this discovery was conducted by Prof. Julian Partridge of Bristol University in England. Partridge developed a computer simulation that illustrates how the orientation of the plates within the spookfish’s eyes are perfectly adapted for focusing reflected light on the fish’s retina.

The scientists’ findings are reported in the January 27 edition of the journal Current Biology. The article, titled “A Novel Vertebrate Eye Using Both Refractive and Reflective Optics,” was co-authored by Hans-Joachim Wagner, Ron H. Douglas, Tamara M. Frank, Nicholas W. Roberts, and Julian C. Partridge.

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