In science fiction, it is common to show “alien” creatures with varying numbers of eyes. After all, all vertebrates that we know of have two eyes, what better way to make an unfamiliar creature by giving it extra eyes?
As it turns out, there are actually animals with three eyes, and I do not mean things like insects, arachnids, and other such creatures. Take the reptile pictured above: the tuatara. Despite its appearance, the tuatara is not a lizard. It belongs to a group of reptiles called Rhynchocephalia. This used to be a diverse group of reptiles, but they are all extinct today, except for one species, Sphenodon punctatus, which lives exclusively on New Zealand.
In case you are wondering how the tuatara is different from a lizard, the biggest difference has to do with the skull. Lizards and snakes both have flexible skulls. Snakes are particularly famous for being able to stretch their skull wide open to swallow things much larger than their heads. Lizards can do a similar thing, though most of them do not have nearly the flexibility of a snake. Nevertheless, all lizards have multiple joints in their skull and jaw that allows for this extra flexibility. In contrast, tuataras are like us, in the sense that they have only a single joint in their skull, which only allows the mouth to open and close.
As you look at the picture of the tuatara, you may note that it doesn’t look like it has extra eyes. Indeed, in an adult tuatara, only two eyes are visible, and these would be the pair of normal eyes. However, tuataras have a structure called a third eye which is situated on top of its skull. In a juvenile, this third eye is visible, but in an adult, it becomes covered with skin.
This third eye has several structures that are reminiscent of a normal eye. There is a covering on the outside that resembles the cornea (the cornea is the outermost clear part of the eye), the inside of the eye is filled with fluid (much like a normal eye), there is a lens-like structure, and there is a something like the retina that contains photosensitive cells (“photosensitive” means that the cells are capable of detecting light).
While this third eye may be covered by skin, its location can be easily found in the skull of a tuatara. There is a gap between the paired parietal bones in the skull, and it is in this gap that the third eye sits. That is why this third eye is also called a parietal eye.
The exact function of this eye is not known. It is thought that it is used to monitor the duration of a day, the help regulate the tuatara’s circadian rhythms. “Circadian rhythms” refers to regular cycles, such as cycles of sleep and being awake, that a creature regular goes through.
Interestingly, the parietal eye is connected with the pineal sac. This sac secretes the hormone melatonin, and this melatonin regulates circadian rhythms. Interestingly, we humans have a modified version of the pineal sac. In us, it is located inside the brain, it is called the pineal gland, and it secretes melatonin, which once again regulates circadian cycles.
Even though the parietal eye of a tuatara has a very different function from normal eyes, it is still considered an eye. We must understand that there are actually a wide variety of eyes found among animals, but the one thing that they have in common in their ability to detect light. Some eyes, such as the parietal eye, are not actually used to form images, as our eyes do, but because they still detect light, they are considered eyes.
Thoughts from Steven