Shark Eyes Designed to Catch Photons in Twilight Zone
FNRS/UCL: Dr J Mallefet
The eyes of bioluminescent sharks are designed to catch as much light as possible.
Kay-africa, Wikimedia Commons
Animals and insects see the world in unique ways. From fish, to dogs, to birds to shrimp, super-eyesight allows them to thrive in places others can't.
Dung beetles, for example, have internal compasses that are sensitive to the sun, Marie Dacke of Lund University and her colleagues have determined. In a paper published in the latest Philosophical Transactions of the Royal Society B, she and her team explain that solar cues and skylight help guide where the beetles roll their coveted balls of poop.Video: 5 Incredible Insect Superpowers
shmoomeema, Wikimedia Commons
Siberian huskies evolved colorful, almond-shaped eyes to see in low light, desolate northern regions. A quirk of genetics is that an individual dog may have two differently colored eyes. A single eye may also feature two colors. It's known as a "parti" or "split" eye.Photos: Ugliest Dog Contenders
Chameleons can rotate and focus their eyes separately to look at two different objects at the same time, according to the San Diego Zoo. This gives chameleons a full 360-degree view around their body.Photos: Chameleon Colors Act Like a Mood Ring
Ants have vision "superpowers," interactive media designers and artists Chris Woebken and Kenichi Okada believe. Using their ant apparatus, humans can see as ants do by placing microscope antennas on their hands (ants have these on their heads) that transmit a 50-fold magnified view of wherever the person's hand is resting.33 Bizarre New Ant Species Discovered
Imagine if you spent most of the day looking up from below. That is what escolar, a large and mysterious deep-sea fish, do, according to a new study by Eric Warrant of the University of Lund and colleagues. Escolar use this technique to "sit and wait" for prey, hoping something tasty will swim over them.
Tomasz Sienicki, Wikimedia Commons
Shrimps have some of the most complex visual systems in the animal kingdom. Justin Marshall of the University of Queensland and his team found that some shrimp stare down prey before attacking with a movement that is so swift that it actually boils water in front of the shrimp. (The other temperate water surrounding the shrimp prevents it from cooking itself to death!)
Stewart Butterfield, Flickr
Most animals, including humans, have round pupils, but the eyes of goats (toads, octopi and a few others too) tend to be horizontal and rectangular with rounded corners. This broadens the horizon that they see, enabling them to better spot predators.
Pen Waggener, Flickr
Bird eyes, such as those of the eagle seen here, feature oil droplets located in the front, Doekele Stavenga of the University of Groningen and colleagues have discovered. The droplets serve as "microlenses" that help to filter and direct light.On the Hunt for Bald Eagles
The eyes of certain animals, such as raccoons and cats, glow in the dark. Their eyes have a light-reflecting surface, known as the tapetum lucidum, which makes this possible. Depending on the animal, the glow takes on certain colors. Cats tend to have eyes that glow green. Miniature schnauzer eyes will sometimes glow turquoise, according to Colorado State University ophthalmologist Cynthia Powell.
Alexander Vasenin, Wikimedia Commons
Cuttlefish, a type of mollusk, are the transformer visionaries of the animal kingdom. They reshape their entire eyes to adjust to what they see. Humans and many other species, in contrast, usually just reshape their eye lenses to get a better look at something.
Giant squid have the largest eyes in the world, according to the Smithsonian National Museum of Natural History. At up to 10 inches in diameter, the human head-sized eyes help giant squid to see in deep water. It's believed that they can detect a moving sperm whale from 394 feet away.Giant Squid Photos
Fernando Mafra, Fotopedia
We create a mental map of our surroundings in our brain. As Michael Land of the University of Sussex explains, "To interact with objects in the world we need to know where they are, whether they are in our field of view or outside it. Objects in memory have to move in the brain as we move through the world, otherwise they would be not be in the right place."
The unique eye structure of deep-sea bioluminescent sharks helps them survive in the twilight zone, a study has found.
Detailed mapping of the eye structure of five species of bioluminescent sharks reveals they have a higher rod densities than other sharks, report a team of international researchers in the journal PLOS ONE.
The twilight -- or mesopelagic -- zone covers ocean depths from 200 to 1000 meters down. Only the shorter wavelengths of light at the blue end of the spectrum reach this far down.
Mesopelagic sharks make up 12 per cent of all sharks and use bioluminescent light to communicate, find prey, and provide camouflage from predators.
Some species of lantern sharks are only 50 centimeters long.
"These amazing little animals aren't well understood, so by studying their eyes we can gain useful insights into how these sharks make a living," says study co-author, Professor Julian Partridge of the University of Western Australia.
Partridge and colleagues examined the eye shape, structure, and retinal cell pattern of four lantern shark species -- Etmopterus lucifer, E. splendidus, E. spinax and Trigonognathus kabeyai -- and one dalatiid or kitefin shark species -- Squaliolus aliae.
The authors found the eyes of bioluminescent sharks are designed to capture as much light as possible.
"We found things which hadn't been seen before in sharks, such as a layer of tissue behind the retina to reflect and increase the light available to photoreceptors," says Partridge.
The authors also found gaps between the lens and iris of these sharks, allowing extra light to reach the retina.
The sharks also had long thin photoreceptors, the rods in the back of their eyes which absorb light.
Previous studies have shown that long rods are very sensitive to light, while thin rods detect fast movement.
Bioluminescence often appears as just a brief flash. The ability of these sharks to see details in the shape of that flash tells them if it's being produced by a potential mate, lunch or a predator.
Partridge and colleagues found the density and distribution of the ganglion cells, which send information from the photoreceptors to the brain, indicates these sharks have relatively good resolution.
"They can see fairly fine detail for a fish, although it's not as good as humans," says Partridge.
"We also found differences in the density patterns of the rods and ganglion cells, depending on whether these sharks lived in mid water, or near the sea floor."
The difference in density patterns indicates whether the sharks seek food directly in front of them or on the sea-bed below.
Partridge and colleagues also found a strange translucent structure in the top of the eye which they believe is used by bioluminescent sharks to balance their own light emissions for camouflage, against the light coming down from above.
"If you're an animal in the sea where there's still some down-welling light, you can look upwards for silhouettes that might then become your lunch," says Partridge.
"So there's a lot of selective pressure for animals to disguise their silhouette because there's nothing to hide behind in the deep sea. One way to deal with that is by disguising their shape by putting lights on their bellies."
These animals need to match the color, brightness and distribution of light filtering down from above by constantly altering their light emitting cells or photophores to disguise themselves.
"That's always intriguing [to me] how an animal monitors both the down-welling light and its own bioluminescence to match the two," says Partridge.
"This teaches us new things about the deep sea which is the biggest living space on the planet, and yet it's also the environment we know least about."