A closer look at that freaky, giant fish eye

Last week, Mark told you about a giant eyeball that washed up on the beach in Florida. Today, the Florida Fish and Wildlife Conservation Commission released their preliminary analysis of who that eyeball once belonged to and how it likely ended up becoming the temporary toast of the Internet.

The Deep Sea News blog called it last week, but the official word from the experts is that this was the eye of a swordfish. The distinction is based on the size, the color, and the fact that there are bits of bone present around the edges (something you wouldn't see attached to a giant squid eye).

How do you get a swordfish eye without a swordfish attached? Simple: It's swordfish season. In the press release, Joan Herrera, curator of collections at the FWC's Fish and Wildlife Research Institute in St. Petersburg, said that,
"Based on straight-line cuts visible around the eye, we believe it was removed by a fisherman and discarded."

But before we pack this mystery away, I think you should take one more close look at the giant eyeball, because it offers a great view a really interesting feature of fish eye anatomy. Fish eyes are similar to those of land-dwelling vertebrates. But there are some key differences. In particular, the shape of the lens…

In the human eye, you've got an iris, you've got an opening in the iris called the pupil, and you've got a flattish lens sitting behind the pupil. In a fish eye — including this one &mash; the lens is much more rounded and it sticks up through the pupil like a little nubbin.

In 2007, Advanced Aquarist magazine had a feature all about fish eye anatomy, which explained how this anatomical difference affects the way the fish eye works.

A prominent characteristic of the fish eye, from the outside at least, is its bulbous nature. Some of the reasons for this will become apparent. The outer layer of the eye, the cornea, is dome-shaped and transparent. It is the first to receive light. With the terrestrial vertebrate eye, light travels through the air and hits the cornea. Because the air and cornea are of differing densities, the light is refracted (bent and directed) into the opening called the pupil. Water and cornea are of about equal densities so there is little refraction with the cornea of a fish eye. Again, with terrestrial vertebrates, the iris is the colored aperture that opens and closes, adjusting the size of the pupil and the amount of light entering the eye.

But in most teleost fishes, because of the protruding lens, the iris is rigid, making the pupil a fixed size. Therefore, to compensate for the amount of light entering the eye, an amount which could be too intense or not intense enough to measure, the retina adjusts the position of the photo receptors. And whereas our terrestrial eyes adjust to light levels within a few moments, fish eyes take much longer. We can observe this in aquarium fish that have been subjected to suddenly having their tank light turned on. Such fish typically hide until their eyes have adjusted to the light, which can take fifteen or twenty minutes.