Congratulations to science writer Ed Yong, who we know from his blogging in National Geographic‘s “Phenomena” Science Salon (which we’ve been sharing for a while now), for his first full feature story in the magazine he writes for on the web! It’s a fascinating piece on the eye, “nature’s most exquisite creation,” as his subtitle affirms. And indeed, we learned a lot about vision in the animal world thanks to Yong’s story:
Some see only in black and white; others perceive the full rainbow and beyond, to forms of light invisible to our eyes. Some can’t even gauge the direction of incoming light; others can spot running prey miles away. The smallest animal eyes, adorning the heads of fairy wasps, are barely bigger than an amoeba; the biggest are the size of dinner plates, and belong to gigantic squid species. The squid’s eye, like ours, works as a camera does, with a single lens focusing light onto a single retina, full of photoreceptors—cells that absorb photons and convert their energy into an electrical signal. By contrast, a fly’s compound eye divides incoming light among thousands of separate units, each with its own lens and photoreceptors. Human, fly, and squid eyes are mounted in pairs on their owners’ heads. But scallops have rows of eyes along their mantles, sea stars have eyes on the tips of their arms, and the purple sea urchin’s entire body acts as one big eye. There are eyes with bifocal lenses, eyes with mirrors, and eyes that look up, down, and sideways all at the same time.
At one level, such diversity is puzzling. All eyes detect light, and light behaves in a predictable manner. But it has a multitude of uses. Light reveals the time of day, the depth of water, the presence of shade. It bounces off enemies, mates, and shelter. The box jellyfish uses it to find safe pastures. You use it to survey landscapes, interpret facial expressions, and read these words. The variety of tasks that eyes perform is limited only by the fecundity of nature. They represent a collision between the constancy of physics and the messiness of biology. To understand how eyes evolved, scientists need to do more than examine their structures. They need to do what Nilsson did with the box jellyfish: understand how animals use their eyes.