The box jellyfish has 24 eyes, which are dark brown and grouped into four clusters called rhopalia. Four of the six eyes in each rhopalium are simple light-detecting slits and pits. But the other two are very sophisticated. They have light-focusing lenses and can see images, albeit at a lower resolution.The box jelly fish uses its lower lensed eyes to spot approaching obstacles, like the mangrove roots that it swims among. The upper lensed eyes serve as a free-floating weight at the bottom of the rhopalium that ensures that the upper eye is always looking forward, even if the jellyfish swims upside down. If this eye detects dark patches, the jellyfish senses that it’s swimming beneath the mangrove canopy, where it can find the small crustaceans that it eats. If it sees only bright light, it has strayed into open water, and risks starving. With the help of its eyes, this brainless blob can find food, avoid obstacles, and survive.
The box jellyfish’s eyes are part of an almost endless variation of eyes in the animal kingdom. 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 photo-receptors-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’ head. 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.
Eyes are tailored to the needs of their users. A sea star’s eyes – one on the tip of each arm – can’t see color, fine details or fast-moving objects; they would send an eagle crashing into a tree. Then again, a sea star isn’t trying to spot and snag a running rabbit. It merely needs to spot coral reefs. Its eyes can do that; it has no need to evolve anything better. The human eye is reasonably fast, adept at detecting contrast, and surpassed in resolution only by birds of prey. Insect eyes have a much faster temporal resolution, two flies will chase each other at enormous speed and see up to 300 flashes of light a second. We are lucky to see 50″. A dragon-fly’s eye gives it almost complete wraparound vision; our eyes do not.
And the elephant hawk moth has eyes so sensitive that it can still see colors by starlight. In some ways we’re better, but in many ways , we’re worse. There ‘s no eye that does it all better. Our camera eyes have their own problems. For example, our retinas are bizarrely built back to front. That’s why we have a blind spot. There ‘s no benefit to these flaws; they’re just quirks of our evolutionary history. Our brains can fill in the missing details in our blind spots but some problems we can’t avoid. Our retinas can sometimes peel away from the underlying tissue, leading to blindness, that would never happen if the neurons sat behind the photoreceptors, anchoring them in place. This more sensible design exists in the camera eyes of octopuses and squid. An octopus doesn’t have a blind spot. It never gets a detached retina. We do, because evolution doesn’t work to a plan. It meanders mindlessly, improvising as it goes.
Most birds and reptiles see color with four types of cone photoreceptors, each carrying an opsin that’s tuned into a different color. But mammals evolved from a nocturnal ancestor that had lost two of these cones, presumably because color vision is less important at night and because cones are most effective in bright daylight. Most mammals are still saddled with these losses, and see the world through limited palette. Dogs have just two cones, one tuned to blue and the other to red. Marine mammals dispensed the blue cone when they became aquatic. Many whales lost the red cone too. They have only rod photoreceptors-excellent for seeing in the deep ocean darkness but useless for seeing color.
The mantis shrimp’s eyes have three separate regions that focus on the same narrow strip of space, providing depth perception without help from the other eye. They can also see ultraviolet parts of the spectrum that are invisible to us, and polarized light that vibrates in a single plane. And while we have three kinds of color receptors in our retinas, mantis shrimp have 12 each tuned to a different color.
Article by Ed Yong for National Geographic – Published January 14th, 2016.
Eyes: more than what you see