In the natural world, survival of an animal is dependent upon how the animal can see, which means his vision should be perfect to match his survival in his habitat. A sharp eye of an eagle to detect the prey, or a capability of a frugivore bird to see color so that it can differentiate between ripe or unripe fruits, or night vision for carnivores to hunt their prey, all are examples of specialized vision. The eyes of each animal group have evolved differently, and even they have got the capability, which we as humans don’t have. In this blog we will discuss the evolution of eyes in different animal groups and how it helps in their survival.
Evolution of eyes
The earliest eye developed in an organism was not complex but was just a cell which can not see the image but can detect or sense the light and darkness. Later during the course of evolution, in millions of years, these specialized cells evolved so much into a complex eye structure, what we see today.
These light sensitive cells or earliest eyes were simple cells and not able to detect the image but only can sense the light or darkness. This was the only requirement of that simple organism but later when the life of the organisms became complex their body started adapting accordingly and these simple organisms had evolved into a complex organism of different types and simultaneously the eyes also evolved.
During the evolution these simple light sensitive spots became slightly curved, forming a cup-shaped structure. This structure allowed the organisms to sense the direction of the light. Later this curved structure developed a small opening, like a pinhole camera, this allows to form a rough image of the objects in front of the eyes.
Later the lens developed which was a big advancement in the development of eyes. This makes the eyes produce sharper and clearer images. This type of eyes was developed by vertebrates and some invertebrates like octopuses. The evolution of eyes has not taken similar path in all organisms; it evolved differently in different animals as per needs. The insects developed very complex eyes which are made up of many tiny units, giving them a wide field of view and better motion detection. On the other hand, vertebrates have developed the camera type of eyes.
Types of Eyes in animals
As discussed above, the eyes of every animal’s group evolved differently, which best suited their needs. Here in this section, we will discuss the broad types of eyes found in animal groups
Simple eyes (Ocelli)
This is the simplest form of visual organ, found in invertebrates like earthworm, flatworm, and in some insects. This type of eyes cannot form images but instead they detect the light or darkness which helps the organisms to avoid predators and stay in suitable environments. In insects, these are small, secondary photoreceptor organs located in a triangle on the head between compound eyes. In insects these eyes detect rapid changes in light intensity polarization to aid in flight stability.
Compound eyes
Compound eyes are specialized visual organs found in arthropods (insects and crustaceans). A compound eye contains thousands of photoreception units called ommatidia. Each of them has their own lens and light receptors which creates a mosaic image which provides wide field of view and helps detect rapid motion.
The numbers of ommatidia ranges from a few to up to 30,000 in dragonfly. These ommatidia combine thousands of images to create a panoramic mosaic picture with the 360-degree view.
Camera type eyes
This is the most complex high resolution imaging type of eyes and are found in all vertebrates and some invertebrates like octopuses. They feature a single chamber, cornea, lens and retina.
Cornea, the transparent outer layer and the internal lens bend the light and focus on light sensitive surface called retina, which is like the camera sensor or film. The Iris and Pupil work as aperture of cameras and control the amount of light entering the eye. Optic nerves work as a data cable send the images to the brain.
Special Adaptation in Animal Eyes
Apart from the normal structural differences in the animal eyes, some animals have evolved with certain kinds of special features in their vision which helps them to survive in their environment.
Night vision in nocturnal animals
Some animals who adapt to night life have developed a unique feature in their eyes which helps them to see in the darkness better than others. These animals are owls, cats, civets etc. Their eyes have more rod cells which are sensitive to low light. Many of them have a reflective layer called the Tapetum lucidum which reflects the light back through the retina, improving visibility and causes the glowing eyes seen in the dark.
Sharp vision in Birds of Prey
Birds like eagles, hawks and vultures have exceptionally sharp eyes so that they can see their prey from a distance. Their eyes have more density of photoreceptor cells, which makes them see the fine details from far. Some raptors can see their prey from high above the ground during their flight because of this adaptation.
Independent eye movement
All animals move their eyes together, but only one animal can move them independently. Chameleons can move their eyes independently, which allows them to observe in two different directions simultaneously. This gives them around 360-degree field of view and helps them to see the prey and predator both. To get an accurate depth perception of the prey while hunting they move both eyes in one direction towards the prey.
Extraordinary color vision
Some animals can see better and far more colors than other animals including us. Bees can detect ultraviolet patterns on flowers while butterflies can see different colors for mate selection. The mantis shrimp is unique in terms of their complex visual systems; they can detect a wide range of colors and even polarized light. This ability helps them to navigate and communicate underwater.
Heat Detection in Snakes
Some snakes like pit vipers have developed a specialized organ near their eyes which can detect the heat of prey even in complete darkness. This helps them to hunt even effectively in complete darkness.
Eyes without vision
Animals who live in deep caves, underground tunnels, murky water or the dark depths of ocean where light cannot reach, have little or no use of eyes, they have spot on the place of eyes, but it does not work the way normal eyes do.
This change in their eyes is because of the no use or less of eyes. During their evolution the eyes disappear, and a spot left at the place. In these dark places vision does not help them to find the food or mate, so they developed another way to detect, like in freshwater dolphins they use SONAR system to navigate and find the food.
Conclusion
In the animal world eyes are more than an organ to just see. They have evolved and developed as a tool of survival and show us that every species sees their world differently. To understand nature, we have to learn to see not just with our eyes but with awareness.
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