Animals with no eyes, but they can still see
There are some animals with no eyes, but they can still ‘see’. This is not a joke, but is serious scientific research.
For example, a researcher from duke University, points out the sea urchin. A sea urchin is like a pincushion that moves, with multi-colored spikes and soft feet that can stretch. However sometimes they behave as though they can ‘see’, but we have not yet discovered how they can do this. Their behavior is so purpose driven, but scientists have not yet found out what this behavior is based on. Some suggest they appear to be like aliens.
Some areas of science in this century have begun to consider that the sea urchin may not have an eye, but actually is an eye. Our planet could be inhabited by big eyes, wandering around on lots of soft feet.
Past scientific research on vision has concentrated on a few vertebrates and some insects. This is no longer the case. The scientists researching this area of vision are no longer studying only eyes that are similar to human eyes.
By studying other creatures, scientists can see how evolution has adapted to the problem of gaining information from light in many ways. These investigations include:
- Creatures that are too tiny to have brains, but have eyeballs,
- Creatures with skin that has its own light sense, and
- Scallops and butterflies, who have developed light sensors and eyes in different parts of their body.
The notion of an eye is much broader than was previously thought. After many years of research, the discovery of opsins (light-catching molecules in animals) changed the way scientists were thinking. Despite this the puzzle of the sea urchin remains.
The Eye as an Icon
Charles Darwin, in his book ‘On the Origin of the Species,’ suggests the eyeball, although almost perfect, is extremely complicated. The fact that it may seem too perfect to think of in evolutionary terms, did not deter Darwin. He notes that simple life forms may have had cells that were sensitive to light, only so they could tell day from night. Of course, Darwin was probably studying the camera-like eye that vertebrates have. Research today is looking far beyond these eyeballs.
Octopuses, for example, have eyes that are quite similar to vertebrates, but their skin has photoreceptors which can detect light, and the octopus can change color. This is used for camouflage. Experiments showed that the color changed even when the skin was removed from the octopus, showing that the brain was not actually needed.
The Asian swallowtail butterfly
The Asian swallowtail butterfly has insect-like compound eyes which are more effective than human eyes in ultraviolet or polarised light. However they also have photoreceptors (a basic organ for sensing light) on their genitals. A Japanese scientist believes these eye spots help with position. He found males had difficulty mating when the eyespot was removed and females without eyespots, even after mating, did not lay eggs.
The diversity of types of eyes is incredible. The Giant Clam has cameras the size of pinholes on the lips of its mantle, and uses them go watch for danger. The fish, known as the ‘four-eyed fish,’ has double eyes, half for looking above and the other half for below the water surface.
In the 1960s an English scientist found that although scallops have eyes that look like the eyes of a vertebrate, they have a surface of mirrors behind the eyeball, which focus on images. It has been suggested that scallops have a lens that is soft and squishy, which makes small adjustments to the amount of light coming in.
A crawling eye is one that can sense light and its direction. It is the ability to fit the bits of information together that creates an image.
It is said that J.D. Woodley first put forward the theory that urchins were like eyeballs, and their surface is mostly sensitive to light. Woodley proposed that the shade from the spines of the urchin would restrict the amount of light. However experimenting with this type of creature is difficult and often yields inconsistent results.
Various students of science have attempted many experiments to try to understand how urchins ‘see’. One experiment showed that although urchins couldn’t discern details, they could do more than simply knowing the difference between day and night.
To test another idea about the spines, suggested by Woodley, two students investigated urchins with different density of spines. In 2010 they reported that those with dense spines could ‘see’ a smaller spot that those with less dense spines. Although these results are showing the use of the spines, they still don’t prove what actually makes vision available to these creatures.
The discovery of opsins was important in this research. They are a protein that enables vision in animals. Human beings have other molecules that are sensitive to light, but it is the opsins that allow the photoreceptors to work.
Opsins are inside cell membranes. By itself an opsin in not useful for vision, but its ability to connect with a molecule, chromophore, is what triggers vision. In most animals and humans they are a type of retinal, which comes from vitamin A. That is one reason carrots are considered useful for vision.
At present scientists are aware of many forms of opsins, mostly through research into DNA. Scientists may know about them, but they remain unaware of what many of them actually do. There may be opsins that have nothing to do with light and vision. Mice and men have opsins in their sperm, but there opsins may be related to heat and consequent navigation.
More about urchins
In an Italian research laboratory in 2006 they detected most of the genes of a purple sea urchin, and this confirmed the urchin has light catching opsins. They actually have eight opsins. This led to further research which revealed that the soft foot of the urchin projects into the spines. They found these were rhabdomeric opsins, which are essential for vision in invertebrates. Because they were not over the surface of the urchin, they did not support the previous suggestions about spine-shading. It was proposed that opsins in the feet are sufficiently shaded to enable the urchin to detect the direction of light.
In more detail…..
Although the sea urchin doesn’t have a centralised brain, it does have opsin proteins that detect light. As research continued, a second system for visualisation was found in the sea urchin. Ciliary opsins were found on the surface, and also on the feet. There are many questions remaining about the sea urchin, as it also has ciliary opsins on the feet. How could the spine notice light and also give shading. The most recent research suggests the feet of the urchin may hole the answer. Does the urchin have ‘eyes?’