Deep Sea Eyes Evolved to Protect from Predators

A recent study of the eyes of giant and colossal squid reveals they would likely have only given an advantage for detecting the presence of large predators, such as the sperm whale.

Giant squid have possibly the largest eyes of any creature that ever lived. At a diameter of 10.5 inches, with a pupil size of 3.5 inches and hard lenses as big as human eyes, these eyes are disproportionally sized compared to almost any other creature, which raises the question, “Why?” Any time a creature develops a highly unusual structure such as this type of eye, the typical assumption is that some selection pressure has driven its development. Now researchers believe they have found the answer to what selection pressure drove the development of squids’ huge eyes.

It has long been assumed that the huge eyes of giant squids were in some way an adaptation to living in very dark water. Larger eyes are able to take in more light and so presumably would allow the monstrous squid, which measure up to 49 feet (15 m) in length, to see better in dim light conditions. However, researchers found that for most situations, an eye size greater than 4 inches, the size of swordfish eyes, provided no benefit. See the chart below for a comparison of the relative size of creatures’ eyes.

However, there was one situation in which the very large eyes of the squid would be very helpful: the detection of approaching sperm whales. As sperm whales cut through the deep water, they disturb numerous smaller organisms, which give off bioluminescence. The massive eyes of the giant squid species would allow it to detect and interpret the pattern of disturbed organisms to see sperm whales approaching from a distance of nearly 400 feet, which would allow the squid to potentially avoid approaching whales.

Looking at the chart above, we notice that another creature had eyes about the same size as the giant squid, the Temnodontosaurus platydon. Based on the models provided by researchers, it seems likely that the function of these eyes was the same: to detect the approach of a very large predator.

But what ate a temnodontosaurus? Shown above in comparison with a human, temnodontosaurus was one of the largest known creatures in Jurassic seas.

One possible speculation is the presence of a very large pliosaur. Some estimate that pliosaurs, similar to liopleurodon (image below), may have been up to 50 feet long, about half again the length of temnodontosaurus, which may very well have made it a viable predator. Another possibility, though, is the existence of a kraken, a gigantic cephalopod predator, which would not be well preserved in the fossil record.

What is remarkable about these eyes is their extraordinary ability to function under demanding conditions. If your eyes are not properly functioning, please contact a local ophthalmologist to talk about options for improving your vision.

Giant Cambrian Predator Had Excellent Vision

Recent analysis of fossils of an ancient predator reveal that the creature had excellent vision. The predator, Anomalocaris, had a worldwide distribution and is though to have been the top predator in the Cambrian seas. The existence of the predator was postulated before the creature was known. In 1979, a paleontologist studying trilobites explained that wounds on the bodies of certain trilobites might be caused by a large predator. At the same time, another paleontologist realized that what had previously been identified as a shrimp body was actually an appendage on a larger creature, which might be responsible for trilobite injuries. When the full body of the creature was described in 1985, it was characterized as “a formidable predator.”

Now we know that vision was an essential tool for this predator. 515 million year old fossils attributed to the creature show extremely complicated compound eyes. The eyes are speculated to have had as many as 16,0000 individual lenses, which is comparable to modern dragonflies, which may have as many as 30,000 lenses per eye. By contrast, triDragonflies use their excellent vision for hunting prey in a three-dimensional space, much like the Anomalocaris, which, with its newly-discovered vision could locate prey more effectively than competitors. In response, it is likely that prey developed anti-vision defenses, such as camouflage.

But what did Anomalocaris eat? Some researcher claim its teeth were too soft to eat trilobites, but fossil feces too large to be from any other known animal contain trilobite pieces. So, until another candidate is discovered, Anomalocaris is the prime culprit.

Because the fossils in question are not directly attached to Anomalocaris, some dispute that the eyes actually belonged to the predator. However, it is unknown what other creature they might belong to. Again, Anomalocaris is the only creature large enough to have owned them. However, because preservation of lenses is rare in fossils of this age, there may be many creatures with similarly sharp eyesight, which had been unknown before this discovery. It also likely means that arthropods likely developed eyes before they developed their characteristic jointed skeletons. For more information on this predator and its vision, see the Discover blog Not Exactly Rocket Science.

This type of eye is very different from human eyes. Instead of having a pair of lenses (the cornea and the crystalline lens) that focuses light onto a bank of nerve endings (the retina), compound eyes have many separate lenses that each focus light onto a separate photoreceptor. Thus, each separate lens represents a pixel in the creature’s vision, creating an arrangement that is ideally suited for capturing movement, but less well suited to capturing detail. An evolutionary arms race between camouflage and vision would drive increasing numbers of pixels that could resolve detail to allow predators to spot hiding prey, even when static.

