This kind of pupil likely provides predators with the sharpest way to estimate distance between themselves and a tasty meal, the study found. Gauging this distance is important, because it helps the predator decide how far it needs to leap out to catch its prey.
[Images: See the World from a Cat's Eyes]
Just like a camera lens with a zoom, chameleons can focus their eyes and enlarge what they are looking at, Live Science reported previously.
When one of these clams detects dark objects moving nearby, it withdraws its mantle toward its shell, the study found.
Another clam, known as the disco clam (Ctenoides ales), has about 40 eyes. But its vision is poor; it couldn't even detect flashing reflected from a nearby disco clam, according to research covered by Live Science.
For instance, the fiddler crab (Uca vomeris) can see all around itself, including overhead, according to a 2009 study published in the Journal of Experimental Biology.
The eye cells on top of the crab's stalks can perceive light and dark, but not fine detail. "The crab only needs to see one dark spot moving in its upward vision to know it must run for its burrow," study co-author Jan Hemmi, a senior lecturer of biology at the University of Western Australia said in a statement.
The crocodile has a layer of reflective, mirrored crystals behind its retinas. During the day, a pigment in these crystals acts like a pair of sunglasses. But at night, the pigment cells retract, allowing the crystals to reflect light back onto the retina, amplifying the strength of the image. This allows the crocodile to essentially see in the dark.
According to a 2013 study in the journal Vision Research, the narrow, W-shaped pupil helps the cuttlefish balance uneven, vertically traveling light. This cuts down the amount of sunlight that is scattered across the cuttlefish's lens, making it easier for the animals to see contrasting images, the researchers of the study said.
Humans have trichromatic vision, a term that describes the red, blue and green light-sensitive proteins in our eyes. Dragonflies, in contrast, can see way more colors, including ultraviolet light. The insects have from 11 to 30 light-sensitive proteins in their eyes, according to a 2015 study in the journal Proceedings of the National Academy of Sciences that looked at 12 species of dragonflies.
The answer has to do with survival. A goat's oddly shaped pupils likely help it scan the horizon for predators, Live Science previously reported.
The place where the optic nerve enters the retina is called the blind spot. But don't despair: You can shrink the blind spot with eye-training exercises, a small 2015 study found.
The jumping spider can see almost 360 degrees around itself with its eight eyes. It uses the large, primary eyes on the front of its head to see detail and the small, secondary eyes to see motion, Live Science previously reported. [See photos of jumping spider eyes]
Geckos clean their gem-like eyeballs — which are either lidless or covered by clear eyelids — with a quick lick of the tongue.
Rather, the shrimp can differentiate colors that are about 25 nanometers apart, a 2014 study in the journal Science found. In contrast, humans can discriminate shades that are as little as 1 nanometer to 4 nanometers apart, Live Science previously reported. In other words, the mantis shrimp can still see a ton of colors, but it can't differentiate between them as well as humans can.
"They're definitely not seeing the world of color in as much detail as other animals," study researcher Justin Marshall, a neurobiologist at the University of Queensland in Australia, told Live Science.
[In Photos: Mantis Shrimp Show Off Googly Eyes]
Owls also have a tapetum lucidum, a structure that reflects visible light back onto the retina, helping the animals see at night, Live Science reported.
Penguins have flattened corneas, which refract (bend) light less than human corneas do. In effect, it's up to a penguin's lens, not the animal's cornea, to focus light onto the retina. Luckily, penguins are well-adapted to do that. They also have strong eye muscles that can change the shape of the lens when the birds are on land or in the water, according to the New England Aquarium.
What's more, the birds have clear eyelids, called nictitating membranes, that protect their eyes when they're swimming underwater, the aquarium said. These adaptations help penguins see wherever they go!