Monday, November 12, 2012

Mystery of the Fish Eye

If you're on Facebook you probably saw the picture of the giant bloody eyeball that washed up on a Florida beach a few weeks ago. Speculation on the source of the softball-sized eye ranged from the reasonable (giant squid or whale) to the unlikely (Big Foot).
Photo by Carli Segelson, Florida Fish and
Wildlife Conservation Commission

Based on the eye's color, size and structure, along with the presence of bone around it, scientists concluded the eye came from a swordfish. They also observed straight cuts around the perimeter, suggesting the eye had not been ripped out in some deepsea wrestling match but removed with a knife. Most likely, the eye was cut out and discarded by a fisherman. Was it a fisherman with a keen sense of humor who thought "won't this freak someone out?" as he tossed it overboard? We may never know.
Giant squid attacking a bait squid.
Photo from National Geographic.
Most people don't realize how big a swordfish or marlin eye can be because most of it is inside the head. As impressive as the eye's size is, it pales in comparison to the giant squid's dinnerplate-sized eye that comes in at 3 times the diameter of the swordfish orb. Scientists speculate the larger eye allows the squid to detect the shimmer of bioluminescent organisms in the dark of the ocean deep. The glitter of light could indicate the approach of the squid's only predator, the sperm whale.

Fish eyes, for the most part, work similar to our eyes. They have rods and cones, and light enters through a cornea and passing through the pupil to reach the lens. Most fish have a fixed pupil size, but cephalopods, like the giant squid, have a pupil that adjusts size and shape: it's w-shaped when contracted and round when fully dilated. There isn't much difference in refractive index between the water and the cornea--light passes in a straight line, no bending as it does when passing through air into our liquid-filled eyes. Human eyes are adapted to accomodate the differences in refraction between air and water and so are more concave than most fish eyes. That's why we need to wear a mask to see underwater, but fish don't.

Four-eyed fish,
 The four-eyed fish might just be the winner for the most unusual eyeballs in the aquatic world. These fish feed on terrestrial insects at the surface so they need to see underwater, where they live and in the air, where they feed. Their two eyes (yes, 4-eyes is a misnomer) are raised above the top of the head and divided in two different parts, allowing them to see below and above the water surface at the same time. The fish floats at the water surface with only the lower half of each eye underwater. The two halves are divided by a band of tissue and the eye has two pupils connected by part of the iris. The upper half of the eye is adapted for vision in air, the lower half for vision in water The lens of the eye also changes in thickness top to bottom to account for the different refractive indices of air versus water. The two pupils allow the 4-eyed fish to search for the food above the water while keeping an eye out for predators below the water at the same time. It also makes them really difficult to catch!

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