As with these early predators, your vision is a great tool, and visual acuity can make the difference in facing your competition. To make sure your vision is at its peak, please contact a local ophthalmologist today.

Dinovision: How We Know What We Know about Dinosaur Vision

Studying the vision of current and prehistoric animals can help us understand the origins of our vision and track down causes for visual defects or shortcomings. Studying the vision of current animals is relatively easy. Researchers have devised a number of vision tests that allow us to test the functional vision of animals. We can also look at the structure of their eyes to determine their vision on the basis of the optical structures they possess.

It is much harder to understand the visual systems of extinct animals, since the eyes, as soft tissue, are rarely preserved, and then only in hard-bodied animals whose eyes were part of their exoskeleton, such as insects or arthropods.

In the case of something like dinosaurs, the problem is much more complicated. We can get a lot of answers from the morphology of the skull. The skull tells us a lot about the structure of the eyes, including whether they had a scleral ring to support their large eyes. A scleral ring is a bone that sits inside the sclera of the eye to give it support and tells us the size of the iris of an animal and therefore how much light it could admit

The shape of the skull also tells us the size and shape of the animal’s brain. We know that the brain performs numerous tricks in helping us to see, including selecting between different images, which is helpful in the case of people receiving multifocal intraocular lenses after cataract surgery.

Another important tool for determining the way extinct animals see is looking at their evolutionary relatives. We know, for example, that dinosaurs are closely related to crocodiles and birds. By looking at the vision of an animal’s living relatives, we can extrapolate how they may have been able to see.

Based on all this evidence, we have actually learned a lot about what and how well dinosaurs see, something we will explore periodically.

To learn more about your own vision, please contact a local eye doctor for a consultation.

Cataracts and Dinosaurs

You’ve probably heard the expression, “When you’re a hammer, every problem is a nail.” Well, it should not surprise you then that when an ophthalmologist considered what caused the extinction of the dinosaurs, he came up with an ophthalmological solution: cataracts. The ophthalmologist in question is L.R. Croft, and in 1982 he forwarded this theory in his slender hardbound volume The Last Dinosaurs. I’ve heard this theory periodically–it tends to make people’s list of possible causes of the K-T extinction following the phrase “some even believe . . .”–but I’ve never given it due consideration. I can’t evaluate the theory in detail because this book did not reach widespread circulation, and it seems like only a few dozen copies are around today. The closest to me is about 572 miles away, a bit far to drive for a blog. In fact, there is so little exploration of this theory that there are few relevant search results returned on Google, most of them being dedicated to Lara Croft’s various encounters with dinosaurs.

But I’ll try to hit all the high points before turning to the problems with the theory.

Croft was  one of the researchers who helped characterize the role that heat and ultraviolet radiation play as causes of cataracts. He realized that when the crystalline lens of the eye is exposed to heat or ultraviolet radiation, cataracts form faster, so he proposed that global warming could have led to an increase in the rate of cataract formation. If the rate increased enough, dinosaurs would go blind before they reached reproductive age.

As a supporting argument, Croft forwards the theory that dinosaur crests partly evolved to shade the eyes from sunlight. This seems plausible for carnotaurus

and triceratops,

but seems much less likely for parasaurolophus

and styracosaurus.

To me, the most powerful argument in favor of Croft’s theory is the discovery that placental mammals lack protective oil drops in our eyes, a lack implying that most mammals on the planet went through a nocturnal phase in evolution. This would explain how mammals survived to take over when all the dinosaurs were going blind–they were active at night and didn’t suffer the same amount of lens damage that diurnal dinosaurs did. It might also account for the survival of true reptiles. As ectotherms, they would tend to be out more at night during a global warming event, so they might escape the worst of the eye damage.

However, the argument still falls short in a number of ways. First, there’s the obvious problem with the head crests just not seeming to be primarily devoted to protecting the eyes. If cataracts are rising to extinction levels, there’s going to be some serious selection pressures to make these structures protect the eyes.

Second, it doesn’t account at all for the extinction of marine reptiles and ammonites. These would be largely shielded from increased damage to their eyes due to either global warming or increases in ultraviolet radiation. Ammonites most likely didn’t even have lenses that could develop cataracts, just primitive pinhole-camera arrangements like the modern-day nautilus.

All this to say that if you are going to ask an ophthalmologist a question, make sure it’s one about the eyes, because that’s the answer you’re likely to get anyway. If you have an eye-related question, please contact a local ophthalmologist today.