Sometimes it would be nice to have a third eye. Think of all the things you could see with a little more perspective. It would certainly be helpful out on the water when things are always happening in every direction — fish jumping, weather changing, boats coming by. Fishermen often talk about having their head on a swivel to make sure they are constantly aware of what is around them. This is particularly critical in the summer when coastal waters are very busy. But, alas, we have not evolved to have a third eye.
Horseshoe crabs have 10 eyes and a series of light receptors on their tails. Glenn Michaels photo
There are many ocean creatures, however, that have more than two eyes. Take the horseshoe crab that has 10 eyes, including several on its shell as well as a series of light receptors along its tail, or the sea urchin that technically has no eyes, but rather its whole body functions as an eye, with every tube foot having a light receptor at its tip. Despite these oddities, for the most part, two eyes are by far the norm for animals living both in and out of the water.
There are, of course, differences in the features of these eyes. For example, a fish eye has a lens that is round in order to focus light coming in from all directions under water. This is an amazing thing to discover if you have ever had the opportunity to dissect a fish eye. The lens, rather than looking like a curved thin contact lens akin to a human eye lens, is a small round translucent hard ball. It’s a bit like a crystal ball taking in the light from every angle and focusing it into a single image for the fish.
But what about the ocean animals that live above the surface but also dive down below it and need to see — how do their eyes work under water? They don’t have a third eye or a round lens but instead have a third eyelid. This is known as a nictitating membrane and it is often barely visible as it is translucent. But, this thin membrane covers most of the surface of the eye. Its purpose above water is to help protect the eye from getting irritants in it. Think of how amazing it would be to have this during a windy day at the beach when it is all too easy to end up with an eye full of sand. Below water, the nictitating membrane helps to protect the eye from the sting of the salt so that it can see beneath the surface. It doesn’t focus the light as clearly as the lens of a fish eye, but it does work well to protect it and provide some visibility.
I recently saw a picture of a black-crowned night heron with its nictitating membrane drawn across its eye just as it was about to dive under the surface in a blog put out by naturalist Mary Holland called “Naturally Curious” (naturallycuriouswithmaryholland.wordpress.com). Her blog is always full of great tidbits, photos and trivia about the natural world and is worth subscribing to. She also has a terrific book of the same name that has a section on each season and features a variety of wildlife and natural occurrences.
Other marine animals, including many fish, also have nictitating membranes and some mammals as well, including bears and cats, which you may have noticed if you’ve looked closely. We humans, however, have to resort to putting on an awkward set of goggles or a snorkel mask that leaves an uncomfortable mark on our face and makes us sound like we have a cold when we talk. It’s just another reason to be impressed and inspired by the ability of nature to adapt and thrive in a variety of environments in ways that far surpass humans’ attempts through technology to mimic them.
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